Download CEP10071 - Evaluation report: X-ray tomographic image guided

Evaluation report
X-ray tomographic image guided
radiotherapy systems
CEP10071
March 2010
Contents
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Summary ................................................................................................ 3 Introduction ............................................................................................. 6 Product description ................................................................................. 9 Methods ................................................................................................ 14 Technical performance.......................................................................... 23 Purchasing ............................................................................................ 48 Acknowledgements ............................................................................... 50 Glossary ................................................................................................ 51 References............................................................................................ 54 Appendix 1: Supplier contact details ..................................................... 58 Appendix 2: Clinical protocols ............................................................... 59 Appendix 3: Imaging dose calibration.................................................... 61 Appendix 4: Imaging dose data ............................................................. 62 Appendix 5: Hounsfield number accuracy ............................................. 64 Appendix 6: CATphan image quality – spatial resolution ....................... 66 Appendix 7: CATphan image quality – CTP404 module ........................ 69 Appendix 8: Evaluation phase 1 summary of results ............................. 72 Appendix 9: User survey questionnaire ................................................. 76 Author and report information................................................................ 84 CEP10071: March 2010
Summary
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The product
X-ray tomographic IGRT systems are designed to acquire images of the 3D
treatment target either immediately prior to or during radiotherapy treatment delivery.
This report describes the evaluation of three different tomographic IGRT systems
currently in use in the UK:
•
•
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Varian OBI v1.5
Elekta Synergy v4.2
TomoTherapy Hi-Art v3.2
Objective
This document forms part of a national evaluation of X-ray tomographic IGRT
systems, specifically those funded by the Department of Health Cancer Equipment
Programmes. The purpose of the evaluation was to investigate whether these
systems are able to provide accurate image guidance for radiotherapy. This report
compares the performance and functionality of the systems assessed from both a
technical and a user perspective.
Field of use
Image guidance systems of this type are used to assist with and improve the delivery
of radiotherapy. They produce 3D images of patients whilst they are set up in the
treatment position. Associated image registration capabilities allow the error in
positioning to be quantified through comparison with a ‘reference’ CT image of the
patient acquired at the planning stage. If this is found to be above a specified action
level, these errors can be corrected via movement of the patient support system or
re-positioning of the patient.
National guidance
As image guidance is a relatively new and developing technology there is little
national or international guidance governing 3D CT-based in-room image guidance
systems. The National Radiotherapy Advisory Group in its 2007 report to the UK
government [1] recommended that all replacement and newly installed machines are
capable of image guided four-dimensional (4D) adaptive radiotherapy.
Methods
The evaluation was carried out in accordance with the CEP protocol [2], developed
by a team in Leeds in consultation with equipment suppliers, users, professional
bodies, and recognised experts in the field. The evaluation included several
phantom-based technical studies assessing image alignment, image dose and image
quality with a total of ten sites contributing data.
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Summary
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A user survey was conducted through a web-based questionnaire, with recruitment of
users of tomographic IGRT performed through the Society of Radiographers.
Technical performance
All systems assessed had accurate alignment between the tomographic image to the
treatment isocentre (within 1mm).
The automatic couch correction features of all systems accurately re-positioned the
treatment isocentre to within 1.5mm following translational shifts. Rotational
accuracies were not measured.
Imaging doses measured for typical clinical protocols ranged from 1.4 mGy per scan
for a low dose head protocol to 25 mGy for the highest exposure pelvis protocol.
These are comparable or lower than the maximum dose associated with most
megavoltage planar portal imaging protocols.
All systems were able to differentiate the contrast between muscle and fat as evident
from images taken of a male pelvic phantom and, from this point of view; all were
able to provide 3D soft tissue anatomical information.
There were differences observed in the measured image quality parameters on each
of the three systems and these were shown to vary with image dose. It was not
possible, in this evaluation, to determine whether these differences affect the
systems ability to perform image guidance.
Operational considerations
Tomographic IGRT systems are still relatively new. Few centres who responded to
the user survey have had them installed for more than two or three years. As a result,
the role of tomographic IGRT is still developing. This is further indicated by the
variety of clinical applications and verification strategies used. Users were generally
happy with the technical features of their systems.
These systems may require additional in-room space, cable ducting and desk space
for the IGRT workstation in the control area. The exact requirements will depend on
the particular system and should be confirmed early in the purchase process so any
refurbishment costs can be included in the business plan.
CEP verdict
All three systems evaluated fulfil the basic requirements of IGRT. They are capable
of imaging 3D volumes within a patient with some definition of soft tissue and they
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Summary
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each have the ability to automatically correct for translational discrepancies in patient
position based on comparison with a reference CT scan. Alignment of scans with
treatment isocentre and automatic correction of the treatment delivery is achievable
within 1.5 mm. Imaging doses are relatively low when compared to the dose from the
treatment itself and are equal to or less than the dose associated with conventional
megavoltage planar portal imaging. Following IRMER regulations, doses from
imaging should still be justified on an individual basis.
Tomographic IGRT is still relatively new and research and development are ongoing.
While the results from this evaluation suggest tomographic IGRT is promising, clinical
trials will likely be required to identify the disease sites where it can best be used and
optimise the functionalities for each site.
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Introduction
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Background
In the UK, over 50% of cancer patients will have radiotherapy as part of their
treatment [1]. This treatment modality involves the delivery of a high dose of radiation
to the tumour, whilst minimising the irradiation of surrounding healthy tissue.
Many significant developments have taken place in the field of Radiotherapy over the
past three or four decades primarily due to several key technological advancements.
The first major landmark was the introduction of computed tomography (CT) with
direct applications within treatment planning. This new imaging technique, coupled
with improvements in computer processing capabilities and speed, meant computer
planning systems rapidly developed to allow individualised patient planning in 3
dimensions (3D). This was followed by the introduction of multi-leaf collimators,
which resulted in an increase in the conformality of the dose distribution achievable
around the treatment target.
More sophisticated methods of planning and beam delivery are now available in the
form of intensity modulated radiotherapy, IMRT, in which the intensity of the radiation
is varied during radiation beam delivery. This enables better sparing of organs at risk
and the possibility of escalating the dose to the target without compromising
surrounding healthy tissue. In many cases IMRT plans are produced using inverse
planning techniques. This involves assigning dose constraints to each defined target
volume or organ at risk with the system automatically calculating the beam
parameters needed to deliver the optimal plan. These benefits can only be fully
realised if the radiation distribution is assured to be delivered where it is planned in
relation to patient structures.
Image-guided radiotherapy (IGRT) uses imaging techniques to improve the accuracy
of radiotherapy delivery to the target tumour, allowing more accurate and precise
targeting of the treatment volume and avoidance of organs at risk (OAR). This may
lead to a reduction in the radiation-induced complications and side effects that are
caused by irradiation of normal tissues. It may also allow an increased dose to be
delivered to the target tissues, thereby maximising the chances of successful control
or eradication of the tumour.
The term IGRT is ill-defined and may have a variety of meanings in different
contexts. Here IGRT refers to the ability:
•
•
•
to visualise the anatomical target and OAR’s in 3D
to identify changes in position, shape and size of target anatomy relative to
that seen when the treatment was planned
to quantify the variation in position of the anatomical target between the
planned and initial setup treatment images
CEP10071: March 2010
Introduction
•
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to correct any patient misalignment by changing the relative geometry of the
treatment machine (couch, gantry) before the treatment is delivered.
This technology is being rapidly deployed throughout the world and while there is, as
yet, little clinical trial evidence of its effect on outcomes, it has great potential to
improve radiotherapy delivery for a significant proportion of patients [3].
IGRT can involve almost any imaging modality, but a common implementation is to
have a “CT- like” scanner closely integrated with the linear accelerator (linac). For the
purposes of this project, these systems have been termed X-ray tomographic IGRT
systems.
Scope
The Department of Health Cancer Equipment Programmes invested £5m in IGRT
supplementary equipment to eight linear accelerators in operation within the NHS
(Varian and Elekta), plus the provision of one integrated IGRT solution
(TomoTherapy). This funding was conditional on recipient NHS trusts participating in
a national evaluation of the technology, the purpose being to verify that these
systems are able to achieve accurate image guidance and to compare their
performance and functionality both from a technical and a user perspective. This
document presents the findings of that evaluation.
The project comprised two phases. Phase 1 concerned the development of an
evaluation protocol, published separately [2] and phase 2 was the implementation of
this protocol at several ‘test’ sites, as reported in this document.
Specifications of tomographic IGRT systems available in the UK are provided in a
separate market review (CEP10072).
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Introduction
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National guidance
As IGRT is a relatively new and developing technology, there is little national or
international guidance governing 3D CT-based in-room image guidance systems.
The following is an excerpt from the National Radiotherapy Advisory Group in its
2007 report to the UK government [1]:
“NRAG: A 3D based environment for imaging, planning and radiotherapy delivery is
the current baseline for linacs. However, 4D radiotherapy takes into account tumour
volume in three dimensions but also takes into account changes with time (the 4th
dimension). Adaptive therapy also allows the treatment set-up and dose delivered to
be verified and then changed as necessary during a course of treatment. NRAG
advises that image guided four-dimensional (4D) adaptive radiotherapy is the future
standard of care for radical radiotherapy treatment that the NHS should aspire to.
NRAG therefore recommends that all replacement and newly installed machines are
capable of image guided four-dimensional (4D) adaptive radiotherapy. There is
evidence (set out in the technology report) that these processes will become more
time-efficient as the technology becomes standard practice.”
Outside of the UK there are work in progress guidelines for the use of IGRT from the
American Society for Therapeutic Radiology and Oncology (ASTRO) and the
American College of Radiology (ACR) [4] and also from the Advanced Technology
QA consortium, based at Washington University School of Medicine in the United
States [5]. The ACR have also published practice guidelines for IGRT [6].
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Product description
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Equipment overview
X-ray tomographic IGRT systems are designed to acquire images of the 3D
treatment target either immediately prior to or during radiotherapy treatment delivery.
The systems enable these images to be compared with the CT reference images
used for planning the patient’s treatment. The supporting document, CEP10070 [2]
provides a framework and detailed protocol for the technical evaluation of all X-ray
tomographic IGRT systems, including in-room kilovoltage CT (kVCT) [7], kilovoltage
and megavoltage cone beam CT (kV-CBCT, MV-CBCT) [8, 9] and megavoltage CT
(MVCT) [10].
This specific report describes the evaluation of two types of tomographic IGRT
system; kV-CBCT and MVCT. KV-CBCT systems have an additional kV X-ray
imaging system mounted onto the gantry of the MV treatment machine, producing kV
cone beam CT images. This is a method used by a number of different
manufacturers [11-12]. MVCT is, at present, unique to the one manufacturer and
employs the technology of standard helical kVCT but uses an MV fan beam [14].
MV-CBCT systems use the electronic portal imaging device (EPID) which is mounted
on the gantry of conventional MV treatment machines. These acquire a 2D projection
image through the patient from the MV treatment beam but can be engineered to
rotate around the patient while acquiring a series of projection images. The images
can be reconstructed into 3D datasets with image intensity related to the electron
density. At the time this study was initiated, no such systems were available in the
UK market and are therefore not covered in this evaluation. Additionally, kVCT
systems which are essentially a CT scanner located in the treatment room with a
couch that allows direct transfer of the patient between the kVCT scanner and the
MV treatment machine were also not evaluated in this study.
Systems evaluated
Three systems were evaluated for this report:
•
•
•
Varian OBI v1.5
Elekta Synergy v4.2
TomoTherapy Hi-Art v3.2
Varian On-Board Imager (OBI) v1.5
The Varian OBI is a kV-cone beam CT system integrated onto the Varian Clinac
series of linacs. The system consists of an X-ray tube and amorphous silicon flat
panel detector, both of which are mounted on robotic arms to the linac gantry system.
The imaging direction is perpendicular to the treatment beam axis. For the tube this
is such that the X-ray source is at 1000 mm from the machine’s isocentre. For 2D
imaging the source to detector distance is variable but for CBCT it is fixed at the
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Product description
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default setting of 1500 mm. Re-calibration for other source to detector distances is
possible, but the default is usually adopted.
Image acquisition is controlled by the on board imager computer from the linac
control room. The system can be set to acquire both static gantry radiographic or
fluorographic images as well as CBCT. In CBCT mode the robotic arms extend
automatically to the required position prior to imaging and can be retracted back to
their folded position once imaging is complete. There are two principal fields of view
in the trans-axial plane called full fan and half fan. In full fan mode the maximum field
of view is 24 cm, while in half fan mode the collimation and detector are shifted
laterally to allow a field of view up to 50 cm diameter1 and a half cone beam is used
resulting in only half of the field of view being irradiated in any one projection.
