The effect and stability of MVCT images on adaptive TomoTherapy

Article (PDF Available)inJournal of Applied Clinical Medical Physics 11(4):3229 · July 2010with51 Reads
Source: PubMed
Abstract
Use of helical TomoTherapy-based MVCT imaging for adaptive planning is becoming increasingly popular. Treatment planning and dose calculations based on MVCT require an image value to electron density calibration to remain stable over the course of treatment time. In this work, we have studied the dosimetric impact on TomoTherapy treatment plans due to variation in image value to density table (IVDT) curve as a function of target degradation. We also have investigated the reproducibility and stability of the TomoTherapy MVCT image quality over time. Multiple scans of the TomoTherapy "Cheese" phantom were performed over a period of five months. Over this period, a difference of 4.7% in the HU values was observed in high-density regions while there was no significant variation in the image values for the low densities of the IVDT curve. Changes in the IVDT curves before and after target replacement were measured. Two clinical treatment sites, pelvis and prostate, were selected to study the dosimetric impact of this variation. Dose was recalculated on the MVCTs with the planned fluence using IVDT curves acquired before and after target change. For the cases studied, target replacement resulted in an overall difference of less than 5%, which can be significant for hypo-fractionated cases. Hence, it is recommended to measure the IVDT curves on a monthly basis and after any major repairs/replacements.
The effect and stability of MVCT images on adaptive
TomoTherapy
Poonam Yadav
1,2,4
Ranjini Tolakanahalli
1,2
Yi Rong
1,3
Bhudatt R Paliwal
1,2
Department of Human Oncology,
1
University of Wisconsin - Madison, Madison, WI;
Department of Medical Physics,
2
University of Wisconsin - Madison, Madison, WI;
University of Wisconsin Riverview Cancer Centre,
3
Wisconsin Rapids, WI; Vellore
Institute of Technology University,
4
Vellore, Tamil Nadu, India.
yadav@humonc.wisc.edu
Received 1 December, 2009; accepted 5 May, 2010
Use of helical TomoTherapy-based MVCT imaging for adaptive planning is
becoming increasingly popular. Treatment planning and dose calculations based
on MVCT require an image value to electron density calibration to remain stable
over the course of treatment time. In this work, we have studied the dosimetric
impact on TomoTherapy treatment plans due to variation in image value to density
table (IVDT) curve as a function of target degradation. We also have investigated
the reproducibility and stability of the TomoTherapy MVCT image quality over
time. Multiple scans of the TomoTherapy “Cheese” phantom were performed over
a period of ve months. Over this period, a difference of 4.7% in the HU values
was observed in high-density regions while there was no signicant variation in the
image values for the low densities of the IVDT curve. Changes in the IVDT curves
before and after target replacement were measured. Two clinical treatment sites,
pelvis and prostate, were selected to study the dosimetric impact of this variation.
Dose was recalculated on the MVCTs with the planned uence using IVDT curves
acquired before and after target change. For the cases studied, target replacement
resulted in an overall difference of less than 5%, which can be signicant for hypo-
fractionated cases. Hence, it is recommended to measure the IVDT curves on a
monthly basis and after any major repairs/replacements.
PACS numbers: 87.55.Qr, 87.56.bd, 87.57.C, 87.57.Q
Key words: TomoTherapy, IVDT, target degradation, MVCT
I. INTRODUCTION
It is more than two decades since 3D conformal radiotherapy that a forward planning meth-
odology was introduced in all modern radiotherapy departments. In 1982, Anders Brahme
(1)
for the rst time proved analytically a possibility of inverse planning to maximize dose to a
target while minimizing dose to the neighboring critical structures. Based on this concept,
most of the radiotherapy departments practice intensity-modulated radiotherapy (IMRT) for
better target coverage and tumor control. Requirements of quality assurance (QA) for IMRT
are very stringent. As a result, most radiation therapy machines are equipped with online
imaging for image-guided radiotherapy (IGRT) and adaptive radiotherapy (ART). ART is a
radiation treatment process where the subsequent delivery can be modied using a feedback
of the geometric and dosimetric information from previously treated fractions. It is a multistep
a
Corresponding author: Poonam Yadav, Department of Human Oncology and Department of Medical
Physics, University of Wisconsin, 600 Highland Avenue, Madison, WI 53792, USA; phone 608-263-5401;
fax:608-263-9167; email: yadav@humonc.wisc.edu
JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 11, NUMBER 4, FALL 2010
4 4
5 Yadav et al.: MVCT images and adaptive TomoTherapy 5
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
process involving dose reconstruction,
(2)
dose accumulation,
(3)
treatment evaluation, recon-
touring and reoptimization.
