Download Physics/Engineering Aspects of Medical Accelerators

Physics/Engineering Aspects of
Medical Accelerators
January 9-11, 2013
Timothy J. Waldron, M.S.
University of Iowa Hospital & Clinics
Iowa City, Iowa, USA
Med Accels Jan2013 Waldron D2 1
Physics/Engineering Aspects of Medical Accelerators,
January 9-11, 2013
Day 2 1130 – Image Guided Radiation Therapy (IGRT)
•
•
•
•
What is Image Guidance?
Types of IG in RT
Image formation and characteristics
Example Systems
Med Accels Jan2013 Waldron D2 2
Image Guided Radiation Therapy
(IGRT) Short History
Earliest External Beam Radiotherapy (i.e., Co-60, 250 kV)
2D (film) Imagery diagnoses.
Clinical setup to bony landmarks and external anatomy
Treatment of large volumes
Non-isocentric treatment machines, “SSD techniques”
Crude Beam Shaping “Portal Verification”
Film portal verification not very useful if Co-60.
• Large source size –geometric penumbra
• Energy limited bone contrast
• Solution to add kV imager to Cobalt machine
Development of Medical Linear Accelerators (1960’s)
Isocentric techniques
Improvements in film imagery –intensifier cassettes for MV.
Beam shaping with custom molded blocks
Conventional simulators
Med Accels Jan2013 Waldron D2 3
IGRT Short History (3)
Development of “Conventional Simulator” (1970’s?)
CT-based diagnostic imagery becoming available
Med Accels Jan2013 Waldron D2 4
IGRT Short History (2)
Development of “Conventional Simulator” (1960’s-1970’s?)
CT-based diagnostic imagery becoming available
Fluoroscopic/Film planning of beam arrangements using
“Conventional simulators”- fluoroscopic and film based
treatment planning.
Computerized treatment planning at some institutions
Positron Emission Tomography (PET)
Development of other Technologies (1980’s)
Magnetic Resonance Imaging (MRI)
Computerized treatment planning widely available (PCs)
Digital image processing for handling and manipulation of
digitized films and CT images.
Image segmentation
Digital imaging in fluoroscopic and interventional use
Med Accels Jan2013 Waldron D2 5
IGRT Short History (3)
Use of “CT Simulator” (1990’s)
CT-simulation
3D Computerized Treatment planning systems
Growth of image registration algorithms
Electronic Portal Imaging Devices (EPIDs)
Multileaf Collimation
Intensity Modulated Radiation Therapy (IMRT)
Multi-modality Imaging (2000’s)
PET/CT
On Board Imaging (OBI)
Cone Beam CT (CBCT)
Magnetic Resonance Imaging (MRI)
4D Imaging and Gated Treatment Delivery
Adaptive Radiation Therapy
At what time was radiotherapy not “image guided”?
Med Accels Jan2013 Waldron D2 6
Imaging Use in Radiation Therapy
3 Major Image Applications
Simulation & Planning
(CT, 4DCT, MRI,
PET, MRSI, fMRI,
US)
Localization/Verification
(CBCT, CT, MRI, CT,
US, IR, Surface, etc.)
Treatment/Response Monitoring
(PET, MRI, MRSI, fMRI, CT)
Med Accels Jan2013 Waldron D2 7
Imaging Use in Radiation Therapy
Simulation and planning:
Imaging data is collected with the patient in the treatment state with respect to
immobilization and planned treatment devices.
Images are transferred to the Treatment Planning System, where they may be
combined with other modality images for organ delineation and beam
planning.
Ultimately a set of deliverable beam parameters and images for use in
treatments are generated that will treat the target as designated by
prescription.
Simulation and planning may be performed once or more than once for a
given course of treatment.
These activites are usually not done in the treatment room.
Imaging modes for simulation and planning are generally CT and often others
such as PET and MRI.
Med Accels Jan2013 Waldron D2 8
Imaging for Simulation and Planning
Xray Computed Tomography (CT)
CT imaging is the mainstay of patient data
for simulation/treatment planning.
