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Journal of Medical and Biological Engineering, 34(4): 377-385
377
Cone Beam Computed Tomography for Adaptive
Radiotherapy Treatment Planning
Kavitha Srinivasan1,*
Mohammad Mohammadi1,2
Justin Shepherd2
1
School of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005, Australia
Department of Medical Physics, Royal Adelaide Hospital, Adelaide, SA 5005, Australia
2
Received 21 Nov 2012; Accepted 23 Jan 2013; doi: 10.5405/jmbe.1372
Abstract
Cone beam computed tomography (CBCT) images obtained from linac-based kV imagers are typically used for
image-guided radiotherapy, in particular to perform three-dimensional image matching. CBCT image sets can also be
used for adaptive radiotherapy where the treatment plan is modified on the basis of periodic imaging throughout the
treatment course. CBCT images provide both anatomical information and Hounsfield unit (HU) values, which are
required for dose calculations. This study evaluates treatment plans based on CBCT datasets calibrated using the
Catphan 504 phantom to investigate the feasibility of using CBCT for adaptive replanning. The CBCT images were
acquired from a Varian On-Board Imager system. Conventional planning CT (PCT) images obtained from a Philips
Brilliance Big Bore CT scanner were used as reference images. The HU-density calibration curves of CBCT were
obtained using a Catphan 504 phantom and a CIRS density phantom and compared with the clinical PCT calibration
curve (obtained using the CIRS density phantom). Treatment plans created using the different calibration curves were
compared. Identical targets were delineated on CBCT and PCT images on four different-sized phantoms and planar
dose maps were generated. The dose-volume histograms of PCT- and CBCT-based plans were compared and evaluated
by gamma analysis. To extend the study to a typical clinical situation, two prostate cases were included. The dose
distribution comparison between PCT- and CBCT-based plans for patients yielded similar results to those obtained
using phantoms. The study also analyzed the effect of phantom dimensions on HU values and its impact on dose
calculations. The isodose distributions computed based on PCT and CBCT using the Catphan calibration curve agree to
within ± 1% compared to that based on CBCT using the density phantom calibration curve. However, for phantoms of
larger diameter, there is a pronounced discrepancy in the 50% and 60% isodose lines, with the dose difference being
about ± 3%. For phantoms whose thickness is less than the cone beam scan length (16 cm) and for phantoms whose
diameter is less than that of the calibration phantom, the variation in HU values is high. The effect of a change in radial
diameter has a larger impact on dose calculations. This study shows that the CIRS density phantom is not suitable for
CBCT calibration and that individual calibration curves obtained using phantoms of appropriate dimensions should be
used for planning individual treatment sites.
Keywords: Cone beam computed tomography (CBCT), Catphan, Adaptive radiotherapy (ART), Hounsfield unit (HU)
1. Introduction
Modern linear accelerators typically have a range of
imaging options for image-guided radiotherapy (IGRT).
Kilovoltage imagers for improved patient set-up are now
commonly used to perform cone beam computed tomography
(CBCT) scans for three-dimensional (3D) matching. Linear
accelerators with CBCT have been applied to adaptive
radiotherapy (ART), where the CBCT dataset is used to adapt
the patient treatment plan based on current patient anatomy.
* Corresponding author: Kavitha Srinivasan
Tel: +61-8-83138353; Fax: +61-8-83134380
E-mail: [email protected]
CBCT allows the monitoring of anatomical deformations
(weight loss, tumour regression) during the course of
radiotherapy treatment, making it a powerful tool for improving
patient positioning at the time of treatment and for improved
target localization throughout the treatment course [1,2]. Use of
CBCT for on-line IGRT applications has been shown to
improve the accuracy of radiotherapy treatment at various
treatment sites by determining and correcting patient set-up
errors down to sub-pixel accuracy [3]. In addition, CBCT
datasets can be used for treatment planning and dose
calculations [4] if the Hounsfield unit (HU) information
provided is reliable and accurate.