(4)
ART uses the MVCT data acquired at the time of treatment, as
well as information obtained during delivery verication. It is important that the information
from MVCT be directly proportional to the photon attenuation in order to use the images for
treatment planning. It has been shown that MVCT images can be used for dose calculation,
the accuracy of which is similar or even superior to that of the initial dose calculation based on
kVCT images.
(5)
For accuracy of dose calculation, reproducibility and stability of image value
to density data are highly important.
The performance of the IGRT systems has been studied extensively in recent years. Per-
formance characteristics of MVCT on Hi·Art TomoTherapy system
(6,7)
have been reported.
These reports include image noise and uniformity, spatial resolution, contrast properties, and
multiple scan average dose from multiple scans. It has been shown that for equivalent doses,
MVCT has equivalent spatial resolution and noise characteristics as that of kVCT images.
However, kVCT systems outperform MVCT in terms of low contrast visibility. To date, there
have not been many studies that track these parameters over time. Recently, Stock et al.
(8)
have
investigated and compared only image quality parameters for multislice CT, linac based cone-
beam CT (CBCT) and simulator over a period of 16 months. The image parameters studied
were noise, spatial resolution, low contrast visibility (LCV) and uniformity. They reported no
signicant trend in any of the three devices in their study. Langen et al.
(9)
tested the stability of
the MVCT numbers by determining the variation of this calibration with spatial arrangement of
the phantom and MVCT acquisition parameters. They found that the largest difference in any
of the dosimetric endpoints was 3.1% but, more typically, the dosimetric endpoints varied by
less than 2%. They also investigated the variation in the MVCT numbers over a period of nine
months, but have not reported dosimetric effects of the IVDT variation over time.
Helical TomoTherapy MVCT uses slip rings to allow delivery of continuous radiation from
the rotating gantry. During MVCT imaging, the nominal energy of the incident electron beam
is reduced from 6 to 3.5 MeV and the eld width (IEC-Y) of 4 mm is used for the MVCT
beam during image acquisition.
(10)
As a standard, helical TomoTherapy MVCT has been used
routinely for daily patient treatments setup.
(6,9,11)
The incorporation of daily images into the
radiotherapy process is essential for ART.
(12)
On the Hi·Art system, due to target degradation, X-ray target replacement is required ap-
proximately every 10–12 months. Over time, the target thins initially causing the beam to be
more forward peaked and, towards the end of target life, the target thinning causes a decrease
in the beam energy and a softening of the beam prole at the lateral edges of the beam. Sta-
ton et al.
(13)
studied the effects of target degradation on IMRT delivery. They have reported
dosimetric differences of about 4% towards the end of the target life. For dosimetry purpose,
the MVCT numbers, or pixel values, need to be converted to electron densities using a CT to
electron density calibration curve.
(14)
While the establishment of a CT to electron density curve
is straightforward with an appropriate phantom, the variation of this calibration with time needs
to be investigated since it impacts dosimetric uncertainties. The overall purpose of this study
is to evaluate the dosimetric effects of variation in image value to density table (IVDT) due to
target degradation. We have tabulated the variation of IVDT over a period of ve months and
studied its dosimetric impact on treatment plans for two clinical sites: prostate and pelvic. We
also have included in this study the change in IVDT due to relocation of the machine after the
relocation beam was matched to the “gold standard”. A similar process is followed after every
target replacement. The dosimetric effect of the above actions on clinical cases was studied. In
addition, we also present the reproducibility and stability of the TomoTherapy MVCT imaging
system with respect to noise and contrast to noise ratio (CNR).
6 Yadav et al.: MVCT images and adaptive TomoTherapy 6
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
II. MATERIALS AND METHODS
A. Stability of image value to density table
MVCTs potentially provide greater accuracy for radiation therapy dose calculations and inho-
mogeneity corrections.