Advantages:
•
•
•
•
Fast acquisition (seconds)
Geometrically accurate
High resolution
Good anatomic
visualization
• Data are useful for dose
computation ( signal level
correlates to electron
density)
• 3 dimensional data set
• Compatible markers,
immobilization aid in setup
Med Accels Jan2013 Waldron D2 9
Imaging for Simulation and Planning
Xray Computed Tomography (CT)
Disadvantages:
•
•
•
•
Patient Radiation Dose –Limit dose = limit signal
Little or no biological function information
High-Z artifacts; these also impede use for dose calculation.
Limited soft tissue contrast
Med Accels Jan2013 Waldron D2 10
Imaging for Simulation and Planning
CT Disadvantages:
Med Accels Jan2013 Waldron D2 11
Imaging for Simulation and Planning
Magnetic Resonance Imaging (MRI)
CT
•
•
MRI Advantages: •
•
•
T1
T2
No Ionizing Radiation Dose to Patient
Good Soft Tissue Contrast
High resolution (depends on magnet strength)
Good anatomic visualization
3 dimensional data set
Imaging for Simulation and Planning
MRI Disadvantages:
• Inductive heating, of both the patient and
any metallic items in the patient.
• Force on implanted metals, i.e. surgical
clips. (Not all patients can be imaged).
• Neurostimulation in very high fields.
• Compatibility with markers,
immobilization, etc.
• Slow acquisition time, compared to CT
• Geometric distortion inherent in image
formation.
• No functional biological information in
standard images.
• Image data not especially useful for
dose calculations.
Med Accels Jan2013 Waldron D2 13
Imaging for Simulation and Planning
Position Emission Tomography (PET) and PET/CT
PET Advantages:
• Image set contains information
about biological function,
i.e.,F18DG provides sugar
metabolism information.
• If attenuation correction CT is
used (PET/CT image set is and
accurate 3 D model of uptake.
Med Accels Jan2013 Waldron D2 14
Imaging for Simulation and Planning
Position Emission Tomography (PET) and PET/CT
PET/CT Disadvantages:
• Resolution: Limited by range of
positrons in tissue (~0.4 cm).
• Sensitivity: Limited by injected
activity/dose to patient.
• Patient must be injected with
radioactive tracer –dose.
• Very long acquisition times.
• Liquid radioisotope
management/radiation safety
issues may be problematic.
Med Accels Jan2013 Waldron D2 15
Imaging for Simulation and Planning
Combining Image Data Sets – “Image Fusion”
Multiple types of imaging may be combined to aid in structure delineation in
the treatment planning process.
This is sometimes referred to as “image fusion”.
When two image sets are “fused”, or co-registered, the secondary image data
set is manipulated so that it is maximally match to the primary data set. The
two data sets may then be viewed together or independently.
Changes to the secondary set are normally “non-deformable”, meaning that
they are limited to translation, rotation and magnification of the entire data set.
No local changes are made.
Typically for treatment planning, the CT image set is selected as primary data,
both for geometric stability and dose calculation.
MRI and/or PET/CT typically are selected as the secondary sets, but additional
CT acquisitions might also be used.
Med Accels Jan2013 Waldron D2 16
Imaging for Simulation and Planning
Combining Image Data Sets – “Image Fusion”
Note PET/CT data are inherently “fused”, but this is due to
machine geometry.
Imaging for Simulation and Planning
Combining Image Data Sets – “Image Fusion”
CT
MRI
PET/CT
3 DATASETS
Slide from John Bayouth
Imaging for Simulation and Planning
The “output” data from Simulation and Planning1: Treatment Data
Calculated Dose
Distribution
“Image”
Machine treatment parameters: The set
of data needed for the machine to deliver
the calculated dose to the patient.
• Couch, gantry collimator setting
• MLC settings
• Energy and MU settings
• Et cetera
Med Accels Jan2013 Waldron D2 20
Margins for target definition
- Add to GTV to get clinical target
volume (CTV)
- For internal motion due to
respiration, bladder filling, etc.