For accurate dose calculations, knowledge of the
relationship between HU and electron density is essential.
CBCT images include a larger amount of scatter compared to
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J. Med. Biol. Eng., Vol. 34 No. 4 2014
that in fan beam CT images [5,6], resulting in a larger variation
in HU values, which limits the HU calibration accuracy and
reliability [7,8]. Thus, to date treatment plans based on
planning CT (PCT) image sets (fan beam geometry) are still
superior to those based on CBCT image sets [9]. The
magnitude of the scattered radiation in CBCT depends on the
size of the scanned object [6,7,10] and hence soft tissue
contrast becomes dependent on object size. In addition, limited
gantry rotation speed and large field-of-view (FOV) in a single
rotation pose problems for image quality. Many methods have
been developed to improve the image quality of CBCT [11-14].
It was reported that the number of projections (400-700) from
cone beam geometry is insufficient for image reconstruction,
leading to variations in HU values [4]. Apart from the
geometrical limitations, the Feldkamp algorithm (FDK) used
for cone beam reconstruction has the limitation of
approximating the line integrals and hence the image quality
degrades as the plane moves away from the central axis [15].
This leads to image artifacts such as streaks, rings, cupping,
and beam hardening, which degrade image quality [10,15] and
reduce soft tissue contrast.
Despite these drawbacks, many studies have investigated
the use of CBCT datasets for dose calculations [4,7,10,16,17].
It was found that the difference in dose can vary from 3-4%
between CBCT- and PCT-based treatment plans using Catphan
calibration curves [7,10,16,17], with the largest difference
(14.5%) observed for head and neck patients [4]. For bone
inhomogeneities, the difference in dose goes up to 20% for two
different beam energies [16]. Other approaches for using CBCT
datasets generate specific HU-density tables [4]. HU values of
PCT have been mapped with CBCT geometry using pixel
correction strategies from look-up tables [8,11] or based on
rigid registration algorithms [14]. Several scatter correction
methods have been developed [18-24] in order to make CBCTbased dosimetry reliable.
CBCT datasets have also been applied to adaptive plans to
reduce the planning and target margins during the course of
treatment [25-28]. The first clinical results of ART applied to
prostate cancer showed that a CBCT-based adaptive plan found
to reduce the dose to the rectum significantly [25]. The
feasibility of CBCT ART for muscle-invasive bladder cancer
was determined and compared with conventional radiotherapy
[26]. It was found that the volume of irradiated normal tissue
was significantly smaller (29% less than conventional) without
reducing the clinical target volume. Hawkins et al. [27]
investigated the potential reduction in normal tissue irradiation
(lungs and heart) for treatment of esophageal cancer for the
adaptive plan. A novel re-optimization technique was
demonstrated for prostate IMRT plans and it was found that a
plan solution can be achieved within 2 min [29] and that it is
feasible to use CBCT for online ART [30].
The present study investigates the use of a Varian Onboard Imager (OBI) for ART. Phantoms with various
dimensions are used, and the variation in HU values along the
phantom Z axis is compared. Parameters that affect the stability
of the HU values and their impact on dose calculations are
investigated. The dosimetric accuracy of kV-CBCT-based dose
calculations is evaluated by extending the study to two clinical
situations. The relationship between HU values, scanned object
dimensions, and the dose is studied.
2. Materials and methods
2.1 Varian OBI
This study used the Varian OBI (Varian Medical Systems,
Palo Alto, CA, USA), which has an integrated linac at 90 °
with respect to the treatment beam. Three modes of imaging are
available in Varian OBI, namely kV radiography, CBCT, and
fluoroscopy. The OBI consists of a kV X-ray source (0.4- and
0.8-mm focal spots) and a kV amorphous silicon digital
imaging detector, which faces the X-ray source. For
reconstruction diameters of up to 24 cm, a full fan mode is
used, where the beam central axis passes through the detector
center in order to acquire full projections of the entire object in
a single rotation. A total of 360 projections during the 200°
gantry rotation are obtained in full fan acquisition. Imaging in
full fan mode uses 100 kVp, 20 mA, and 20 ms. For larger
reconstruction diameters (up to 45 cm), the CBCT acquisition
is switched to half fan mode, in which the detector is offset by
14.8 cm to acquire 655 projections during 360 ° gantry rotation.