(5)
The stability of the CT to electron density calibration is an indicator
of the CT number integrity and a prerequisite for dose recalculation.
To establish CT to electron density calibrations, TomoTherapy “Cheese” phantom (Gammex
RMI, Middelton, WI) was used. The phantom is an 18 cm thick solid water cylinder with a
diameter of 30 cm. The phantom consists of two semi-cylindrical halves, in between which a
lm can be placed. There are 20 plugs, including four solid water plugs plus 16 tissue substitute
plugs that range in electron density relative to water from 0.29 g/cm
3
to 4.59 g/cm
3
. These can
be inserted into 28 mm diameter holes in the “Cheese” phantom. The inserted plugs as shown
in Fig. 1 are representative of the range of inhomogeneities observed in the clinical environ-
ment. The “Cheese” phantom was scanned multiple times over a period of ve months which
included an X-ray target change. Hence, the data provided compare the performance of MVCT
towards the end of the target life to that taken after a target replacement. The MVCT images
acquired were then imported to the Pinnacle
3
8.1 treatment planning system (TPS) (Philips
Medical Systems, Fitchburg, WI) to measure image values. Regions of interest with diameter
of 20 mm were contoured at the center of each of the phantom plugs and the mean HU values
within the contours were recorded. The electron densities of each phantom plug were recorded
from the manufacturer specications, and the physical density corresponding to the mean CT
values recorded was plotted as the IVDT curve.
A potential change in IVDT curve is due to changes in beam characteristics caused by X-ray
target wear. A Hi·ART TomoTherapy system requires X-ray target replacement approximately
every 10–12 months. The IVDT curve was recorded for a period of ve months over the life of
the target. Near the end of target life, the target thins resulting in beam softening and changes
in the beam prole at the lateral edges of the beam of up to 6%, as shown by Langen et al.
(9)
To
Fi g . 1. Picture of “Cheese” phantom with plugs of known electron densities inserted into the built-in twenty cavities.
7 Yadav et al.: MVCT images and adaptive TomoTherapy 7
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
study the changes in the IVDT curve with respect to the target change, the “Cheese” phantom
was scanned before and after the target change.
A system move was also performed during the ve months period. The system was recom-
missioned to match the “gold model” treatment beam. The system experienced a table top/
couch upgrade and a corresponding software upgrade, which accounts for the changes in the
couch. However, no other major parts in the beam line were replaced. Changes in the IVDT
curve before and after the system move were also tabulated.
B. Dose recalculation
The effect of IVDT curve variability was studied on two clinical cases: prostate and pelvic.
The treatment plan parameters (prescription, number of fractions, eld width, pitch, modula-
tion factor and sinogram) for each plan are shown in Table 1. Each case had one kVCT image
(called reference image) that was used as a planning CT. MVCT images (20–40) were acquired
for daily registration purposes. A very low dose (approximately 2 cGy) was delivered for each
MVCT dataset. The MVCT was acquired in the normal mode with a slice thickness of 4 mm.
A treatment plan with appropriate tumor coverage and minimum dose to sensitive structures
was planned on kVCT. The dose distribution based on the daily MVCT images was calcu-
lated using Planned Adaptive module on TomoTherapy Planning Station with different IVDT
curves. The MVCT of the patient acquired just prior to treatment was used for recalculation.
The MVCT images were aligned with the kVCT images using the rigid-body alignment tool
in the TomoTherapy image registration panel.
(15)
Then, the intended uence pattern was used
to recalculate the dose distribution on the MVCT image set using the corresponding IVDT
curves. Dose recomputations for each case were recalculated for the acquired IVDT curves:
prior to target change (end of target life), post target change, before system move, after system
move and recommissioning.
Ta b l e 1. Treatment-planning parameters: prescription, number of fraction, eld width, pitch and planning modulation
factor, and sinogram segments for prostate and pelvis.