- Accounts for spatial uncertainties
in the radiation delivery process
- Presence of organs at risk can decrease margin size
(spinal cord, optic nerve, etc.)
Figure from ICRU Report 62 (1999)
Imaging for Simulation and Planning
The “output” data from Simulation and Planning 2: Localization
Localization Data are the mages and parameters necessary to accurately
place the patient such that the pose and location of the target with respect to
the treatment machine is consistent with the dosimetric treatment plan.
Parameters
• Couch and fiducial
coordinates.
• Patient marking notes and
diagrams
• Shifting instructions
• Data for use with in-room
localization tools, such as
lasers and non-radiographic
imaging systems.
Med Accels Jan2013 Waldron D2 22
Images
• 2D Digitally Reconstructed
Radiographs (DRR).
• Surface renderings from TPS
• 3D Simulation/planning CT with
delineated contours, fiducials
indicated.
DRR/Portal imaging example
DRR
Double-exposure EPID image
Slide from Junyi Xia, Ph.D.
Imaging Use in Radiation Therapy
3 Major Image Applications
Simulation & Planning
(CT, 4DCT, MRI,
PET, MRSI, fMRI,
US)
Localization/Verification
(CBCT, CT, MRI, CT,
US, IR, Surface, etc.)
Treatment/Response Monitoring
(PET, MRI, MRSI, fMRI, CT)
Med Accels Jan2013 Waldron D2 24
Imaging for Localization and Verification
Now in the treatment room…
Localization:
The process (in the treatment room) of accurately placing the patient such
that the pose and location of the target with respect to the treatment
machine is consistent with the dosimetric treatment plan. (Each fraction).
Verification:
Radiographic confirmation of correct localization. Images collected in
the treatment pose are compared with reference image data from
treatment planning. This may be daily or less frequent, depending upon
modality used.
While localization may be performed using either radiographic or nonradiographic tools, verification is an inherently radiographic process.
Med Accels Jan2013 Waldron D2 25
Imaging for Localization and Verification
NonRadiographic
Tools
Treatment
Room, Daily
Coordinate transform
from TPS to Tx room
(isocenter).
Verify tumor
location unchanged
from plan
Beam data (MLCs,
MU’s, Machine
parameters)
Med Accels Jan2013 Waldron D2 26
This
seems to
be where
the
“guidance”
part comes
in.
•
•
•
•
•
•
•
Lasers
Light Field
Gating Devices
IR Tracking
Surface Monitors
RF Transponders
Ultrasound
Radiographic
Tools
•
•
•
•
•
•
•
•
Portal Films
EPID
CT on Rails
CT Built In
(Tomo)
kV X-ray (2D)
Stereoscopic
kV X-ray
kV CBCT
MV CBCT
Which of these are
“image” tools?
Non-Radiographic Image Guidance Tools
(Systems acquire data in 2D or 3D, can be considered image guidance, even
though images may or may not be presented to the user.)
Advantage and Limitations
• Can be used for maintenance of localization, generally after initial
radiographic verification. +
• No additional radiation dose to patient. +
• Generally not significant machine modifications, potential for cost
containment. +
• Accuracy is ok when applied correctly. +
• Variable level of “clearance” issues with couch, machine, room clutter,
etc. ~
• No information about internal anatomy, so efficacy of any technology
is site-dependent. • Non-radiographic tools cannot be used for initial verification of setup.
Radiologic images are still necessary. Med Accels Jan2013 Waldron D2 27
Non-Radiographic Image Guidance
Infrared (IR) light based systems
Machine vision systems capable of measuring the location of emitters
or markers whose pose and location are known with respect to the
treatment room coordinate system (“Isocenter”).
• Markers are placed on the patient at the time of CT and
incorporated into the planning data set.
• The coordinate transform from the CT coordinate system to
the room coordinate system is computed and sent to the
sensor computer in the treatment room.
• The treatment room sensor is calibrated to isocenter, and
when the patient is set up for treatment, the in-room sensor is
used to localize the patient with relative accuracy (0.5-1 mm).