In this mode, one half of the projections are obtained from one
half fan projections and the other half is obtained from the
other half fan projections from the other direction. Imaging in
half fan mode uses 125 kVp, 80 mA, and 13 ms. A bow-tie
filter is added to the kV source while scanning. The CBCT
version used is 2.1 and the OBI version is 1.5.
2.2 Planning CT
A Philips Brilliance Big Bore CT scanner (Philips Medical
Systems, Cleveland, OH, USA) version 2.2 was used in this
study to acquire reference CT images. It is a 16-slice helical
scanner that has an 85-cm-diameter bore and modulates the
tube current during the scan based on patient anatomy. Table 1
shows the scan protocols for CBCT and PCT for the head and
body scans used here. The slice thickness can be selected based
on the study and depends on which collimation aperture and
resolution mode are chosen. There are several filters, including
standard, high resolution, and ultra-high resolution, available
for each resolution mode to optimize image quality. Depending
on the FOV and resolution mode, the pixel resolution is
Table 1. Image acquisition parameters for CBCT and PCT for head and
body scans.
Parameter
Scan mode
Bow-tie filter
X-ray voltage (kVp)
X-ray current (mA)
Exposure time* (ms)
SID (cm)
Number of projections
Detector configuration (cm2)
Field of view (cm2)
CBCT
Head
Body
Full
Half
100
125
20
80
20
13
150
150
360
655
30 × 40 30 × 40
25 × 25 45 × 45
PCT
Head
Body
--90
120
250
133
800
750
16 × 1.5 16 × 1.5
25 × 25 35 × 35
* Exposure time for CBCT is per projection and that for PCT is per rotation
CBCT-Based Treatment Planning
determined by choosing the size of the matrix, which can be
512 × 512, 768 × 768, or 1024 × 1024.
2.3 Treatment planning system
The treatment planning system (TPS) used was Pinnacle³
(ADAC Laboratories, Milpitas, CA, USA) version v.8.0m.
Collapsed cone convolution (CCC) is the model-based
algorithm [31] used in this study for dose calculations. It has
built-in tissue heterogeneity corrections, handles 3D anatomical
information, and allows accurate dose calculations in
heterogeneous conditions.
2.4 Phantoms and Patients
The Catphan 504 phantom (The Phantom Laboratory, NY,
USA) uses tissue substitute materials and is cylindrical, with a
20-cm diameter and a 20-cm thickness. It has several modules.
The module used in this study (CTP404) contains inserts of
different densities. The CIRS density phantom model 062
(CIRS Tissue Simulation Technology, Norfolk, VA, USA) uses
water and tissue equivalent epoxy materials. It is a
homogeneous ellipsoidal phantom (33 cm × 5 cm × 27 cm)
with 17 small cylindrical parts that can be separated out. The
dense bone insert in this phantom is represented by
hydroxyapatite, whose mineral composition is similar to that of
animal bones. It has a removable central circular section that
enables this phantom to be used for both the head and the
abdomen. Other homogeneous phantoms were also studied to
evaluate HU uniformity: a water phantom (8 cm thick and 25
cm in diameter), a Varian Norm phantom for head scans (made
of polyethylene 27 cm thick and 25 cm in diameter) and a
Varian Norm phantom for body scans (made of polyurethane 28
cm thick and 45 cm in diameter).
Two prostate patients were selected for evaluating CBCTbased dose calculations. Both patients had similar dimensions,
with an anterior-posterior separation of 21 cm and a lateral
separation of 34 cm.