Collimation in the long axis is continuously variable and can be set by the operator
with the maximum lengths achievable being dependent upon the fan mode and slice
thickness.
During image acquisition approximately 650 2D projection images are acquired while
the gantry rotates through 360º, taking approximately 1 minute. In full fan mode, it is
possible to reduce the number of projection images to approximately 360, by
reducing the gantry rotation range to only 200º. This is an effective way of reducing
patient dose and increasing image acquisition speed. Scans can be acquired with or
without a bow tie filter [13] (with separate filters available depending on whether
operating in full fan or half fan mode). The direction of gantry rotation can be either
clock-wise or anti-clockwise. The direction defaults to that with a start angle nearest
the gantry angle immediately prior to the start of image acquisition.
There are six standard pre-defined CBCT modes with three different tube voltages;
100 kV for those involving the head, 110 kV for the thorax and 125 kV for pelvic
acquisitions. These modes have standard exposure settings ranging from 72 mAs
(low dose head protocol) to 720 mAs (high quality head protocol and pelvis spotlight
mode). By varying the combination of tube current and pulse length, it is possible to
create user customised image acquisition modes at non-standard exposures.
Once acquired, the projections are stored on a separate computer system which
reconstructs the CBCT slices and sends them to the OBI computer for display.
Images can be reconstructed on pixel matrices of 128 x 128, 256 x 256, 384 x 384 or
512 x 512. The pixel size depends on the field of view and the size of the pixel
matrix. Slice separation can be selected with a choice of 1 to 5 mm in 0.5 mm
increments and 10 mm. However, the default setting when used in a pre-defined
1
Note that these FOVs apply for the default source to detector distance of 1500 mm.
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Product description
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clinical mode is a pixel matrix of 384 x 384 with 2.5 mm slice thickness. There is a
choice of two image reconstruction filters, sharp and standard which affect the
characteristics of the noise in the reconstructed images.
Image registration of the CBCT image with a reference image is also performed on
the OBI computer. Image registration can be performed either manually or
automatically and is restricted to rigid body transformations and rotations. Assuming
a standard Varian couch, the degrees of freedom for the registration are the three
perpendicular translation directions, (lateral, vertical and longitudinal) and one
rotation about the vertical axis corresponding to an isocentric couch rotation.
Correction of patient position is achieved by translation of the couch in lateral, vertical
and longitudinal directions as well as isocentric couch rotation and all can be
performed automatically and remotely from outside the room, including yaw. There
are no in-built facilities for correcting rotations about the horizontal two axes, ‘pitch’
and ‘roll’.
Elekta Synergy x-ray volumetric imager (XVI) v4.2
The Elekta Synergy system is a kV-cone beam CT system integrated onto an Elekta
Precise linear accelerator. The system consists of an X-ray tube and amorphous
silicon flat panel detector both of which are mounted with a view direction that is
perpendicular to the treatment beam axis. The tube is deployed manually for imaging
while the detector unfolds from its stored position against the face of the gantry under
motorised control. The configuration of the system has the X-ray source at 1000 mm
from the machine’s isocentre while the X-ray source to imager distance is fixed at
1536 mm.
Image acquisition is controlled by the XVI computer from the linear accelerator
control room. The system can be set to acquire static gantry, radiographic or
fluorographic images as well as CBCT. There are three principal fields of view in the
trans-axial plane called small, medium and large. The small field of view has a
diameter of approximately 25 cm with the detector centred on the source-isocentre
axis. The large field of view has a diameter of approximately 50 cm and requires a
half fan offset of the detector and corresponding offset collimation of the radiation
field. The medium field of view has a diameter of approximately 40 cm and is a
compromise between the improved image quality of the small field of view and an
increased field of view to sufficiently image the majority of patients. Collimation for
the small, medium and large fields of view is achieved by a set of removable lead
collimator inserts with fixed aperture sizes. These have to be interchanged manually
when swapping between fields of view. Collimation in the long axis is also achieved
using the removable collimators with a choice of 3 to 4 lengths depending on the field
of view (S, M, L). These are labelled 2, 10, 15 and 20 but are approximately 3.5,
13.6, 17.6 and 27.7 cm (giving corresponding cone angles of 1.0º, 3.9º, 5.0º and
7.9º) and can be set by the operator.
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Product description
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During image acquisition approximately 630 2D projection images are acquired while
the gantry rotates through 360º, taking approximately 2 minutes. The direction of
gantry rotation can be either clockwise or anti-clockwise.
The tube voltage for image acquisition can be chosen from a range of 70 kV to
150 kV and the exposure level can be chosen using discrete combinations of tube
current and pulse length. The range of exposure levels is from 0.1mAs/pulse to
80mAs/pulse. Details of image acquisition such as tube voltage, current, pulse
length, gantry start and stop are chosen using predefined clinical presets.
Once acquired the image is reconstructed on the XVI computer. Image
reconstruction size, position and voxel size are controlled using reconstruction
presets. Standard reconstruction presets provided with the system have three
resolutions, described as small medium and large corresponding to 0.5 mm3, 1 mm3
and 2 mm3. Voxel sizes are isotropic hence slice thickness is linked to the in-plane
resolution. The image viewing application allows slices to be viewed as an average of
slices on either side to simulate thicker slices. The flexibility to adjust image
reconstruction, filter parameters and scatter correction exists, but is not usually
adjusted from the default values by the user.
Image registration of the CBCT image with a reference image is also performed on
the XVI computer. Image registration can be performed either manually or
automatically using one of two algorithms. The Bone algorithm uses the chamfer
matching algorithm [15] and the Grey algorithm uses correlation ratio [16, 17]. Image
registration is rigid body with six degrees of freedom. These are the 3 perpendicular
translation directions, lateral, vertical and longitudinal, and 3 rotations about the three
major axes.
Correction of patient position is achieved by translation of the couch in lateral, vertical
and longitudinal directions, as well as isocentric couch rotation, and all can be
performed automatically and remotely from outside the room. There are no in-built
facilities for correcting rotations. However, additional equipment, such as the
Hexapod robotic couch, may be purchased to correct rotations about the three axes.
TomoTherapy Hi-Art v3.2
The TomoTherapy Hi-Art system is a fully integrated IMRT/IGRT system. The
megavoltage treatment delivery system is delivered using a helical fan beam with a
continuous couch feed similar to a diagnostic CT scanner. The design of the
TomoTherapy Hi-Art system is optimised for the delivery of complex IMRT
treatments, the IMRT capabilities replacing the features of conventional linear
accelerator systems such as electron beams and non-coplanar beam arrangements.
Imaging is performed using an X-ray beam generated from the same linear
accelerator system as the treatment beam; the 6MV beam is de-tuned to 3.48 MeV
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Product description
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peak (average of 700-800 keV) for imaging purposes [18]. The beam is collimated by
264 dynamic multi-leaf collimators to a width of 1 cm and length sufficient to give a
maximum transverse field width of 38.4 cm at isocentre. Maximum field length is
160 cm, achieved by continuous translation of the treatment couch through the
isocentre as the radiation is delivered. The image is acquired using an arc shaped
Xenon gas detector which rotates with the source. The source to detector distance is
145 cm and the source to axis distance is 85 cm. The default pitch settings of the
helical acquisition are 1.0, 1.6 and 2.4 which correspond to slice widths of 2 mm,
4 mm and 6 mm. These are given the terms 'fine', 'normal' and 'coarse'. The beam
on time to acquire a single slice is 5s with an initial 16s overhead at the start of the
procedure.
Images are reconstructed with an in-plane matrix of 512 x 512 for the 38.4 cm width
field of view giving rise to a pixel spacing of 0.75 mm.
Rigid body image registration of the MVCT with the reference kVCT can be
performed manually or automatically using a mutual information based algorithm [14].
Image registration provides the three perpendicular translations (lateral, vertical and
longitudinal) and three rotations about the three major axes, required to correct the
patient. Vertical, longitudinal and lateral couch movements can be performed
remotely from the control area. Correction for rotation about the longitudinal axis
('roll') can also be performed automatically and remotely by correcting the gantry
position of the treatment prescription. There are no facilities for correcting rotations
about the other two axes, 'pitch' and 'yaw'.
MVCT images acquired before a treatment is administered can be used in the
Planned Adaptive module to recalculate the dose to the patient from that particular
fraction. This enables any positional corrections or variations in patient anatomy that
may be present, (for example tumour shrinkage), to be taken into account. Once
recalculated, a dose difference map can be generated, allowing the effect of these
anatomy or positional changes on the plan to be assessed. If desired, regions of
interest (ROIs) can be re-contoured to allow the analysis of DVHs for these deformed
structures. In addition, hotspots and coldspots within a particular structure can be
isolated and segmented out as separate ROIs, thus local top up or reduction of dose
can be performed. The MVCT data can then be exported to the planning system
incorporating any modifications or additions to the ROIs and a new plan can be
generated.
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Methods
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User evaluation
A web based user survey (appendix 9) based on previous KCARE survey forms and
designed in consultation with the Society of Radiographers was used to gather user
feed back on tomographic IGRT systems. Where possible, the survey used multiple
choice answers and rating scales to ensure focussed responses, but provision was
also made for more general comments.
Technical evaluation
The objective of the technical evaluation was to test each of the IGRT systems to
assess their capability in delivering accurate image guidance. A secondary objective
was to perform a subset of the tests on more than one system of the same type to
determine the variation across multiple systems from the same manufacturer where
possible. To achieve these objectives the evaluation examined three key aspects of
performance: image alignment, image dose and image quality.
Image alignment
For image guidance, geometrical accuracy of the image is as important as the ability
to see an object in the image. In this context, geometrical accuracy can be divided
into a number of components:
•
•
•
•
initial spatial registration of the image data to the treatment delivery system
spatial integrity of the data within the image volume
image registration of the IGRT image to the pre-treatment planning scan
the correction of any patient misalignment.
The initial spatial registration identifies the position of the image volume with respect
to the position of the isocentre of the megavoltage beam delivery system; often this is
in a different plane with a defined offset. This measures the radiation isocentre which
is a better representation of the delivered treatment centre and is quicker than
measurement of the physical isocentre. To assess this alignment, for CT based
imaging a kV-MV coincidence test has been devised specifically for this evaluation
[2, 19]. For an integrated CT-treatment machine, as is the case with TomoTherapy,
the imaging and treatment beams are one and the same and are therefore assumed
to be registered.
Once the imaged volume has been registered to the isocentre of the treatment
delivery system it is also necessary to know that all points within an imaged volume
are in the correct position relative to each other and that there are no rotations of the
volume. By using structures in an image quality test phantom, image scaling, rotation
and skew have been measured. While these do not demonstrate spatial integrity of
the data over the whole imaged volume, they do at least demonstrate the integrity of
a sample of points. Since image reconstruction relies on correct knowledge of the
CEP10071: March 2010
Methods
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projection data geometry, identification of these few points goes some way to
demonstrating the integrity of the whole volume.
In this evaluation, image registration of the IGRT image to the pre-treatment planning
scan has been performed manually using visualisation. Although automatic image
registration algorithms are provided on some IGRT systems and have the potential to
reduce time spent on image registration and reduce intra-observer variability, the
performance of these is not easily measured and is the subject of current research
[20]. It therefore was not considered to be within the scope of this report to evaluate
the performance of the automatic image registration algorithms.
To demonstrate the ability to measure and correct for the misalignment of a patient,
an ‘image-shift-verify’ test has been employed using a geometric phantom. The
ability to correct the patient position is usually restricted to three orthogonal axes of
translation (plus one axis of rotation for some systems) and these are evaluated in
this report. However, it is acknowledged that it does not demonstrate that the
treatment dose is delivered to the correct target. This test only verifies corrections
performed by a translation or rotation shift of the phantom using the patient support
system and does not verify corrections made by adjusting treatment delivery
parameters.
Image dose
The X-ray dose produced by the imaging equipment, which affects normal tissue as
well as tumour, is an important parameter which affects the usability of these
systems, as in some circumstances a high image dose may prohibit the repeated use
of such systems for daily online image guidance. The associated image dose has
therefore been measured on each system for a selection of clinically relevant imaging
protocols.