Med Accels Jan2013 Waldron D2 28
InfraRed/Fiducial Stereo Position Sensor
Example: NDI Polaris
Moore et al, “Opto-electronic sensing of boyd
surface topology changes during radiotherapy for
rectal cancer, IJROBP 56:1 p248, 2003.
IR/Fiducial Position Sensor
NDI Polaris
•This sensor is not an RT specific device but
is present in RT products and other medical
“navigation” systems.
•Active or passive markers, singly or
arranged in “tools” for 6DOF output.
•Up to 60 frames per second, depending upon
markers being tracked.
•Volume of view: Domed cylinder approximately 1 m dia x
1 m long.
•Accuracy: Manufacturer specifies 0.35 mm within the
effective VOV, <0.2 mm in literature.
1m
Patern Projection Stereo Correspondence
(Image Feature Correspondence)
•Another method to enhance image search space is via projection of
patterned or structured light onto the scene.
•A known pattern provides unique features for search and correlation.
•Depending upon the geometry, apparent distortion of the projected
pattern might also be used to compute distances.
Images from: Siebert et al, “Human body 3D imaging by speckle texture
projection photogrammetry,” Sensor Review 20:3, p 218, 2000.
Patterned Light Projection Stereo Example
VisionRT Ltd., ALIGNRT
Stereo camera
pair
•RT Vision sensor system
arranged in pods, each is capable
of stereo “vision”.
•A pod contains 1 stereo pair of
cameras and a speckle pattern
projector.
flash
•A texture camera and white flash
are also present.
•Phantom static accuracy <0.54 mm dev
and 0.2° in computed 6DOF shifts.
Static/dynamic accuracy in humans 1.02
±0.51 mm upper thorax. (Schöffel et al
2007).
Speckle
projector
Speckle Pattern
Schöffel et al, “Accuracy of a commercial optical 3D surface
imaging system for realignment of patients for radiotherapy of
the thorax,” Phys. Med. Biol. 52 (2007) 3949-3963.
Texture
camera
Patterned Light Projection Stereo Example
VisionRT Ltd., ALIGNRT
•Typically 2 pods are used in clinical installation,
mounted lateral to couch to ensure full view of patient
(~240°).
•The psuedorandom speckle pattern is projected
onto the patient during image acquisition. This
pattern provides a set of defined “objects” in the
image to facilitate the stereo search/correspondence
process.
•The illuminated surface positions are computed
at approximately 5 mm spacing, and the
combined model from the two pods is stored.
Bert et al, “A phantom evaluation of a stereo-vision surface imaging
system for radiotherapy patient setup,” Medical Physics 32:8, p.2753
(2005).
Laser Line Projection Methods
(Laser Triangulation Method)
Laser spot
in image at
I(x,y)
Laser line
projected
at P( x, )
CCD
Camera
Galvanometer –
laser fan line
scanning projector
•Here, the invariant geometry
is the camera-laser projector
geometry and the projections
of the laser as a function of
time.
•Under reference conditions,

each pixel I(x,y) in the image
can be associated with a laser
projection
P(
).
x,
•Apparent change in pixel
location can then be related to
surface distance from
reference.
Laser Line Projection Methods
Laser spot
in image at
I(x’,y’)
CCD
Camera
s
D i
(deflected ΔI)
Laser spot
projected
at P
x,( )
I
z
Galvanometer –laser
fan line scanning
projector
•For
surface displaced
•Spota deflection,
ΔI is related
normally
fromchange
the reference
by
to the height
z and the
xspot
,canP(be
z,
the reflected
laser
overall
geometry,
and
)computed
now projects
to a as
different
so long
the pixel
I(x’y’)
in the
image.
incident
angles
and camerainterferometer geometry are
well known.
FS cos2 i
I 
D  S sin i cosi
Moore et al, “Opto-electronic sensing of body
surface topology changes during radiotherapy for
rectal cancer, IJROBP 56:1 p248, 2003.
Line Projection/Triangulation Example
C-RAD Sentinel
•The Sentinel device consists of a line scanning laser and
CCD camera, pictured at right.