2.5 Image acquisition
Initially images of the above-mentioned phantoms were
acquired from two CT volume imaging systems: the PCT as a
reference and the CBCT (from Varian OBI). Both systems
were operated under similar scanning conditions (tube voltage,
tube current, and phantom positioning). Once the images were
acquired, they were imported into Pinnacle³ TPS for HUdensity calibration.
2.6 HU calibration
Dose distributions were calculated based on the density
distribution of the irradiated tissue. Since a CT scanner
measures attenuation coefficient values and hence HU values, a
calibration function must be derived to determine the physical
densities. In order to obtain a calibration curve for a CT scanner,
the CIRS density phantom, which is specifically manufactured
to obtain a precise relationship between CT numbers and
electron densities, was used. Since the volumetric imaging of
CBCT includes large fields of view, the use of the thin CIRS
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density phantom for CBCT calibration leads to more scatter
contribution compared to PCT. Hence, this study uses both the
density phantom and the Catphan for CBCT calibration and
compares these with the PCT calibration to evaluate a suitable
CBCT calibration curve for treatment planning.
Central slices from the density phantom and from the CTP
404 module of Catphan were selected in Pinnacle3 and the
circular regions of interests were drawn to measure the average
HU values of each tissue insert. Once the HU values were
measured, they were plotted against the known physical density
of the tissue inserts for both PCT and CBCT data. Thus, the HU
values in each voxel of the reconstructed image were calibrated
to the relative physical density for tissue inhomogeneities.
2.7 Dose calculations and evaluations
A planning target volume (PTV) of same size (3cm
diameter) and shape was contoured in both PCT and CBCT
datasets to ensure similar planning conditions for comparison
purposes. The PTV for the Catphan, water, and Norm body
phantoms (polyurethane) were chosen to be at the center of the
phantoms. For the density phantom, the PTV is the breast
equivalent insert material (physical density = 0.99 g/cc) at the
periphery of the phantom. To calculate the dose distributions in
the region of interest, a single 6-MV photon beam with a field
size of 10 × 10 cm2 was generated using the source to axis
distance (SAD) technique for PCT and CBCT data. The
resultant dose distributions and the plot of cumulative dosevolume histograms (DVHs) that summarizes the radiation
distribution within the volume of interest were obtained and
compared.
Planar dose distributions of various phantoms obtained
from Pinnacle³ were imported into MapCHECK™ (SUN
NUCLEAR Corporation, Melbourne, FL, USA) in order to
compare and analyze the PCT and CBCT dose distributions
quantitatively. The comparisons were made using a density
phantom calibration curve (curve 1) and a Catphan calibration
curve (curve 2). The gamma analysis method [32] with a dose
difference threshold of 3% and distance-to-agreement (DTA)
criterion of 3 mm was used to compare the CBCT dose
distributions with that obtained using PCT.
To evaluate CBCT-based dose calculation, two prostate
patients were recruited. The patients were CT-scanned and the
images were transferred to the Pinnacle3 TPS. The target region
included the prostate gland with a margin of 6 mm. Femoral
heads were included as organs at risk during planning. Static
plans using five 6-MV photon beams of equal weighting with
gantry angles of 0 °, 55 °, 130 °, 230 °, and 310 ° were adopted
for the two cases. CBCT images were acquired with the patient
in the treatment position. The images were saved to the record
and verify (R&V) system and then analyzed. The PCT- and
CBCT- based treatment plans were compared under conditions
similar to those for the phantoms.
The variation in HU values along the Z axis (thickness of
the phantom) is of more concern when dose delivery is based
on 3D dose distributions. Therefore, Z profiles of HU were
studied for three different phantoms. The HU profiles along the
thickness of the homogeneous phantoms were extracted using
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J. Med. Biol. Eng., Vol. 34. No. 4 2014
ImageJ software (National Institutes of Health) from PCT and
CBCT data and compared. The variation in HU values through
the slices of the phantom was determined and the difference in
percentage HU variation as a function of phantom dimensions
(thickness and radial diameter) was examined.
that of the calibration phantom, the HU variation is high even
for small differences.