Image quality
As an indicator of a system’s ability to image soft tissue, the image quality must be
sufficiently good to resolve contrast between muscle and fat. It is accepted that the
inter-dependence of contrast, resolution and dose is a complex relationship which
limits the size of an object that can be seen at the same level of contrast. This is
further complicated by the ability to change the reconstruction filters on some
systems which can have large effects on this relationship. It is beyond the scope of
this evaluation to perform tests that comprehensively characterise the imaging
system. Instead, basic image quality measures of contrast, noise and resolution have
been measured on each system using a selection of clinically relevant imaging
protocols. The additional imaging of an anthropomorphic phantom has also been
included to enable the image quality of a clinically realistic object to be visually
assessed and allow subjective comparison between different systems.
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Methods
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Participating centres
The technical evaluation was carried out in two stages; the first, performed between
February and May 2008, took place at the DH funded ‘evaluation sites’. In addition
some evaluation work was performed at other sites with similar systems. These sites
formed part of an informal research network which permitted the testing of the
evaluation protocol. In this document these have been called the ‘test sites’.
Due to significant changes in software and system upgrades in the subsequent
period, a follow-up evaluation was carried out in February 2010 using the most up to
date clinical systems currently available in the UK. The main body of this evaluation
report presents the data acquired during this second wave of evaluations.
Results from the first round of evaluations are given in appendix 8. This data
provides an indication of the variation in performance that exists between multiple
systems from the same manufacturer and highlights the degree of standardisation
between them.
Table 1. List of participating sites
Phase 1. Evaluation sites
Equipment
Guy's and St Thomas NHS Foundation Trust
Elekta Synergy
Maidstone and Tunbridge Wells NHS Trust
Varian OBI
Poole Hospital NHS Trust
Elekta Synergy
Southampton University Hospitals NHST
Elekta Synergy
Ipswich Hospital NHS Trust
Varian OBI
University Hospital Birmingham NHSFT
Elekta Synergy
Royal Free Hampstead NHS Trust
Varian OBI
Cambridge University Hospitals NHS Foundation Trust
TomoTherapy Hi-Art
Phase 1. Test sites
Equipment
Ipswich Hospital NHS Trust
Varian OBI
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Methods
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Cambridge University Hospitals NHS Foundation Trust
TomoTherapy Hi-Art
Leeds Teaching Hospitals NHS Trust
Elekta Synergy
Phase 2. Test sites
Equipment
Clatterbridge Centre for Oncology NHS Foundation Trust
Varian OBI
Cambridge University Hospitals NHS Foundation Trust
TomoTherapy Hi-Art
Leeds Teaching Hospitals NHS Trust
Elekta Synergy
Test 1: Registration of image volume to treatment isocentre
Overview
The purpose of this test is to assess whether the centre of the imaging volume is
registered to the treatment isocentre to within ± 1 mm in all directions. This is
particularly critical if the system is to be used as guidance for intra-cranial
stereotactic treatments such as SRS and SBRT. A greater tolerance of ± 2 mm may
be acceptable for other clinical sites [51]. The design of this test is not suitable for
systems where the imaging beam is the same as the treatment beam and therefore
share the same focal spot and machine isocentre
The test relies on a phantom that can be imaged with both the imaging device and
the treatment device. The MODUS Penta-Guide Quasar Phantom [2, 19] was chosen
for this task. Its design is such that the megavoltage treatment isocentre is inferred
from an anterior-posterior and lateral portal image, however this method does not
measure the MV treatment beam isocentre sufficiently accurately to determine
whether the IGRT system is suitable for treatments which require high levels of
accuracy and precision, e.g. stereotactic cranial irradiation and SBRT. The method
as detailed in [19] is summarised below.
Materials
1. Modus QUASAR Penta-Guide phantom (Modus Medical Inc, Ontario, Canada)
[21]
2. Analysis software [2, 19]. For further details contact the authors.
Method
1. The Pentaguide phantom was scanned at high resolution on a CT scanner
and a simple plan was created with a single isocentre positioned at the
geometric centre of the central air-cavity within the Penta-Guide phantom.
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2. Eight ,12cm x 12cm square beams were created at gantry angles of 0°, 90°,
180° and 270°, two at each gantry angle with opposing head angles (e.g. 90°
and 270°). 10 cGy was set for each beam to provide sufficient quality MV
images for automated image analysis.
3. The DICOM images, DICOM-RT structure and plan data were sent and
imported into the IGRT system(s) under evaluation and the plan data was sent
to the treatment delivery system.
4. The phantom was aligned to the room lasers of the IGRT system ensuring that
the phantom was level.
5. Eight MV images were acquired for the beams defined in the treatment plan.
6. The phantom was then scanned using the CBCT system with a standard field
of view.
7. Analysis software was used to determine the shift of the centre of the
Pentaguide phantom with the MV isocentre.
8. The IGRT system was then used to determine the shift required to align the
central air-cavity in the CBCT image of the Pentaguide phantom with the aircavity in the CT images of the treatment plan.
9. The vector (lateral, longitudinal and vertical) difference between the phantom
centre measured with the MV-imaging system with that of the kV-imaging
system was then calculated.
Test 2: Image-shift-verify test
Overview
This test demonstrates the ability of an IGRT system to perform a basic correction of
patient misalignment to within ±2 mm. This is achieved through imaging a misaligned
object, performing an image registration to determine the misalignment and
correcting the misalignment. Verification of the correction is then achieved by a
further image and image registration procedure. This test was performed using the
geometric Pentaguide phantom as used in test 1.
Materials
1. Modus QUASAR Penta-Guide phantom (Modus Medical Inc, Ontario, Canada)
[21].
Method
1. The Pentaguide Phantom was scanned at high resolution on a CT scanner
and a basic plan was created with the treatment isocentre at the centre of the
phantom.
2. The DICOM images, DICOM-RT structure and plan data was imported into the
IGRT system under evaluation.
3. Using external lasers on the IGRT system, the phantom was aligned to a set
of offset markers on the surface of the phantom and a standard IGRT image
was acquired.
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4. Manual image registration was then performed and the required registration
shifts as displayed by the IGRT system were recorded.
5. The phantom position was then corrected as far as possible by movement of
the patient support system using automated methods where available and
manual movement when this was not possible.
6. A repeat image was then acquired followed by another image registration to
enable residual errors to be quantified.
7. This process was performed as described at each of the evaluation sites and
repeated three times at each of the test sites to give an indication of system
reproducibility.
Test 3: Imaging dose
Overview
Using a standard method, this test enables the dose in air and the dose (to water) in
two CTDI phantoms to be measured for a range of scanner settings and clinical
protocols.
Currently no standards exist for the measurement of concomitant dose associated
with IGRT procedures and there is no consensus as to what measurements should
be made or what information should be reported. For standard diagnostic CT
scanning procedures, a 10 cm long CTDI chamber is used, which due to the
elongated sensitive volume, enables all scatter contribution to be collected during the
imaging of a single slice. Measurements are made both at the centre and at the
periphery of a CTDI phantom. Resultant doses are then expressed as a weighted
CTDIw (1/3 x central measurement + 2/3 x peripheral measurement). However,
CBCT systems do not have narrow beam geometry thus it is no longer instructive to
measure over 10 cm. The situation is further complicated by the fact that for larger
fields of view, there is an offset between the tube and the panel thus resulting in a
complex dose profile in the transverse plane. Consequently, for this evaluation, dose
is sampled at a single point within the field using a Farmer chamber inside a CTDI
phantom and measurements are acquired at an intermediate position (~7 cm from
the centre) in addition to the standard central and peripheral positions.
Materials
1. Two computed tomography dose index (CTDI) phantoms (head and body,
16 cm and 32 cm diameters) (ImPACT, London, UK) [22]
2. A Farmer chamber with calibration traceable to NPL standard at the
appropriate beam quality obtained by measuring the half-value thickness.
3. Calibrated electrometer.
Method
This protocol closely follows the methods employed by Song et al [23].
1. For in air dose measurements, the ion chamber was suspended over the end
of the couch and positioned at isocentre.
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2. For scan protocols involving smaller FOV imaging fields, dose measurements
were made in two 16cm diameter CTDI phantoms. These phantoms were
positioned back to back with the ion chamber positioned longitudinally offset
by approximately 1.5 cm to ensure that measurements were not affected by
the join between phantoms. The CTDI phantoms were aligned such that the
midline of the phantom coincided with the imaging isocentre.
3. For scan protocols involving larger FOV imaging fields, dose measurements
were made in two 32 cm diameter CTDI phantoms set up in the same way.
4. On the Elekta Synergy system, 6 standard clinical protocols were measured
using the S20 collimator for the head protocols and M10, M15 and M20 for the
pelvic acquisitions. On the Varian OBI system, the five manufacturer supplied
standard clinical protocols were measured and on the TomoTherapy Hi-Art,
measurements were made for each of the three pitch settings (fine, medium
and coarse).
5. In all cases, measurements were made at the centre and periphery of the
CTDI phantom. For the larger FOV protocols, a further measurement was
acquired at an intermediate position (~7 cm from the centre).
For further details of the image acquisition settings adopted see appendix 2.
Test 4: Pseudo-clinical image quality
This test is intended to assess image quality using an anthropomorphic phantom
which has realistic soft-tissue organs in order to simulate a clinical scenario as
closely as possible.
Materials
1. Virtually Human Male Pelvis Phantom, CIRS (Norfolk, Virginia ) [24])
Method
1. The anthropomorphic phantom was aligned centrally to the isocentre.
2. An image of the phantom was acquired on each of the test systems using
typical clinical protocols for prostate IGRT, for specific acquisition and
reconstruction settings, see Appendix 2.
3. The images were then displayed in each of the sagittal, coronal and
transverse planes ready for visual inspection.
Test 5: Image quality
Overview
This test is intended to measure various indicators of image quality for 3D images
measured for a selection of acquisition and reconstruction settings representative of
clinical practice. As there are no standard ways of measuring image quality over the
whole volume, this method measures image quality at specific points in the FOV
CEP10071: March 2010
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using a standard image quality phantom [25], thus enabling the following image
quality indicators to be measured: contrast to noise ratio, Hounsfield number
accuracy, axial plane resolution (lp/cm), modulation transfer function, slice sensitivity,
uniformity and spatial integrity.
Materials
1. CATphan 504 (The Phantom Laboratory, Salem, NY, USA) [25].
2. Image analysis capability either on the system under test, or a separate Dicom
viewer such as ImageJ [26] or IQWorks [27].
Method
1. The phantom was positioned at the isocentre of the imaging volume. On the
upper surface of the CATphan there are five alignment markers to indicate the
position of the centre of each module. In each case, the CATphan was aligned
longitudinally with the 2nd marker (CTP528). Care was taken to ensure that
the CATphan was aligned laterally and vertically with the room lasers and to
avoid any tilt of the phantom.
2. The phantom was then scanned on each system using the acquisition
parameters dictated by each system’s respective clinical protocol; see
Appendix 2 for further details.
Image analysis
1. Contrast-to-noise ratio (CNR): Contrast-to-noise ratio gives an indication of
the ability of a system to distinguish the difference between two materials in
the presence of image noise. It is given by the ratio of the difference in mean
grey level between two objects (contrast) by the standard deviation of the
noise. Using the image analysis function, the mean and standard deviation of
the mean pixel values occurring within square ROIs (approx 7mm x 7mm)
placed on each of the 8 contrast inserts of the CTP404 module was measured
and thus CNRs were calculated for each insert relative to polystyrene using
the equation below, where ‘mat’ indicates the material of the ROI, ‘ x ’ is the
mean of the pixels in the ROI and ‘s’ is the standard deviation of the pixel
values in the ROI.
CNRmat =
(x
(s
mat
2
mat
− x polystyrene )
+ s 2polystyrene
)
2. Hounsfield number accuracy: The mean pixel values found during the
analysis above are related to the Hounsfield numbers for each of the inserts.
To convert the raw image data into actual Hounsfield numbers, it is necessary
to apply a scaling factor, ‘Rescale slope’ and an offset equal to the ‘Rescale
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Methods
22
intercept’ [28]. Using the Dicom header files, these factors were determined
and applied to the data thus enabling the Hounsfield numbers to be
calculated. Through comparison with the actual values specified by the
phantom supplier, the Hounsfield number accuracy was determined by taking
the difference between the two.