•The projected laser fan beam incident angle is stepped
via a galvanometer with approximately 1 cm pitch at the
patient surface.
•Each line step is captured and analyzed by
searching the image for the laser line.
•Since the incident angle for each line is known, the
displacement from the reference is readily computed
via triangulation.
•Resolution/accuracy: Vertical resolution < 0.1
mm, transverse < 0.5 mm (Brahme (2008).
Line Projection/Triangulation Example
C-RAD Sentinel
•Normal detection VOV 40x40x20 cm3.
•Frame rate at full FOV ~1 fps. Higher
rates available with reduced FOV.
•Resolution/accuracy: Vertical resolution <
0.1 mm, transverse < 0.5 mm (Brahme
(2008).
Brahme et al, “4D laser camera for accurate
patient positioning, collision avoidance, image
fusion and adative aproaches during diagnostic
and therapeutic procedures,” Medical Physics 35
(5) p. 1670 (2008).
RF Transponder Marker-Tracking Systems
Calypso System
EM-excitable
transponders
Transponders can
be located many times
per second with better
than 1 mm accuracy.
Slide from Junyi Xia, Ph.D.
Radiographic Image Guidance (In tx room)
Radiographic Tools in IGRT
Advantage and Limitations
• Can be used for initial localization, verification and maintenance of
localization. +
• May provide information about internal anatomy. +
• Useful for monitoring anatomic changes relative to tx plan +
• Generally significant machine modifications, costly add-on • Radiographic tools deliver radiation dose to patient • Accuracy may also depend upon selected technology and anatomic
site. ~
2D Images:
• Emulsion Films
• kV and MV Electronic Imaging
Med Accels Jan2013 Waldron D2 40
3D Image Data:
• CT On Rails/In Room
• kV and MV Cone Beam CT
Portal imaging with film
- Permanent record of beam's eye view
- Film system used at UIHC (Kodak EC-L screen)
- Megavoltage films inferior to kilovoltage films
- As beam energy increases, contrast decreases
Image from www.kodak.com
Slide from Junyi Xia, Ph.D.
Flat panel detector systems
EPID image from: Hell E, et al, Nucl. Instrum. Meth. Phys. Res. A (2000)
Slide from Junyi Xia, Ph.D.
Indirect detection flat panel
systems
- Detectors sensitive to visible light
- X-rays are converted to visible light in scintillator
- Visible light scatters in scintillator, causing blurring
Image from Bushberg et al, “The Essential Physics of Medical Imaging” (2002), p. 304
Slide from Junyi Xia, Ph.D.
Direct detection flat panel
imagers
• No scintillator => no visible light scatter prior to detection
• Direct detection system*: similar image quality as indirect, 55% less efficient
• Direct detection system more sensitive to x-ray scatter from patient
• Scatter sensitivity (direct) and visible light scatter (indirect) offset each other
Image from Bushberg et al, “The Essential Physics of Medical Imaging” (2002), p. 304
*Partridge
et al, Nucl. Instrum. Meth. Phy. Res. 484, 351-63 (2001):
Slide from Junyi Xia, Ph.D.
Flat panel detector array
40 cm,
1024 pixels
Image from Bushberg et al, “The Essential Physics of Medical Imaging” (2002), p. 301
Slide from Junyi Xia, Ph.D.
Flat panel readout
- Detectors collect charge produced by radiation when switches open
- When switches closed, charge goes out to digitizer through multiplexer
Switches closed first
Row of detectors read
Switches closed second
Row of detectors read
Switches closed third
Row of detectors read
Image from Bushberg et al, “The Essential Physics of Medical Imaging” (2002), p. 301
Slide from Junyi Xia, Ph.D.
Radiographic Image Guidance (In tx room)
“3D” In-Room Imaging: Fan beam CT
CT On Rails: Treatment couch
modified to pivot patient from
treatment location to CT location.
CT then moves to scan patient on
treatment couch.
MVCT: Tomotherapy performs
spiral CT acquisition using
reduced energy (~4 MV) linac
beam.