3. Results
3.1 HU calibration
The relationship between HU values and physical density
was established for PCT and CBCT using the density phantom
and the Catphan. Routine planning was performed using a CT
calibration curve obtained using the CIRS density phantom (i.e.,
the reference curve). The calibration curve for PCT and CBCT
obtained using the density phantom (curve 1) and for CBCT
obtained using the Catphan (curve 2) is shown in Fig. 1. Error
bars in HU are the standard deviation of the values within the
inserts of the phantom. The results show that curve 2 is closer
to the PCT curve, with agreement in HU values for densities up
to 1 g/cc, whereas curve 1 has much reduced values of HU,
agreeing only up to 0.5 g/cc.
Figure 2. Scan along the thickness of phantom (Z axis).
(a)
(b)
(c)
Figure 3. HU profiles along the thickness of (a) water phantom, (b)
Norm head phantom, and (c) Norm body phantom
superimposed on their corresponding XZ orthogonal views.
Figure 1. Calibration curves for PCT and CBCT.
3.2 HU profiles and dependence on object size
The HU profiles of different-sized homogeneous
phantoms were measured along the thickness of the phantoms
(Z axis) (Fig. 2) and compared in order to verify image
uniformity. Figure 3 shows the HU profiles of the water
phantom (8.8 cm thick), Norm head phantom (15.8 cm thick),
and Norm body phantom (28 cm thick). The HU profile of the
water phantom exhibits large fluctuation with reduced HU
values along the periphery. With an increase in the thickness of
the phantoms (Norm head and body phantoms), the HU
fluctuation is significantly reduced (Figs. 3(b) and 3(c)),
leading to flatter profiles.
The HU profiles were dependent on scanned object size
and showed minimum percentage of variation for scanned
phantoms whose dimensions are larger compared to the
calibration phantom. Figure 4 shows that when the thickness of
the object is less than the scanning length (standard cone beam
scan length = 16 cm), the scatter contribution leads to high HU
variation. Similarly, when the diameter of the object is less than
Figure 4. HU dependence on phantom dimensions, along thickness (L)
and along Radial diameter (D) of phantom.
CBCT-Based Treatment Planning
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3.3 Dose evaluations - DVHs and gamma maps
The isodose lines were calculated using a single 6-MV
10 × 10 cm2 photon beam. The dose distributions in the axial
slice of the density phantom, water phantom, Catphan, and
Norm body phantom obtained when irradiating an identical
spherical target with a 3-cm diameter and of similar volume
delineated in PCT and CBCT images are shown in Fig. 5. The
dosimetry results were evaluated qualitatively for each study
using DVHs, which are shown in Fig. 6. The results show that
the dose distribution obtained from CBCT using the Catphan
calibration curve (curve 2) agrees well with that of PCT
compared to using density phantom calibration curve (curve 1).
To further evaluate the results quantitatively, the planar dose
distributions in each phantom obtained from PCT and the
corresponding slice from CBCT were extracted and compared
by gamma analysis. Gamma analysis with the routine clinical
threshold levels of 3% and 3 mm was utilized using
MapCHECK™. The results are shown in Fig. 7. In the 4window comparison panel of MapCHECK™ software, the dose
distributions obtained from PCT and CBCT are in the top two
panels. In the lower-left panel, the comparison dose map
(a)
(b)
(c)
(a1)
(a2)
(b1)
(b2)
(d)
Figure 6. DVH comparison of PTV for (a) density phantom, (b) water
phantom, (c) Catphan phantom, and (d) Norm body phantom
obtained with PCT and CBCT using density phantom
calibration curve (curve 1) and Catphan calibration curve
(curve 2).
(c1)
(c2)
(d1)
(d2)
Figure 5. Dose distributions on an axial slice of (a) density phantom, (b)
water phantom, (c) Catphan phantom, and (d) Norm body
phantoms acquired using PCT (a1, b1, c1 and d1) and CBCT
(a2, b2, c2 and d2).