3. Uniformity: Uniformity is a measure of a system’s ability to produce a uniform
image across the field of view of an object with uniform density. To calculate
uniformity, five adjacent trans-axial image slices sampled from a large uniform
region of the CTP468 module were selected within each image set. For each
of these slices, a central ROI of size 10mm x 10mm, was chosen with four
other identical ROIs placed 45 mm above, below, left and right of the central
ROI. Two definitions were used to calculate the overall uniformity as defined
below, where ‘max’ and ‘min’ refer to the maximum and minimum pixel value
averaged over each ROI respectively, ‘Ave(peripheral)’ is the average pixel
value within all peripheral ROIs and ‘Centre’ relates to the mean pixel value
occurring within all central ROIs.
U1 =
( Max − Min)
( Max + Min) / 2
U2 =
Ave( peripheral ) − Centre
Centre
4. Axial plane resolution: The ability of the system to resolve two lines of high
contrast placed close together was quantified by recording the greatest
number of line pairs within the CTP528 module that were fully resolved when
the images were viewed at similar magnification and window settings.
5. Spatial integrity: To assess the system’s ability to accurately represent the
object imaged without scaling, rotation or distortion, the four rods spaced at
the corners of a 50 mm square within the CTP404 module of the CATphan
were used. Linearity was quantified by recording the side length with the
greatest deviation from 50 mm and the aspect ratio was recorded as the
maximum ratio of perpendicular side to length.
6. Slice sensitivity: Slice sensitivity is a measure of slice width. This was
assessed by measuring the full width at half maximum (FWHM) across the
wire ramps present in the CTP404 module of the CATphan. This was
performed using the image analysis software IQworks [27].
7. Low contrast visibility: Within the central region of the CTP404 module of
the CATphan, there are acrylic spheres with diameters ranging between 2 mm
and 10 mm. To assess low contrast visibility, the images were assessed to
see if any of the spheres were visible.
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23
Test 1: Registration of image volume to treatment isocentre
This test was only performed on Elekta Synergy v4.2 and Varian OBIv1.5, being
unnecessary for TomoTherapy Hi-Art as previously discussed. The test was also
performed at each of the phase 1 evaluation centres; see Table 21, Appendix 8. The
resultant vector (magnitude) has also been calculated.
Table 2. kV/MV alignment (mm)
Elekta Synergy v4.2
Lateral
Longitudinal
Vertical
Magnitude
0.4
0.4
0.0
0.6
Lateral
Longitudinal
Vertical
Magnitude
-0.3
-0.8
0.6
1.0
Varian OBI v1.5
Test 2: Image-shift–verify test
This test has demonstrated the ability of all systems to perform a basic correction of
patient misalignment to within ±1.5mm using a geometric phantom. Both the Elekta
Synergy system and the TomoTherapy Hi-Art system were able to position the
phantom to within 0.6mm as shown in table 3. Despite the Varian OBI recording a
maximum residual error of 1mm, the digital display for this system only reads to the
nearest mm therefore residual errors up to 1.5mm may have occurred. The image
matching was performed by current users of each of the systems with previous
experience.
Table 3. Residual errors after image-shift-verify test (mm)
Elekta Synergy v4.2
Lateral
Long
Vertical
Set 1
0.2
0.2
0.5
Set 2
-0.6
0.0
0.0
Set 3
-0.3
-0.2
0.5
Lateral
Long
Vertical
Set 1
-1
0
0
Set 2
1
0
0
Set 3
0
0
0
Varian OBI v1.5
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24
TomoTherapy Hi Art v3.2
Lateral
Long
Vertical
Set 1
0.2
-0.3
-0.4
Set 2
0.2
-0.2
-0.1
Set 3
0.0
-0.3
-0.3
Test 3: Imaging dose
Tables 4-5 display the resultant doses that are representative of each scan protocol.
The values reported correspond to the average measurement made across central
and periphery positions in the head phantom, and between central, periphery and
mid-periphery positions in the body phantom. The comprehensive list of
measurements is provided in appendix 4. Dose measurements are accurate to within
±10%.
Table 4. Elekta Synergy v4.2 dose measurements in mGy
Protocol 1 Protocol 2 Protocol 3
Low head Med Head High Head
Head
1.4
5.4
Protocol 4 Protocol 5 Protocol 6
Pelvis
Pelvis
Pelvis
M10
M15
M20
10.7
Body
Air
1.5
5.9
11.7
12.7
14.0
15.3
37.8
38.2
38.6
Table 5. Varian OBI v1.5 dose measurements in mGy
Head
Low Dose
Head
Standard
Dose
Head
High
Quality
Head
2.8
5.6
27.8
Body
In Air
5.2
10.5
52.4
Pelvis
Pelvis
Spotlight
24.9
20.2
78.9
42.9
Table 6. TomoTherapy Hi-Art v3.2 dose measurements in mGy
Coarse
Medium
Fine
Head
9.5
14.7
29.9
Body
7.2
9.8
21.4
In Air
11.6
18.4
34.8
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25
Test 4: Pseudo-clinical image quality
Figures 1-3 display the images of the VHMP as acquired on each of the systems
using typical clinical pelvic protocols.
Elekta Synergy v4.2:
There is clear differentiation between the muscle and fat soft tissues. In the transaxial images there is a degree of streak artefact which increases with the longitudinal
field of view. Some shading artefacts are present, an example being the shadowing
visible between some of the pelvic bones, demonstrating non uniformity of the CT
number within the imaged volume. There is also some evidence of a circular artefact
which is likely to be related to the interface between regions exposed throughout the
360° scan and those regions only exposed through 180° (this artefact was also
observed within the CATphan images).
Varian OBI v1.5:
There is clear differentiation between the muscle and fat soft tissues. In the transaxial images there are some streak artefacts originating from high density objects
within the image i.e. bones and surface markers. In the sagittal and coronal sections
the noise characteristics are elongated in the longitudinal direction due to the slice
thickness (2.5mm). The spotlight protocol gives acceptable images as long as the
image of the skin surface is not required. For this protocol, a prominent shading
artefact between the brighter centre of the image and the darker outer region is
visible; however this is not likely to affect image guidance of the central prostate.
TomoTherapy Hi-Art v3.2:
There is clear differentiation between the muscle and fat soft tissues. The CT number
uniformity across the image is free from shading and streaking artefacts. However,
this absence makes the stochastic noise more apparent and in the sagittal and
coronal sections the noise characteristics are elongated in the longitudinal direction
due to the slice thickness. This is increasingly noticeable as the pitch increases.
There is also an artefact extending along the longitudinal rotation axis of the system
which again is predominantly visible in the sagittal and coronal views.
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Figure 1. Elekta Synergy v4.2 VHMP images displayed in the transverse, sagittal and coronal planes for a) M10, b) M15 and c) M20 collimators
a)
b)
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c)
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27
Technical performance
28
Figure 2. Varian OBI v1.5 VHMP images displayed in the transverse, sagittal and coronal planes for a) pelvis and b) pelvis spotlight protocol
a)
b)
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Figure 3. TomoTherapy Hi-Art v3.2 VHMP images displayed in the transverse, sagittal and coronal planes for a) coarse b) normal and c) fine
pitches
a)
b)
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c)
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30
Technical performance
31
Test 5: Image quality
Tables 7-9 list the various image quality parameters that were measured on each
system for their respective clinical protocols. The full data set regarding Hounsfield
number accuracy can be found in Appendix 5 and example images taken from both
the CTP404 module of the CATphan and the CTP528 module of the CATphan are
displayed in Appendix 6 and 7. A summary of the image quality data for each system
is presented below.
TomoTherapy Hi-Art v3.2
The spatial integrity of the images was found to be good in all cases; linearity was
below 1% and the maximum aspect ratio was 1.017. On visual inspection, although
the images appeared uniform the presence of a brighter region occurring at the
centre of the imaged volume resulted in relatively high uniformity values (of the order
of 5%). This artefact was noticeable in all image sets.
The noise present within the polystyrene ROI and hence the CNR for polystyrene did
not vary significantly with pitch. Spatial resolution was also constant with pitch at
3 lp/cm resolved.
Estimates of the Hounsfield numbers of each of the density inserts were within 13%.
No variation with pitch was observed.
Elekta Synergy v4.2
The spatial integrity of the images was found to be good in all cases; linearity was
below 1% and the maximum aspect ratio was 1.018. For all protocols investigated,
the slice sensitivity was overestimated by an average of 0.7mm. The head protocols
produced the most uniform images with uniformity figures ranging between 1.8% and
3.2%. Using the pelvis protocols with the panel offset to produce the medium field of
view, resulted in a loss of uniformity with a ring artefact becoming more dominant at
~5cm from the centre of the image. This led to the uniformity figures increasing up to
a maximum of 4.9%, corresponding to the protocol with the largest field length (M20).
This artefact is not visible in the VHMP images and may only be present in CATphan
images.
Increased exposure and therefore increased dose for a given protocol resulted in an
expected reduction in noise and thus a higher CNR. Spatial resolution remained at
3 lp/cm for all protocols assessed.
The Synergy system overestimated the Hounsfield numbers for all the density inserts
with a maximum error of 123%. Although the accuracy increased significantly with
image dose, even the highest exposure protocols (corresponding to the pelvis) still
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suffered large errors ranging between 29% and 63% depending upon the insert
density.
Varian OBI v1.5
The spatial integrity of the images was found to be good in all cases; linearity was
below 0.7% and the maximum aspect ratio was 1.005.The slice width was estimated
to within 0.15mm for all protocols tested except the low dose head protocol which
underestimated it by 0.6mm, however, it is likely that this may indicate an error in the
actual measurement.
The Varian OBI system accurately estimated the Hounsfield numbers of each of the
density inserts to within 6%. Discounting the first clinical protocol, the calculated
Hounsfield numbers from the low dose head protocol (2.8 mGy) and the half fan
pelvis protocol showed excellent agreement with the true values, with maximum
variations of 2% and no further variations with imaging protocol were observed.
Spatial resolution was very good with 7lp/cm resolvable for all clinical protocols
assessed except for the pelvis protocol for which a maximum of 6lp/cm were
resolved. This was the only protocol which utilises the half fan mode. Uniformity was
also relatively constant across each imaging protocol with a maximum value of 2.8%.
However, a distinct ring artefact was present within the images using the half fan
pelvis protocol which resulted in poorer uniformity (3.9%). This artefact is not visible
in the VHMP images and may only be present in CATphan images.
Surprisingly, the noise was slightly higher for the standard dose head when
compared with the low dose head protocol resulting in a lower CNR. This result
suggests that despite the increase in dose by a factor of two no additional benefit in
image quality is gained from using the standard head protocol over the lower dose
protocol.
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Table 7. Varian OBI v1.5: comparison of image quality parameters as measured using different imaging protocols
Dose
(mGy)
Low Dose
Head
Standard
Dose Head
High Quality
Head
CNR for
polystyrene
Noise for
polystyrene
†
CT
Number
Error
Min Max
Resolution
lp/cm
Linearity
%
Aspect
ratio
Slice
sensitivity
(mm)
Uniformity§
%
U1.
U2.
2.8
2.8
32.6
0%
6%
7
0.1
1.004
1.91 (0.30)
2.3
-1.3
5.6
2.9
34.5
0%
-2%
7
0.3
1.002
2.47 (0.25)
2.8
-1.9
27.8
6.7
14.0
0%
-2%
7
0.1
1.000
2.59 (0.23)
2.3
-1.5
Pelvis
24.9
11.5
9.0
0%
5%
6
0.7
1.005
2.62 (0.33)
3.9
3.7
Pelvis
Spotlight
20.2
8.4
12.0
0%
-2%
7
0.3
1.002
2.43 (0.21)
2.2
-1.5
Notes: † Noise defined as the standard deviation of the polystyrene density insert.
§ Two different definitions of uniformity used.
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Table 8. Synergy XVI: comparison of image quality parameters as measured using different imaging protocols
Low Dose
Head
Med Dose
Head
High Dose
Head
Pelvis
M10
Pelvis
M15
Pelvis
M20
CT number
Error
Min
Max
123
74%
%
Dose
(mGy)
CNR for
polystyrene
Noise for
polystyren
e†
1.4
3.3
19.4
5.4
8.3
9.2
60%
10.7
9.4
7.9
12.7
15.3
14.0
15.3
Linearity
%
Aspect
ratio
Slice
sensitivity
(mm)
3
-0.2
1.013
1.53 (0.25)
2.6
0.0
92%
3
0.1
1.003
1.69 (0.41)
3.2
-2.2
52%
81%
3
0.1
1.005
1.79 (0.64)
2.7
-1.8
4.3
30%
61%
3
-0.1
1.002
1.73 (0.16)
4.2
-3.5
12.5
5.8
29%
63%
3
-0.6
1.018
1.74 (0.22)
3.3
-2.7
13.7
7.0
36%
56%
3
-0.2
1.008
1.61 (0.33)
4.9
-4.3
Notes: † Noise defined as the standard deviation of the polystyrene density insert.