Radiographic Image Guidance (In tx room)
“3D” In-Room Imaging: Cone Beam CT
Fan Beam CT: A narrow fan beam of radiation is used in conjunction with
a row of detectors to acquire a “slice” of data each rotation. The patient
is translated to collect a full data set.
Cone Beam CT: A large field of radiation is used in conjunction with a
2D detector to acquire a full 3D data set in 1 rotation.
Radiographic Image Guidance (In tx room)
“3D” In-Room Imaging: Cone Beam CT
kV CBCT: A kilovoltage (kV) x-ray source and image receptor are
incorporated into the treatment machine, typically mounted orthogonal to
the treatment beam.
MV CBCT: A megavoltage beam is used (6X) with an electronic portal
imaging panel to acquire data.
Megavoltage cone beam CT (MVCBCT)
• Treatment beam used
for 3-D volumetric imaging
• Flat panel with a-Si detectors
used for acquisition
• Streaking artifacts from high-Z
materials reduced rel. to kVCT
• Additional on-board imaging
system not required
Slide courtesy of Ryan Flynn, Ph.D.
Video from Siemens
Oncology Care Systems
UNIVERSITY of IOWA Carver College of Medicine
Megavoltage cone beam CT (MV CBCT)
Helical
kV CT
• Images acquired using
conventional equipment: Flat
panel imager + 6 MV Tx beam
• Imaging and treatment
isocenters are the same
MV CBCT
• Streaking artifacts from high-Z
materials reduced rel. to kVCT
• Imaging dose trivial to model in
treatment planning system and
can be incorporated into Tx plan
Slide courtesy of Ryan Flynn, Ph.D.
Images from J. Pouliot
UNIVERSITY of IOWA Carver College of Medicine
Reduced high Z artifacts
Kilovoltage fan beam CT
Megavoltage cone beam CT
Images from Jean Pouliot, UCSF
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Reduced high Z artifacts
Image from Jean Pouliot, UCSF
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
MVCBCT Positioning System
MVCBCT
kVCT
• An automatic image registration algorithm is applied following
MVCBCT acquistion
• kVCT used as a guide each day for MVCBCT patient positioning
• User can then fine-tune the registration manually
• User has freedom to select contoured structures to aid in process
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Day-to-day position of a single patient
UNIVERSITY of IOWA Carver College of Medicine
Spinal cord location over
5 fractions
Images from Kevin Bylund, M.D.
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Daily MVCBCT workflow
kVCT imaging
Delineation
Treatment
planning
Yes
Correct
for shift
Correction
required?
MVCBCT
Daily
Imaging
No, go to next Tx day
No
Daily
Treatment
Last
Treatment?
No, go to next Tx day
Slide courtesy of Ryan Flynn, Ph.D.
Yes
Done with
Treatment
UNIVERSITY of IOWA Carver College of Medicine
MVCBCT Shifts Correct Dose Errors
Planned
Uncorrected
Corrected
70 Gy
75
63
56
45
30
20
Shifts: 0 mm Lateral
-6 mm Anterior
4 mm Inferior
Data from Ann Morris, M.D.
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
MVCBCT Shifts Correct Dose Errors
Planned
70 Gy
Uncorrected
Corrected
75
63
56
45
30
20
Shifts: 0 mm Lateral
-6 mm Anterior
Data from Ann Morris, M.D. 4 mm Inferior
UNIVERSITY of IOWA Carver College of Medicine
Slide courtesy of Ryan Flynn, Ph.D.
MVCBCT Shifts Correct Dose Errors
Planned
Uncorrected
Corrected
75 Gy 70 63
45
56
30
Shifts: 0 mm Lateral
-6 mm Anterior
4 mm Inferior
Data from Ann Morris, M.D.
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Delivered doses to organs at risk match
planned dose after shift correction
Volume Fraction
Spinal Cord
CTV
Planned:
Uncorrected:
Lt Parotid
Rt Parotid
Corrected:
Dose (cGy)
Data from Ann Morris, M.D.