(PCT-CBCT) based on gamma analysis indicates the measured
dose points that failed the criteria in red and blue for values
above and below the criteria, respectively. In the lower-right
panel, depth-dose curves along the high-dose profile region
between the compared sets are compared. The gamma analysis
comparison shows a dose discrepancy within ± 1% using the
Catphan calibration curve (curve 2). The same procedure was
adopted for the other four phantoms. The overall results show
that curve 2 is in better agreement with PCT. For the Norm
body, there is a shift in the isodose lines that causes variations
in dose of up to 5% (figure not shown). However, the dose
discrepancy is reduced to within 3% (Fig. 7(c)) by using the
Catphan calibration curve.
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Figure 8 shows PCT and CBCT axial slices of the two
prostate studies with isodose curves embedded on it. A
comparison of DVHs of the prostate and femoral heads is
presented in Fig. 9. The gamma analysis with 3%/3mm criteria
for both the patients (Figs. 10(a) and 10(b)) show 100% pass
when the Catphan calibration curve was used, with dose
differences within ± 1% . The dose agreement is still clinically
acceptable (98% pass) even with 2%/2.5 mm criteria.
(a)
(b)
(a1)
(a2)
(b1)
(b2)
Figure 8. Dose distributions superimposed on PCT and CBCT axial
slices of patient 1 (a1 and a2) and patient 2 (b1 and b2) for
prostate cases using five photon beams. The blue and pink
shaded regions are the PTVs and the organ at risk (OAR) is
contoured in orange and purple in patient 1 and 2,
respectively.
(c)
(a)
(d)
(b)
Figure 7. Dose distribution, gamma map, and dose profile comparisons
for (a) density phantom, (b) Catphan phantom, (c) Norm body
phantom, and (d) water phantom obtained using Catphan
calibration curve.
Figure 9. DVH comparison of PTV and femoral heads for two prostate
patients (a and b) obtained with PCT and CBCT using density
phantom calibration curve (curve 1) and Catphan calibration
curve (curve 2).
CBCT-Based Treatment Planning
(a)
(b)
Figure 10. 4-panel dose distribution comparison window from
MapCHECK software. The upper two windows show the
dose distribution charts from PCT and CBCT, the lower-left
window shows the gamma comparison map, and the
lower-right window shows the dose profile comparisons for
(a) patient 1 and (b) patient 2.
4. Discussion
In this study, CBCT HU calibration curves were obtained
using the CIRS density phantom (curve 1) and the Catphan 504
phantom (curve 2) and compared with the PCT calibration curve.
The CBCT density phantom calibration curve (curve 1) was
found to diverge from the corresponding PCT curve, showing
reduced values of HU due to a large scatter contribution from
CBCT. The density phantom, which is designed for fan beam
CT, is very thin (5 cm) compared to the large FOV of CBCT.
This causes a greater amount of scattered radiation to reach the
OBI flat panel detector. As a result, the HU values are reduced
and the noise level is increased, degrading image quality.
Comparing the PCT and CBCT calibration curves derived from
the density phantom (curve 1), the HU values agree up to
~0.5 g/cc. The deviation in HU linearity for curve 1 is greater
than that for curve 2. Since the HU calibration on the Varian
OBI was performed using the Catphan phantom, the CBCT
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calibration curve obtained using Catphan (curve 2) agrees well
with the PCT curve for the low-density region up to 1 g/cc
(water), after which the values gradually reduce and diverge
from the PCT curve. This reduction in HU values is due to the
contribution of scattered radiation around high-density materials.
However, the variations in HU values are within the typical HU
value tolerance of ± 40. Thus, curve 2 is more reliable than
curve 1 for treatment planning based on CBCT datasets.
The variation in HU values was determined as a function of
thickness (in Z direction) and the diameter of the scanned object
to study the impact of phantom (scanned object) dimensions on
HU. Figure 3(a) shows the HU profile along the thickness of the
water phantom. There is a large variation in HU values, with
higher values at the center and lower values towards the
periphery. This difference in HU values is suggestive of the
“capping” artifact caused due to overcorrection for beam
hardening effects. This artifact reduces the reconstructed image
quality, leading to poor low-contrast resolution. When the
thickness of the phantom is increased (Figs. 3(b) and 3(c)), the
HU profiles become flatter.