§ Two different definitions of uniformity used.
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Uniformity§
%
U1.
U2.
Resolution
lp/cm
Technical performance
35
Table 9. TomoTherapy Hi-Art v3.2: comparison of image quality parameters as measured using different imaging protocols
Protocol
Dose
(mGy)
CNR for
polystyrene
Coarse
Normal
Fine
9.5
14.7
29.9
3.6
3.7
3.5
Noise for
polystyrene
†
22.7
25.3
21.7
CT
number
Error
Min Max
5% 13%
3% 12%
4% 13%
Notes: † Noise defined as the standard deviation of the polystyrene density insert.
§ Two different definitions of uniformity used.
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Resolution
lp/cm
Linearity
%
Aspect
ratio
Slice
sensitivity
(mm)
3
3
3
-0.3
0.2
0.9
1.009
1.018
1.004
7.1
5.3
4.6
Uniformity§
%
U1.
5.5
4.6
5.0
U2.
-4.8
-4.4
-4.8
Technical performance
36
Discussion
In this evaluation the ability of the systems to accurately assess and correct
patient misalignment has been measured with a well defined geometric
phantom. However, these measurements represent an upper limit on the
clinical performance achievable as many factors come into play when
assessing misalignments on real patients. These include the patient’s size,
shape and the definition of the target anatomy or its surrogates in addition to
any motion occurring during image acquisition. Image quality, as determined
by the calibration of the system, exposure settings and reconstruction
settings, also plays a major role. Finally the visualisation tools and image
registration algorithms are important in facilitating the measurement of
misalignments. These technologies are relatively new and research has
tended to focus on the measurement of patient motion and correction
strategies. As such, there has been less research on quantifying realistic limits
of IGRT accuracy or optimisation of these systems in performing specific
image guidance tasks.
The answer to the question “what image quality is required to perform a
specific image guidance task?” is difficult. All three systems evaluated are
able to differentiate the contrast between muscle and fat as evident from the
VHMP phantom images and, from this point of view, all are able to provide 3D
soft tissue anatomical information that would not otherwise have been
obtainable. Therefore, it is true that they are all able to localise mobile soft
tissue targets better than, for instance megavoltage portal images. However,
there are clear differences in image quality between the three systems with
different contrast to noise ratios, trans-axial plane resolution, slice sensitivity
and image acquisition dose. These factors may all affect the degree of
accuracy to which a particular target can be localised. Without further
research, it is not possible to determine whether the observed image quality is
either sufficient or optimal for a specific image guidance task for example to
localise the prostate to within 3mm.
In the following discussion, various factors affecting image quality and
therefore the potential accuracy and their impact on the measurements
performed in this evaluation are discussed in greater depth.
As has been confirmed for all systems, an increase in exposure results in an
increase in image quality as defined by contrast to noise ratio. However,
according to IRMER legislation [29] patient doses should be kept ‘as low as
reasonably practicable’ (ALARP). It is therefore important that images are
optimised and decisions made as to what level of image quality is acceptable
for the clinical purpose, especially for techniques in which there is potential for
multiple irradiations. Essentially this results in compromises being made or
justification being presented in terms of the potential clinical benefit. Where
image alignment is guided by high contrast structures such as bone-soft
CEP10071: March 2010
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37
tissue interfaces then the image dose can be reduced significantly by reducing
the acquisition exposure parameters without significant impact on the image
guidance accuracy [19, 30]. The Varian OBI and Elekta Synergy systems both
have the flexibility to reduce the dose by lowering the tube current and
exposure time. Furthermore, image dose can be reduced by lowering the tube
voltage where applicable for example in the head and neck region, where
there is inherently good contrast. This means that the associated reduction in
contrast to noise is unlikely to significantly affect image matching. Image dose
can be reduced in the TomoTherapy Hi-Art by increasing the slice thickness,
but this compromises the spatial resolution in the longitudinal direction.
Patient size affects the image quality for a given image acquisition dose. A
larger patient size or section of anatomy will lead to greater attenuation and
therefore reduced signal at the detector. The reduced signal will lead to a
reduction in the signal to noise ratio and consequent reduction in contrast to
noise ratio. A large patient size also leads to greater patient scatter which is a
particular problem for the cone beam CT systems. The relationship between
imaging beam quality, contrast, signal to noise ratio and scatter is complex
[31, 32].
The contrast to noise ratio is a relatively crude measure of an imaging
system’s ability to image clinical anatomy. The low contrast detail that can be
seen in an image is a function of both the modulation transfer function, which
is qualitatively measured by the line pair test object in the CATphan, and the
frequency components of the image noise, not measured here [33, 34]. It is
possible to reconstruct an image from the same projection data twice with
different reconstruction filters to give two images with very different visual
characteristics. One may have a high contrast to noise ratio i.e. low image
noise with good visibility of large low contrast objects, whereas the other
image may have a lower contrast to noise ratio with greater image noise, but
with finer detail visible in the image.
Similarly, contrast to noise ratio is also dependent on the reconstructed image
voxel size i.e. trans-axial plane pixel size and/or slice width. A set of projection
images reconstructed with a large voxel size will have lower levels of noise
and therefore a higher contrast to noise ratio while an image reconstructed
with a small voxel size will have higher noise and therefore a lower contrast to
noise ratio. However, the small voxel size will allow smaller objects to be
resolved as long as the imaging dose is sufficient to reduce the image noise to
an acceptable level. Further advantages of reconstructing with larger voxel
sizes include the speed of reconstruction, speed of automatic image
registration and both disk and memory storage requirements of the hardware.
For these reasons the choice of optimal reconstruction filters and voxel size is
complex.
CEP10071: March 2010
Technical performance
38
The standard settings of both reconstruction filters, trans-axial plane pixel size
and slice width are quite different when comparing the Varian OBI and Elekta
systems, however, both have provided a degree of user flexibility as to the
choice of reconstruction filters and the voxel size that can be employed. In the
absence of good quality research in this area most users are reluctant to
deviate from the manufacturers default settings. In this evaluation it was
proposed that the two CBCT systems were compared by attempting to match
the acquisition and reconstruction parameters to provide comparable images.
This proved to be technically challenging and so image quality was assessed
with standard clinical protocols, thus the data provided in this report can be
used as a national reference for the comparison of similar measurements both
on new installations and developments of existing systems, or for the
comparison of new IGRT systems.
The CATphan, which is designed for measurements of image quality on multislice helical CT scanners, is not particularly well suited to CBCT systems
where noise, resolution and image uniformity may vary throughout the imaged
volume. The phantom is also less than ideal for other reasons; the contrast, of
the low contrast visibility objects, is too low for a reasonable measurement on
these CBCT systems and are not visible at all on the TomoTherapy Hi-Art
system and the resolution test object is good for assessment of the trans-axial
plane but for a reconstruction where voxel size is isotropic the test object
needs to be truly 3D in nature [20]. To address these problems, there is a
need for better phantoms, however, in the absence of such, the CATphan was
chosen as it is typically supplied by the manufacturer with the purchase of the
Varian and Elekta CBCT systems.
The accuracy of the IGRT systems CT number was assessed in this
evaluation; however, the Elekta Synergy system does not specify its accuracy,
which is reflected in the results presented in this report. CT number accuracy
is also likely to be affected by non-cylindrical geometries, particularly in
regions of rapidly changing surface contours and heterogeneities. This is likely
to be a particular problem for CBCT systems where the scatter conditions will
vary from the calibration conditions [35, 36].
Image artefacts also affect the CT number accuracy and these are evident in
the VHMP images.
Ensuring CT number accuracy is a challenge [37, 38], particularly for the
CBCT systems in their current state of development. However, it is not
necessarily a requirement of IGRT where the task is to assess positional
alignment and this may be Elekta’s reasoning for not specifying the accuracy
of this parameter. Despite this, image registration algorithms used in the
image guidance systems and those that might be employed for future
adaptive radiotherapy techniques [39-43] are likely to perform better if the CT
number corresponds more closely to the reference CT images.
CEP10071: March 2010
Technical performance
39
The effect of patient movement during image acquisition will be different on
each of the systems. The presence of a transient movement of bowel gas in
the rectum when imaging the prostate, may only affect one or two slices on
the slice based Hi-Art system, while such movement on a CBCT system can
cause a severe loss in definition of the surrounding anatomy. The Hi-Art
system will suffer from the well reported distortion of slice based systems
when imaging in the presence of respiratory movement [44], although its 5s
per slice acquisition time is likely to give results more typical of slow CT [45].
For the CBCT systems, respiratory motion will be averaged during the
acquisition leading to a blurring of the motion, again similar to slow CT. The
advantage of the CBCT system is that the reconstruction can be binned into
phases and reconstructed to produce a 4D-CBCT without significant extra
dose. Furthermore, this can be achieved by tracking movement within the
projection images and without requiring an external surrogate of respiratory
phase [46]. The release of functionality for 4D-CBCT reconstruction is
expected in the near future on the two CBCT systems evaluated in this report.
The measurements of dose reported here are only indicative of the dose.
When considering patient dose, the patient’s size, shape and the presence of
inhomogeneities need also be considered. On a CBCT system, a factor of 2
might be expected for the difference in dose due to patient size between a
paediatric and large adult [47, 48] and the dose to bone will be approximately
2.5 times that of the soft tissue [48 - 50] for kV systems. For MV systems, the
dose difference due to patient size will be less as will the increase in dose to
bone.
CEP10071: March 2010
Operational considerations
40
Clinical impact
There is little evidence in the literature to date on the direct patient benefit of using
IGRT. The equipment allows visualisation of soft tissues and there are many
publications reporting the movement and deformation of these tissues allowing
treatment margins to be calculated with greater confidence. Image guidance enables
a greater accuracy of treatment delivery to these tissues and therefore enables
treatment margins to be reduced. However, well designed trials are required to
determine if these margins can be reduced without compromising tumour coverage,
particularly when the uncertainty of delineation of the target is taken into account.
Reduction of margins should lead to lower rates of normal tissue toxicity but could
also enable increases in delivered dose with expected increases in local tumour
control.
Alternative technologies
The most common imaging technique used in the past was megavoltage (MV)
radiograph, electronic portal imaging device (EPID) and planar kilovoltage (kV)
imaging. Due to the advances in imaging technologies over the past decade, such as
conformal radiotherapy and IMRT, where dose distributions become more complex
and dose gradients become steeper, the focus is now to improve the ability to
localize the target for treatment with millimetre accuracy. Several new enhanced
methods of imaging are currently being introduced to improve treatment guidance
and verification.
The emerging technologies of IGRT imaging techniques are:
•
•
•
•
Cone beam computed tomography (CBCT) and megavoltage CT, the subject
of this evaluation, as well as in room kVCT and MV CBCT
Optical Tracking – Uses virtual simulation (camera), real-time image guidance
using surface landmarks attached to the skin
Ultrasound Imaging – Provides anatomical information using high frequency
sound waves. It has minimal side-effects and allows real-time imaging
Implanted fiducial markers – Uses markers that are implanted in the body, but
very few types of cancer are accessible for this method.
User evaluation
A user survey was carried out using a web-based questionnaire. 10 confirmed results
were received for 3D IGRT systems (eg CT or CBCT). Figure 4 shows the distribution
of responses by IGRT system and figure 5 shows the year systems were installed.
CEP10071: March 2010
Operational considerations
41
Figure 4. User response by supplier
Siemens CTVision
0%
Siemens MVision
0%
Tomotherapy HiArt
(CTrue)
0%
Varian OBI Advanced
Imaging
45%
Elekta Synergy
55%
Figure 5. Year of installation
40%
35%
Proportion of responses
30%
25%
20%
15%
10%
5%
0%
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
Year of installation
The users were asked to rate a variety of features of their IGRT system. The ratings
are presented in figures 4 to 12. As can be seen from the graphs, the users were
generally happy with the technical features of the system, but scored the ease of use
and user manual slightly lower.