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Plot from
John Bayouth
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Plot from
John Bayouth
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Plot from
John Bayouth
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
MVCBCT Dose Management
• Daily MVCBCT imaging dose calculated for
all patients at UIHC
– For pelvic cases, could be 5-10% of total dose
• Imaging dose incorporated into 3D-CRT
planning during prescription assignment
• Imaging dose incorporated into optimization
process for IMRT
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Treatment beam MVCBCT imaging dose
Head and neck case
5.0 cGy
4.5
4.0
3.5
15.0 cGy
13.5
Prostate case
12.0
10.5
9.0
7.5
6.0
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
Incorporating imaging dose into
IMRT treatment plans
• Imaging protocol is selected, and imaging dose
is calculated
• Inverse planning process is run with imaging
dose as a baseline dose
• Imaging beam functions as a treatment beam
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
MVCBCT treatment planning workflow for IMRT
Cone Beam
Imaging Dose
Generate
Optimized Plan
73 Gy
Conventional
Planning
Method
MVCBCT
Dose
Integration
Method
10
Dose Actually
Delivered
>75 Gy
55
Ideal Plan, with
zero imaging dose
Dose (cGy):
MVCBCT
Imaging (10 cGy
for this case)
60
Similar
70 Gy
9.0
8.0
7.0
+5-7 minutes
calc time
Slide courtesy of Ryan Flynn, Ph.D.
UNIVERSITY of IOWA Carver College of Medicine
CT Image Quality
CT dose index (cGy) listed
1.5
kV CT
(Siemens Somatom Emotion® )
3.0
1.5
1.0
3.0
1.5
1.0
16.0
8.2
3.0
kV Cone Beam CT
(Siemens Artiste® )
MV CT
(TomoTherapy Hi-Art II® )
MV Cone Beam CT
(Siemens Artiste® ,
using treatment beam)
UNIVERSITY of IOWA Carver College of Medicine
Images shown with permission from Simeon Nill, DKFZ
MV CBCT and kV CBCT
Significant difference between kV-based CBCT
and MV-based CBCT
• Image Quality
MVCB Images tend to have poor high-Z (bony) contrast. This is due to
the primary interaction of the photon beam with matter as Compton
scatter.
On the other hand, MV images tend to have better soft-tissue contrast
than kVCB for the same reason.
In general MV imaging is a low-efficiency signal process, so images tend
to be “noisy”.
MVCB beams may be accurately modeled in the TPS and the dose
accounted for in treatment planning.
• Equipment and QA
kVCB imaging systems add a complete subsystem to the treatment
machine.
The kVCB has a separate, but hopefully coincident, isocenter from the
treatment machine.
Imaging Use in Radiation Therapy
3 Major Image Applications
Simulation & Planning
(CT, 4DCT, MRI,
PET, MRSI, fMRI,
US)
Localization/Verification
(CBCT, CT, MRI, CT,
US, IR, Surface, etc.)
Treatment/Response Monitoring
(PET, MRI, MRSI, fMRI, CT)
Med Accels Jan2013 Waldron D2 71
Imaging for Treatment Response and
Monitoring
Monitoring for changes to the patient that might influence dosimetric
objectives. These may be expected or unexpected:
Weight loss
Depth to target/SSD
Location of target
Shape of target
Some changes are fully expected responses to treatment, and typical
regimens have multiple prescriptions (boost) based on etiology and
expected tumor response.
Some changes can be observed by treatment personnel, but some can
only be observed with frequent radiographic imaging such as in-room CT.
Even then, this triggers a return to Imaging for Simulation and Treatment
Planning.
(Welcome back)
Imaging Use in Radiation Therapy
3 Major Image Applications
Simulation & Planning
(CT, 4DCT, MRI,
PET, MRSI, fMRI,
US)
Localization/Verification
(CBCT, CT, MRI, CT,
US, IR, Surface, etc.)
MOTION
MANAGEMENT?
Treatment/Response Monitoring
(PET, MRI, MRSI, fMRI, CT)
Med Accels Jan2013 Waldron D2 73