When the thickness of the scanned object is less than that
of the calibration phantom (Catphan length = 20 cm), there is
about 8-10% variation in HU (Fig. 4). However, this variation
has little impact on the resulting dose calculation (as was the
case for the water phantom with a dose discrepancy within 1%).
When the diameter of the scanned object is larger than that of
calibration phantom, although HU values are stable (Fig. 3(c)),
there is a maximum dose discrepancy of 3% (as was the case
for the Norm body phantom). Thus, the diameter of the object
has a larger impact on dose calculations. This shows that HU
values depend on phantom geometry and hence appropriate
HU-density curves are required for specific anatomical sites.
Different phantoms with the size of the scanned object
(anatomical site) are required in order to use different
calibration curves for different clinical studies.
The Catphan calibration curve (curve 2) was used for
several phantom studies to investigate various situations that
limit the use of CBCT datasets in clinical treatment planning. It
is worth mentioning that the HU calibration curves for PCT and
CBCT were identical in previous studies [7,10,17], which
indicates the possibility that higher-density materials (scatter
component) were not included in the calibration. In clinical
cases, the calibration curve is only useful when inhomogeneities
are included. The dose calculated using CBCT curve 1 and
curve 2 calibration curves was evaluated using gamma analysis
and compared with PCT datasets to study the status of Varian
OBI CBCT for treatment planning without correcting for HU
values. The doses agreed well (within ± 1%) with that of PCT if
the CBCT Catphan calibration curve (curve 2) was utilized for
dose computations. However, there was a maximum dose
discrepancy of 3% (Fig. 7(c)) on the Norm body phantom with a
diameter twice as large as that of the calibration phantom. The
reason is, in order to cover large FOVs, half fan acquisition
mode was adopted. In this mode, the detector is offset to the
center of rotation to cover the large volume. As a consequence,
the central part of the phantom is scanned over a complete 360 °
rotation while the peripheral parts are scanned only over a 180 °
J. Med. Biol. Eng., Vol. 34 No. 4 2014
384
half-rotation. This resulted in an artifact in the images of the
Norm body phantom (Fig. 3(c)), leading to underestimation of
the overall HU values during cone beam reconstruction. This
relative variation in HU values when compared to those
obtained with PCT causes a shift in the isodose curves, resulting
in pronounced variation (± 3%) in the dose distributions when
compared to those obtained with PCT.
In the patient studies, the resultant dosimetric deviations
were found to be ± 5% in the high-gradient dose region for
both the cases when the density phantom calibration curve
(curve 1) was used. The discrepancy was reduced by using the
Catphan calibration curve (curve 2). This further confirms that
the use of suitable calibration curves results in better dose
agreement. Thus, for major clinical treatment sites such as the
prostate, lung, and head and neck, at least three different CBCT
calibrations curves based on dimensions of that region are
required for planning the corresponding anatomical site.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
5. Conclusion
[13]
In order to use CBCT image sets for dose calculations,
reliable HU-density calibration is required. This study showed
that the density phantom used for PCT calibration is not
appropriate for CBCT dose calculations. Treatment plans
created with CBCT images using calibration curves derived
from the Catphan (curve 2) agreed well with PCT plans.
However the dose distributions of plans created from CBCT
images were found to depend on the calibration phantom
dimensions. This indicates the need for calibration phantoms
that match the size of the treatment site to be imaged (e.g.,
prostate). When CBCT HU calibration curves that are
appropriate for a particular treatment site are used for planning,
CBCT can potentially be used for ART.
Acknowledgements
This work was supported by the University of Adelaide
funded by the Australian Postgraduate Award and by Royal
Adelaide Hospital.
[14]
[15]
[16]
[17]
[18]
[19]
[20]
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