CEP10071: March 2010
Operational considerations
42
Figure 6. Overall ease of use
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Very poor
Poor
OK
Good
Very good
Poor
OK
Good
Very good
Figure 7. Speed of use
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Very poor
CEP10071: March 2010
Operational considerations
43
Figure 8. Image quality
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Very poor
Poor
OK
Good
Very good
Poor
OK
Good
Very good
Figure 9. Image registration
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Very poor
CEP10071: March 2010
Operational considerations
44
Figure 10. Ease of image correction
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Very poor
Poor
OK
Good
Very good
Poor
OK
Good
Very good
Figure 11. Reliability
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Very poor
CEP10071: March 2010
Operational considerations
45
Figure 12. Information and usefulness of the user manual
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Very poor
Poor
OK
Good
Very good
Clinical applications
Responders were asked for which clinical applications they used tomographic IGRT.
One responder used tomographic IGRT for breast therapy and a small number of
centres used tomographic IGRT for a few patients for limb, paediatric or palliative
radiotherapy. A small number of centres used tomographic IGRT for some spine
radiotherapy patients, while several centres used tomographic IGRT for some thorax
and mediastinum radiotherapy patients. The only patient groups for which
tomographic IGRT was used for many patients were the head and neck group and
the pelvis patient group.
Verification strategies
A number of different verification strategies were used by different centres. Table 10
shows the responses for each strategy. Centres may use multiple strategies and
were permitted to select as many answers as were appropriate.
Table 10. Verification strategies used
Verification strategy
Number of
responses
Off-line verification (images analysed after treatment and corrections applied at a
subsequent treatment)
5
On-line verification (images analysed immediately and correction applied before
the treatment)
9
CEP10071: March 2010
Operational considerations
46
Interfractional verification (compare setup between fractions)
6
Infrafractional verification (compare accuracy during a single fraction)
3
Real-time treatment verification (compare accuracy as the radiation is being
delivered)
0
Registration strategies
One responder used manual registration. Automatic registration using bone was
most common, being used by more than twice as many responders as automatic
registration using soft-tissue only.
Training
The number of staff provided training varied greatly between responses. Typically a
number of radiographers were trained (between 2 and 12) and at least one physicist
was trained at each centre. Technicians and clinicians were also trained at some
centres, but the number varied considerably from centre to centre.
Instructions for use
The IGRT system should be provided with instructions for use. These may be
hardcopy or softcopy. Due to the complex nature of IGRT, in addition to these
documents, staff should be trained on the use of IGRT features of the system. Initial
training will most likely be provided by the supplier of the equipment.
Service requirements
The addition of IGRT will increase the power consumption of the radiotherapy system
by a small amount. Access conduits may be needed for control and data transfer to
and from the IGRT system. If the IGRT system is being added as an upgrade to an
existing linac the availability of cabling space should be confirmed and if it is not
present then the additional work and cost should be considered in the business plan.
Some IGRT systems use imaging units which are separate to the linac. This may
require a larger treatment room and this should be considered during the preparation
of a business case.
The use of IGRT will require additional time per patient. This will have an impact on
throughput and workflow which should be considered when introducing IGRT.
CEP10071: March 2010
Operational considerations
47
Connectivity
The IGRT system will need to connect to the treatment planning system (TPS) used.
Confirmation that the specific IGRT system to be purchased can fully and
appropriately interact with the local TPS should be sought during the purchase
process.
Image analysis may be conducted on the IGRT workstation. Alternatively, additional
workstations may be provided or the data may be accessible from a generic PC. The
particular options available should be confirmed during the purchase process.
Consumables
The consumable used in the application of this technology is electrical, but this is
quite small when considered against the power requirements for running the
radiotherapy system.
Maintenance and servicing
There are no user-serviceable parts in the system. All servicing must be performed
by certified qualified personnel. A maintenance contract is recommended for this
equipment. No reliability problems have been identified in the published literature or
during the interviews with device users.
Calibration and quality control
Routine quality assurance will need to be undertaken. Daily tests will likely be
performed by a radiographer, while weekly and monthly tests will likely be performed
by specialist technical staff, such as medical physicists. Daily tests would be
expected to take around 10 to 20 minutes, weekly tests around 30 minutes and
monthly tests up to half a day.
Staff requirements
Specific staffing levels for IGRT equipment vary from centre to centre. Typically 2 or
3 radiography staff might be involved in delivering the treatment, with an additional 2
or 3 staff checking and preparing the treatment prescriptions.
During implementation of IGRT, additional time will be required from specialist staff
such as clinical oncologists and medical physicists.
CEP10071: March 2010
Purchasing
48
Purchasing procedures
The Trust Operational Purchasing Procedures Manual provides details of the
procurement process [52].
European Union procurement rules apply to public bodies, including the NHS, for all
contracts worth more than £90,319 (from January 1st 2008) [53]. The purpose of
these rules is to open up the public procurement market and ensure the free
movement of goods and services within the EU. In the majority of cases, a
competition is required and decisions should be based on best value.
NHS Supply Chain (www.supplychain.nhs.uk), a ten year contract operated by DHL
on behalf of the NHS Business Services Authority, offers OJEU compliant national
contracts or framework agreements for a range of products, goods and services. Use
of these agreements is not compulsory and NHS organisations may opt to follow
local procedures.
Purchasing options
In the case of the Elekta Synergy and Varian OBI systems the image guided units
can be purchased for retrospective fitting to specific Elekta and Varian treatment
units. Certain pre-conditions on the specification of the linac will apply. Otherwise, the
IGRT systems can be purchased as add-on components to the linac purchase. In the
case of the TomoTherapy Hi-Art system the IGRT components are integral to the
treatment unit and as a result can only be purchased along with the treatment unit.
Since this evaluation has been performed Siemens Medical Systems have
introduced other X-ray tomographic IGRT systems to the market; the MVision system
which is based on megavoltage cone beam CT imaging in addition to CTVision.
Sustainable procurement
The UK Government launched its current strategy for sustainable development,
Securing the Future [54] in March 2005. The strategy describes four priorities in
progressing sustainable development:
• sustainable production and consumption – working towards achieving more with
less
• natural resource protection and environmental enhancement – protecting the
natural resources and habitats upon which we depend
• sustainable communities – creating places where people want to live and work,
now and in the future
• climate change and energy – confronting a significant global threat.
CEP10071: March 2010
Purchasing
49
The strategy highlights the key role of public procurement in delivering sustainability.
End-of-life disposal
Consideration should be given to the likely financial and environmental costs of
disposal at the end of the product’s life. Where appropriate, suppliers of equipment
placed on the market after the 13th August 2005 should be able to demonstrate
compliance with the UK Waste Electrical and Electronic Equipment (WEEE)
regulations (2006) [55]. The WEEE regulations place responsibility for financing the
cost of collection and disposal on the producer. Electrical and electronic equipment is
exempt from the WEEE regulations where it is deemed to be contaminated at the
point at which the equipment is scheduled for disposal by the final user. However, if it
is subsequently decontaminated such that it no longer poses an infection risk, it is
again covered by the WEEE regulations, and there may be potential to dispose of the
unit through the normal WEEE recovery channels.
CEP10071: March 2010
Acknowledgements
50
We should like to thank the following for their contribution to this evaluation report.
Liz Adams, Medical Physicist, Cambridge University Hospitals NHS Foundation Trust
Charlotte Beardmore, The Society and College of Radiographers
Dan Emmens, Medical Physicist, Ipswich Hospital NHS Trust
Jamie Fairfoul, Medical Physicist, Cambridge University Hospitals NHS Foundation
Trust
Hayley James, Medical Physicist, Ipswich Hospital NHS Trust
Philip Mayles, Clatterbridge Centre for Oncology
Donna Routsis, Addenbrooke's Hospital, Cambridge
John Shakeshaft, Clatterbridge Centre for Oncology
Tim Wood, Leeds Teaching Hospitals NHS Trust
Leeds Teaching Hospitals NHS Trust
Christie Hospital NHS Foundation Trust
UCL, St Luc, Brussels
Guy's and St Thomas NHS Foundation Trust
Maidstone and Tunbridge Wells NHS Trust
Poole Hospital NHS Trust
Southampton University Hospitals NHST
Ipswich Hospital NHS Trust
The Royal Wolverhampton Hospitals NHST
University Hospital Birmingham NHSFT
Royal Free Hampstead NHS Trust
Cambridge University Hospitals NHS Foundation Trust
Elekta AB
Siemens Healthcare
TomoTheraphy Incorporated
Varian Medical Systems Inc
CEP10071: March 2010
Glossary
51
BTF
Bow tie filter - A filter that is inserted in front of the source to
compensate for variations in path length and subsequent
differential attenuation along the beams trajectory.
CATphan
A test phantom used for measuring image quality characteristics of
3D X-ray imaging systems.
CBCT
Cone beam computed tomography - Similar to CT, but instead of
using a linear fan beam, a cone shaped X-ray beam is used and
captured by a 2D array of detectors.
CNR
Contrast to noise ratio - The difference in the mean grey level
between two objects (contrast) divided by the standard deviation of
the noise. The measure gives an indication of the ability of a
system to distinguish the difference between two materials in the
presence of image noise.
CT
Computed tomography - Method of X-ray imaging in which a series
of 2D projection images acquired around a single axis of rotation
are reconstructed to produce a 3D image data set consisting of a
set of axial slices.
CTDI phantom
A standard Perspex phantom used for measuring CT dose that can
be configured for estimates of head (16cm diameter) and body
(32cm diameter).
CT/HU number A relative scale relating the grey level of the pixel in an image to
the linear attenuation of the object imaged. For Hounsfield Units
(HU), air = -1000 and water = 0.
DH
Department of Health
EPID
Electronic portal imaging device - A digital imaging system which
uses an amorphous silicon flat panel detector.
Flexmap
A pre-measured lookup table used during image reconstruction to
correct for shifts in alignment of the kV source and detector due to
kV source arm and detector arm flex.
FOV
Field of view - region over which data is acquired and
reconstructed to display an image. Usually refers to the dimensions
in the trans-axial imaging plane.
IGRT
Image guided radiotherapy - The process by which some form of
imaging is performed during a patient’s radiotherapy treatment and
this additional information is used to either apply corrections to
patient position, or to instigate plan modifications, with the overall
aim being to improve treatment accuracy.
Image
acquisition
The process of acquiring an IGRT image.
CEP10071: March 2010
Glossary
52
Image review
The process of reviewing an IGRT image. This involves
comparison of the IGRT image with a reference CT scan and
maybe performed on- of off-line
IMRT
Intensity modulated radiotherapy - A form of conformal
radiotherapy that utilises several intensity modulated beams to
yield a highly conformal dose distribution, which can be of a
concave shape.
IRMER
Ionising Radiations (Medical Exposures) Regulations 2000
Isocentre
The unique point in space around which all the major components
of the Linear accelerator rotates. This point also defines the frame
of reference of the imaging system.
kV-CBCT
Kilovoltage cone beam CT - Cone beam CT performed utilising kV
energy photons produced by an X-ray tube.
Lat
Lateral - Direction along the minor axis of the couch system, the xaxis (often also described as East/West and anatomically as
left/right).
Long
Longitudinal - Direction along the major axis of the couch system,
the y-axis (often also described as North/South and anatomically
as superior/inferior,).
MV-CBCT
Megavoltage cone beam CT - Cone beam CT performed utilising
MV energy photons produced by the linear accelerator and imaged
using an electronic portal imaging device.
MVCT
Megavoltage computed tomography
NPS
Noise power spectrum - This gives the powers of the spectrum of
spatial frequencies found in an image sample.
Offline
For offline correction strategies, reaction to an image is delayed to
a subsequent fraction. The correction applied can vary between
being a straightforward translation of the patient couch system to
the initiation of a complete re-plan.
Offline review
The process of image review performed after treatment delivery
with the aim of assessing systematic errors in patient position. Any
measured systematic error is used to correct subsequent fractions
of treatment delivery
Online
Online correction strategies involve images being acquired,
assessed and any adjustments to patient positioning are applied
immediately, prior to the patient being treated.
Online
correction
The process of image review and if necessary the correction of
patient position performed line immediately prior to treatment
delivery
CEP10071: March 2010
Glossary
53
Pentaguide
A test phantom for measuring geometrical alignment of 3D X-ray
imaging systems
Pitch
Pitch - Rotation around the lateral axis (x)
QA
Quality assurance - A planned and systematic approach to
monitoring, assessing and improving the quality of services
provided on a continuous basis.
ROI
Region of interest - An area or volume in which data is
preferentially selected.
Roll
Roll - Rotation around the longitudinal axis (y)
SBRT
Stereotactic body radiation therapy - Highly precise radiation
therapy which delivers a high dose to the target volume within a
small number of treatment fractions.
Slice
sensitivity
A measurement of slice thickness. The term has been inherited
from assessment of CT scanners which are slice based and is not
ideal for cone beam CT X-ray imaging systems.
Spatial
Integrity
The ability of a system to accurately represent the object imaged
without scaling, rotation or distortion.
Spatial
resolution
The ability of the system to resolve two lines of high contrast
placed closely together.
SRS
Stereotactic radiosurgery - A highly precise form of radiation
therapy used primarily to treat tumours and other abnormalities of
the brain.
Uniformity
The uniformity of image grey-level across the field of view of the
image of an object with uniform density.
Vert
Vertical - Direction perpendicular to the horizontal plane of the
couch, the z-axis (often described anatomically as
posterior/anterior).
VHMPP
Virtually Human Male Pelvic Phantom - An anthropomorphic
phantom of a male pelvis modelled using data from the Virtual
Human project (National Institute for Health, US). The phantom is
rigid and has materials that mimic both bone and soft tissues with
differing radiological properties such as muscle and fat.
Yaw
Yaw - Rotation around the vertical axis (z)
CEP10071: March 2010
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31. Stutzel, J., U. Oelfke, and S. Nill, A quantitative image quality comparison of four different
image guided radiotherapy devices. Radiother Oncol, 2008. 86(1): p. 20-4.
32. Groh, B.A., et al., A performance comparison of flat-panel imager-based MV and kV
cone-beam CT. Medical Physics, 2002. 29(6): p. 967-975.
33. Siewerdsen, J. and D. Jaffray, NEQ description of 3-D imaging performance in flat-panel
cone-beam CT. Medical Physics, 2002. 29(6): p. 1321-1321.
34. Siewerdsen, J.H. and D.A. Jaffray, Three-dimensional NEQ transfer characteristics of
volume CT using direct and indirect-detection flat-panel imagers, in Medical Imaging
2003: Physics of Medical Imaging, Pts 1 and 2, M.K. Yaffe and L.E. Antonuk, Editors.
2003, Spie-Int Society Optical Engineering: Bellingham. p. 92-102.
35. Siewerdsen, J., et al., A direct, empirical method for X-ray scatter correction in digital
radiography and cone-beam CT. Medical Physics, 2005. 32(6): p. 2092-2093.
36. Siewerdsen, J.H., et al., The influence of antiscatter grids on soft-tissue detectability in
cone-beam computed tomography with flat-panel detectors. Medical Physics, 2004.
31(12): p. 3506-3520.
37. Richter, A., et al., Investigation of the usability of conebeam CT data sets for dose
calculation. Radiation Oncology, 2008. 3(1): p. 42.
38. Katrina, Y.T.S., et al., The effects of field-of-view and patient size on CT numbers from
cone-beam computed tomography. Physics in Medicine and Biology, 2009(20): p. 6251.
39. Guckenberger, M., et al., Evolution of surface-based deformable image registration for
adaptive radiotherapy of non-small cell lung cancer (NSCLC). Radiat Oncol, 2009. 4: p.
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40. Nijkamp, J., et al., Adaptive radiotherapy for prostate cancer using kilovoltage cone-beam
computed tomography: first clinical results. Int J Radiat Oncol Biol Phys, 2008. 70(1): p.
75-82.
41. Zhang, T., et al., Automatic delineation of on-line head-and-neck computed tomography
images: toward on-line adaptive radiotherapy. Int J Radiat Oncol Biol Phys, 2007. 68(2):
p. 522-30.
42. Malsch, U., C. Thieke, and R. Bendl, Fast elastic registration for adaptive radiotherapy.
Med Image Comput Comput Assist Interv, 2006. 9(Pt 2): p. 612-9.
43. Burridge, N., et al., Online adaptive radiotherapy of the bladder: small bowel irradiatedvolume reduction. Int J Radiat Oncol Biol Phys, 2006. 66(3): p. 892-7.
44. Gagne, I.M. and D.M. Robinson, The impact of tumor motion upon CT image integrity
and target delineation. Med Phys, 2004. 31(12): p. 3378-92.
CEP10071: March 2010
References
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45. Lagerwaard, F.J., et al., Multiple "slow" CT scans for incorporating lung tumor mobility in
radiotherapy planning. Int J Radiat Oncol Biol Phys, 2001. 51(4): p. 932-7.
46. Sonke, J.J., et al., Respiratory correlated cone beam CT. Medical Physics, 2005. 32(4):
p. 1176-1186.
47. DeMarco, J.J., et al., Evaluation of patient dose using a virtual CT scanner: Applications
to 4DCT simulation and Kilovoltage cone-beam imaging. Journal of Physics: Conference
Series, 2008. 102.
48. Ding, G.X. and C.W. Coffey, Radiation dose from kilovoltage cone beam computed
tomography in an image-guided radiotherapy procedure. Int J Radiat Oncol Biol Phys,
2009. 73(2): p. 610-7.
49. Chow, J.C.L., et al., Evaluation of the effect of patient dose from cone beam computed
tomography on prostate IMRT using Monte Carlo simulation. Medical Physics, 2008.
35(1): p. 52-60.
50. Ding, G.X., D.M. Duggan, and C.W. Coffey, Accurate patient dosimetry of kilovoltage
cone-beam CT in radiation therapy. Medical Physics, 2008. 35(3): p. 1135-1144.
51. AAPM Task Group 142 Report: Quality assurance of medical accelerators, 2009
52. http://nww.pasa.nhs.uk/PASAWeb/Guidance/TOPPM/LandingPage.htm (NHS intranet
only)
53. http://www.ogc.gov.uk/procurement_policy_and_application_of_eu_rules_eu
_procurement_thresholds_.asp
54. UK Government Strategy for Sustainable Development; Securing the Future
http://www.defra.gov.uk/sustainable/government/publications/index.htm
55. EC Directive on Waste Electrical and Electronic Equipment
http://www.berr.gov.uk/files/file35992.pdf
CEP10071: March 2010
Appendix 1: Supplier contact details
58
Table 11. Supplier details
Manufacturer
Elekta AB,
Box 7593,
Stockholm
SE-103 93
Varian Medical Systems, Inc
Corporate Headquarters
.
3100 Hansen Way
Palo Alto, CA 94304-1038
TomoTherapy Incorporated
1240 Deming Way
Madison, WI 53717-1954 USA
Tel: +1 608 824 2800
Fax: +1 608 824 2996
CEP10071: March 2010
System
Synergy
Supplier
UK Sales and Service
Elekta Limited,
Linac House,
Fleming Way,
Crawley, West Sussex
RH10 9RR
UK Sales/Service
enquiries:
Tel: 01293 654 700
Fax: 01293 654 711
OBI
Hi-Art
Varian Medical Systems
UK Ltd.
Crawley, West Sussex, UK
Tel: 01293.531.244
TomoTherapy Europe
GmbH
Park Lane
Culliganlaan 2A
1831 Diegem, Belgium
Tel: +32 (0)2 400 4400
Fax: +32 (0)2 400 4401/02
Appendix 2: Clinical protocols
59
Table 12. Elekta Synergy v4.2 clinical image acquisition and reconstruction parameters
Protocol 1
Low head
Protocol 2
Med Head
Protocol 3
High Head
Protocol 4, 5 & 6
Pelvis
kVp
100
100
100
120
mA
10
20
32
32
ms
10
20
25
40
Ave No.
Frames
360
360
360
640
Exposure
(mAs)
36
144
288
819
Bow Tie Filter
No
No
No
Yes
Collimator
S20
S20
S20
M10, M15 & M20
Slice
Thickness
(mm)
1.0
1.0
1.0
1.0
In-plane
dimensions
(mm)
1.0 x 1.0
1.0 x 1.0
1.0 x 1.0
1.0 x 1.0
Imaging
Parameters
Table 13. Varian OBI v 1.5 clinical image acquisition and reconstruction parameters
Imaging
Parameters
Low Dose
Head
Standard
Dose Head
High Quality
Head
Pelvis
Pelvis
Spotlight
kVp
100
100
100
125
125
mA
20
10
80
80
80
ms
20
20
25
13
25
Ave No.
Frames
360
360
360
655
360
Exposure
145
72
720
680
720
Fan type
Full
Full
Full
Half
Full
Slice
Thickness
(mm)
2.5
2.5
2.5
2.5
2.5
In-plane
dimensions
(mm)
0.65 x 0.65
0.65 x 0.65
0.65 x 0.65
0.65 x 0.65
0.65 x 0.65
CEP10071: March 2010
Appendix 2: Clinical protocols
60
Table 14. TomoTherapy Hi-Art v 3.2 clinical image acquisition and reconstruction parameters
Imaging
Parameters
Fine
kVp
Normal
Coarse
3.5 MeV peak
Field Length
Up to 300 slices
Slice
Thickness
(mm)
2
4
6
In-plane
dimensions
(mm)
0.75 x 0.75
0.75 x 0.75
0.75 x 0.75
CEP10071: March 2010
Appendix 3: Imaging dose calibration
61
Initial calibration
Cone beam CT systems
On the Elekta Synergy and Varian OBI systems, dose measurements were made
using a Farmer chamber with a calibration factor traceable to national standards. A
single HVL measurement was made of the beam quality of the Elekta Synergy
system to enable suitable calibration factors to be determined. A measurement of
HVL was not performed on the Varian OBI as part of this evaluation; however, it was
assumed that this would not vary from that of the Elekta Synergy by more
than1.5 mm [23]. This results in a further uncertainty in the calibration factor of a few
% which is an acceptable degree of error for dose measurements of this type.
TomoTherapy Hi-Art
The TomoTherapy Hi-Art measurements were performed using a Standard Imaging
A1SL chamber. The absorbed dose to water calibration factor used was the NPL
alanine-derived factor as obtained for the clinical treatment beam (~6MeV peak). The
University of Wisconsin 60Co absorbed dose to water factor for the same chamber
only varied by ~1.2%. Thus as the imaging beam for TomoTherapy lies somewhere
between these two energies the use of the NPL derived factor introduces a maximum
error of ~1%.
CEP10071: March 2010
Appendix 4: Image dose data
62
All dose measurements are given in mGy. To remove variations due to differences in
the number of frames (projection images) or output levels, a normalised dose per
100mAs has also been presented in brackets for the Elekta Synergy and Varian OBI
systems, however, this is not applicable for the TomoTherapy Hi-Art as output cannot
be expressed in terms of mAs.
Table 15. Elekta Synergy v4.2 dose measurements (mGy)
Protocol 4
Pelvis M10
Protocol 5
Pelvis M15
Protocol 6
Pelvis M20
Centre
9.9 (1.2)
11.3 (1.4)
12.9 (1.6)
Top
16.0 (1.9)
17.0 (2.1)
17.5 (2.1)
Mid Top
12.1 (1.5)
13.6 (1.7)
15.2 (1.9)
Mid Bottom
-
-
14.2 (1.7)
Bottom
-
-
16.8 (2.1)
Protocol 4
Pelvis M10
Protocol 5
Pelvis M15
Protocol 6
Pelvis M20
2 x Body
Protocol 1
Low head
Protocol 2
Med Head
Protocol 3
High Head
Protocol 1
Low head
Protocol 2
Med Head
Protocol 3
High Head
Centre
1.2 (3.3)
4.8 (3.3)
9.6 (3.3)
Top
1.4 (3.8)
5.5 (3.8)
11.0 (3.3)
Bottom
1.5 (4.0)
5.8 (4.0)
11.6 (4.0)
Protocol 1
Low head
Protocol 2
Med Head
Protocol 3
High Head
Protocol 4
Pelvis M10
Protocol 5
Pelvis M15
Protocol 6
Pelvis M20
1.5 (4.1)
5.9 (4.1)
11.7 (4.1)
37.8 (4.6)
38.2 (4.7)
38.6 (4.7)
2 x Head
In Air
Isocentre
CEP10071: March 2010
Appendix 4: Image dose data
63
Table 16. Varian OBI v1.5 dose measurements (mGy)
Pelvis
Pelvis
Spotlight
Centre
19.8 (2.9)
17.9 (2.5)
Top
29.0 (4.3)
1.6 (0.2)
Mid Top
23.7 (3.5)
7.7 (1.1)
Mid Bottom
23.4 (3.4)
31.1 (4.3)
Bottom
28.7 (4.2)
42.6 (5.9)
Pelvis
Pelvis
Spotlight
2 x Body
Low Dose
Head
Standard
Dose Head
High Quality
Head
Low Dose
Head
Standard
Dose Head
High Quality
Head
Centre
3.0 (4.2)
6.1 (4.2)
30.4 (4.2)
Top
1.0 (1.3)
1.9 (1.3)
9.6 (1.3)
Bottom
4.3 (6.0)
8.7 (6.0)
43.5 (6.0)
Low Dose
Head
Standard
Dose Head
High Quality
Head
Pelvis
Pelvis
Spotlight
5.2 (7.3)
10.5 (7.3)
52.4 (7.3)
78.9 (11.6)
42.9 (6.0)
2 x Head
In Air
Isocentre
Table 17. TomoTherapy Hi-Art v3.2 dose measurements (mGy)
Fine
Medium
Coarse
Centre
19.6
8.4
6.9
Periphery
23.7
11.4
7.4
Mid
20.8
9.7
7.2
Fine
Medium
Coarse
Centre
29.0
14.0
9.4
Periphery
30.7
15.3
9.5
In Air
Fine
Medium
Coarse
Isocentre
34.8
18.4
11.6
2 x Body
2 x Head
CEP10071: March 2010
Appendix 5: Hounsfield number accuracy
64
Table 18. Varian OBI v1.5 measured Hounsfield numbers. Percentage difference from expected
given in brackets
Low Dose
Head
Standard
Dose Head
High Quality
Head
Pelvis
Pelvis
Spotlight
Air
-942 (6%)
-993 (1%)
-998 (0%)
-998 (0%)
-999 (0%)
PMP
-195 (1%)
-195 (1%)
-202 (0%)
-192 (1%)
-194 (1%)
LDPE
-97 (0%)
-101 (0%)
-108 (-1%)
-97 (0%)
-100 (0%)
Poly
-51 (-2%)
-56 (-2%)
-56 (-2%)
-50 (2%)
-54 (-2%)
Acrylic
91 (-3%)
106 (-1%)
102 (-2%)
126 (1%)
106 (-1%)
Delrin
303 (-4%)
328 (-1%)
322 (-2%)
356 (2%)
325 (-2%)
Teflon
928 (-6%)
980 (-1%)
982 (-1%)
1037 (5%)
985 (-1%)
Table 19. Elekta Synergy v4.2 measured Hounsfield numbers. Percentage difference from
expected given in brackets
High
Head
Pelvis
M10
Pelvis
M15
Pelvis
M20
-78 (92%)
-191
(81%)
-386
(61%)
-375
(63%)
-438
(56%)
734 (93%)
544 (74%)
440 (64%)
235 (44%)
235 (44%)
251 (56%)
795 (90%)
618 (72%)
651
(69%)
522 (62%)
562
(60%)
296 (40%)
296 (40%)
311 (41%)
339 (37%)
339 (37%)
365 (40%)
786 (67%)
695 (58%)
476 (36%)
466 (35%)
520 (40%)
942 (60%)
1394
(40%)
855 (52%)
1319
(33%)
638 (30%)
1105
(12%)
628 (29%)
703 (36%)
1248
(26%)
Low head
Med Head
230
(123%)
PMP
LDPE
Air
Poly
Acrylic
Delrin
Teflon
827 (86%)
945 (83%)
1084
(74%)
1443
(45%)
CEP10071: March 2010
1080 (9%)
Appendix 5: Hounsfield number accuracy
65
Table 20. TomoTherapy Hi-Art v3.2 measured Hounsfield numbers. Percentage difference from
expected given in brackets
Fine
Medium
Coarse
Air
-914 (9%)
-913 (9%)
-908 (9%)
PMP
-93 (11%)
-98 (10%)
-104 (10%)
LDPE
-16 (8%)
-5 (10%)
2 (10%)
Poly
49 (8%)
39 (7%)
45 (8%)
Acrylic
168 (5%)
162 (4%)
168 (5%)
Delrin
384 (4%)
368 (3%)
390 (5%)
Teflon
860 (13%)
866 (12%)
856 (13%)
CEP10071: March 2010
Appendix 6: CATphan image quality - spatial resolution
66
Figure 13. Varian OBI v1.5 images of the CTP528 module of a CATphan. Images have been displayed at the same window and level settings
Low dose head
Standard dose head
Pelvis
Pelvis spotlight
CEP10071: March 2010
High quality head
Appendix 6: CATphan image quality - spatial resolution
67
Figure 14. Elekta Synergy v4.2 images of the CTP528 module of a CATphan. Images have been displayed at the same window and level settings
Low dose head
Medium dose head
High dose head
M10
M15
M20
CEP10071: March 2010
Appendix 6: CATphan image quality - spatial resolution
68
Figure 15. TomoTherapy Hi-Art v3.2 images of the CTP528 module of a CATphan. Images have been displayed at the same window and level
settings
Coarse pitch
CEP10071: March 2010
Normal pitch
Fine pitch
Appendix 7: CATphan image quality - low contrast visibility
69
Figure 16. Varian OBI v1.5 images of the CTP404 module of a CATphan. Images have been displayed at the same window and level settings
Low dose head
Standard dose head
Pelvis
Pelvis spotlight
CEP10071: March 2010
High quality head
Appendix 7: CATphan image quality - low contrast visibility
70
Figure 17. Elekta Synergy v4.2 images of the CTP404 module of a CATphan. Images have been displayed at the same window and level settings
Low dose head
Medium dose head
High dose head
M10
M15
M20
CEP10071: March 2010
Appendix 7: CATphan image quality - low contrast visibility
71
Figure 18. TomoTherapy images of the CTP404 module of a CATphan. Images have been displayed at the same window and level settings
Coarse pitch
CEP10071: March 2010
Normal pitch
Fine pitch
Appendix 8: Evaluation phase 1 summary of results
72
Test 1: Registration of image volume to treatment isocentre
Table 21. kV/MV alignment (mm)
Elekta Synergy v 4.2
Varian OBI v 1.4
Site 1
0.65
Site 6
0.94
Site 2
0.85
Site 7
0.73
Site 3
0.64
Site 8
0.94
Site 4
0.10
Site 5
0.54
Mean
0.56
Mean
0.87
Max
0.85
Max
0.94
Test 2: Image-shift–verify test
Table 22. Residual errors after image-shift-verify test (mm)
Elekta Synergy v 4.2
Site
Lateral
Long
Vertical
Magnitude
1
-0.5
-0.7
-0.3
0.9
2
-0.3
-0.3
-0.3
0.5
3
-0.2
0.0
0.6
0.6
4
-0.8
-0.2
1.0
1.3
5
-0.5
-0.2
-0.7
0.9
Lat
Long
Vertical
Magnitude
6
0
0
1
1
7
0
0
0
0
8
0
1
0
1
Varian OBI v1.4
Site
TomoTherapy Hi-Art v3.1
Measurement not performed
CEP10071: March 2010
Appendix 8: Evaluation phase 1 summary of results
73
Image dose and Image quality
Table 23. Image acquisition and reconstruction parameters as used for the investigation of
image dose and image quality at the phase 1 evaluation and test sites
Elekta Synergy
v4.2
Varian OBI v1.4
TomoTherapy
Hi-Art v3.1
Collimation
S10
Full-Fan, FOV=24 cm,
Length =13.6 cm
Length = 9.4 cm
Pitch = 1
Bow Tie
No
Yes
N/A
120 kV
125 kV
3.5 MV
40 mA, 25 ms;
High dose (40mA,10ms)
N/A
Reconstruction
High resolution (0.5 mm
x 0.5 mm)
0.5 mm x 0.5 mm
0.75 mm x 0.75 mm
Slice thickness
1 mm
1 mm
1 mm
kV/MV
mA & ms
†
†
Note: It should be noted that the Elekta Synergy does not permit the reconstruction of rectangular
voxels. Consequently, 0.5 mm slices were reconstructed initially which were then compressed on
DICOM export to increase slice thickness to the desired 1mm.
Test 3: Image dose
Table 24. Dose measurements made in air and in a single full body CTDI phantom. Results
displayed as mean (standard deviation)
Normalised dose (mGy/100mAs)
Centre
Periphery
In air
Elekta Synergy
1.9 (0.1)
4.5 (0.1)
6.5 (0.1)
Varian OBI
1.9 (0.2)
3.6 (0.2)
10.7 (0.6)
N/A
N/A
TomoTherapy Hi-Art
†
N/A
Note: † This is not applicable as the output for TomoTherapy Hi-Art cannot be
expressed in terms of mAs.
CEP10071: March 2010
Appendix 8: Evaluation phase 1 summary of results
74
Figure 19. CT number accuracy for each density insert within the CTP404 module of a CATphan
for each of the phase 1 evaluation sites, defined as measured CT number minus nominal
(centres 1-5 Elekta Synergy, centres 6-8 Varian OBI and centre 9 TomoTherapy Hi-Art)
800
Air
PMP
LDPE
Polystyrene
Acrylic
CT number accuracy
600
Delrin
Teflon
400
200
0
-200
0
1
2
3
4
5
Evaluation centre
CEP10071: March 2010
6
7
8
9
Appendix 8: Evaluation phase 1 summary of results
75
Test 4: Image quality*
†
Resolution
lp/cm
Linearity
%
Aspect
ratio
Slice
sensitivity
(mm)
8.2
8.9
6-7
-0.3
1.000
11.7
2.5
26.7
9
-0.1
Site 3
10.9
5.9
10.4
7-8
Site 4
11.8
8.2
8.4
Site 5
13.6
5.2
Site 6
24.0
Site7
Site 8
Dose
(mGy)
CNR for
polystyrene
Site 1
11.0
Site 2
Noise for
polystyrene
Uniformity§
%
U1.
U2.
1.34
0.43
-0.08
1.002
0.93
1.32
1.18
0.0
1.000
1.06
3.37
3.13
7
0.3
1.000
1.15
0.75
-0.49
10.6
8
-0.6
1.002
0.99
3.67
3.58
7.6
13.8
9
-0.3
1.000
1.34
0.27
-0.16
28.8
6.5
15.8
9
0.6
1.002
1.42
0.68
-0.47
22.8
6.4
16.3
9-10
-0.3
1.000
1.38
1.24
-1.09
3.6
20.1
4
-0.1
1.003
‡
5.55
-5.13
Elekta Synergy
Varian OBI
TomoTherapy Hi-Art
Site 9
Notes:
12.5
* It should be noted that the Elekta Synergy images were attained without a bow tie filter in contrast to the Varian OBI images which were also
acquired at effectively double the mAs.
† Noise defined as the standard deviation of the polystyrene density insert.§ Two different definitions of uniformity used.
‡ Angled wire not visible in MV-CT images of CATphan, therefore measurement not made
CEP10071: March 2010
Appendix 9: User survey questionnaire
CEP10071: March 2010
Appendix 9: User survey questionnaire
CEP10071: March 2010
Appendix 9: User survey questionnaire
CEP10071: March 2010
Appendix 9: User survey questionnaire
CEP10071: March 2010
Appendix 9: User survey questionnaire
CEP10071: March 2010
Appendix 9: User survey questionnaire
CEP10071: March 2010
Appendix 9: User survey questionnaire
CEP10071: March 2010
Appendix 9: User survey questionnaire
CEP10071: March 2010
Author and report information
Evaluation report:
X-ray tomographic image
guided radiotherapy systems
J Sykes, R Lindsay, S Stanley*,
D Thwaites
Radiotherapy Physics Group
Medical Physics and Engineering
& *Radiotherapy Department, NonSurgical Oncology
St James's Institute of Oncology
Leeds Teaching Hospitals/University of
Leeds
St James's University Hospital
Leeds LS9 7TF
Tel: 0113 2067921
Email: [email protected]
www.medphysics.leeds.ac.uk
PJ Clinch, DP Emerton, M Kazantzi,
JA Cole, CP Lawinski
KCARE
Department of Medical Engineering
and Physics
King’s College Hospital
Denmark Hill
London SE5 9RS
Tel: 020 3299 1620
Email: [email protected]
www.kcare.co.uk
R Dickinson
MagNET Evaluation Centre
Department of Bioengineering
Imperial College London
Exhibition Road
London SW7 1NA
Tel: 020 7594 6305
Email: [email protected]
www.magnet-mri.org
CEP10071: March 2010
84
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