Number of Field Width
Planning
Sinogram
Site Prescription
Fractions (cm)
Pitch Modulation
Segments
Factor
Prostate
70 Gy to 99%
of the PTV
28 2.5 0.43 2 1.7
Pelvic
45 Gy to 99%
of the PTV
25 2.5 0.29 2 5.6
The largest variation in the MVCT calibration curve was found for the high-density values
and this is demonstrated more in the results section. The existence of this effect prompted us
to select the PTV in the pelvis case proximal to the pelvic bones. Changes in the dose volume
histograms (DVHs) and the dose distribution due to changes in IVDT were used to compare the
recalculated plans. DVH points such as D
99
(dose to 99% of target volume), D
5
(dose to 5% of
target volume) for the planning target volume (PTV), and D
50
(dose to 50% of target volume)
and D
5
for the critical structures were calculated for all plans. In addition, target coverage (TC)
and Homogeneity Index (HI) for the PTVs were calculated as explained below.
8 Yadav et al.: MVCT images and adaptive TomoTherapy 8
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
Target Coverage: TC describes the fraction of the target volume receiving at least the
prescription dose and is dened as:
(1)
For perfect coverage, TC equals 1.0.
Homogeneity Index: Dose homogeneity in the target volumes was quantied by the HI, as
recommended by the International Commission on Radiation Units and Measurements. The
HI is dened as the greatest dose delivered to 2% of the target volume (D
2
) minus the dose
delivered to 98% of the target volume (D
98
) divided by the median dose (D
median
) of the target
volume:
(16)
(2)
Smaller values of HI correspond to more homogenous irradiation of the target volume. A
value of zero corresponds to absolute homogeneity of dose within the target.
C. MVCT image performance characterization
We retrospectively studied the MVCT images (Fig. 2) acquired daily with a TomoTherapy
Hi·ART II machine (TomoTherapy Inc. Madison, WI) over a period of ve months. The study
included a target change and a system move. MVCT image performance was characterized
Fi g . 2. MVCT image of QC-3 phantom.
9 Yadav et al.: MVCT images and adaptive TomoTherapy 9
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
using PIPSpro QC-3 phantom (Standard Imaging, Madison, WI). The QC3 phantom (Fig. 3)
consists of ve sets (1–5) of high-contrast rectangular bars with spatial frequencies of 0.1,
0.2, 0.25, 0.45 and 0.76 lp/mm and bar thickness 15 mm. The frame of the phantom is made
of aluminum, and the ve test sections are made of lead and Delrin (Acetal) plastic (density
1.42 g/cm
3
). The phantom is 15 mm thin and has 3 mm acrylic and 2 mm aluminum cover
plates on the top and bottom, respectively.
The QC-3 phantom can be used to perform the following tests: relative modulation transfer
function (RMTF), spatial resolution, signal to noise ratio, and contrast to noise ratio (CNR).
The use of QC-3 phantom to determine the quality assurance of the electric portal imaging
device (EPID) using the software PIPSpro, which provides the frequencies at 50%, 40% and
30% (f
50
, f
40
and f
30
) of the maximum RMTF, has been extensively studied in literature.
(17,18)
We have used the software to study the consistency of noise and CNR in the MVCT images
acquired using TomoTherapy.
Noise, denoted as σ, is the random image noise calculated as the average standard deviation
of pixel values from six different regions.
The contrast to noise ratio (CNR) as calculated by the PIPSpro software is dened as:
CNR= (P
b
-P
d
)/σ (3)
where P
b
and P
d
are the average pixel values of the brightest and darkest region of interest
obtained from the uniform regions of the phantom respectively.
Fi g . 3. QC-3 phantom with frame made up of aluminum and the sections made of lead and plastic.
10 Yadav et al.: MVCT images and adaptive TomoTherapy 10
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
III. RESULTS
A. Stability of IVDT
The MVCT to physical density curve was established from axial scans of the “Cheese” phantom.
It was noted that over a period of ve months, there was no signicant variation for the lower
densities in the calibration curve. The normalized percent variation in HU for the low-density
plugs (less than 0.95 g/cc) was 0.72%. However, a discrepancy of 4.7% was observed in the
curve at densities of 1.561 g/cm
3
and 1.824 g/cm
3
or higher. The effect of a target change and
TomoTherapy machine move in the IVDT curve with respect to the IVDT curve plotted by
taking an average of IVDT curve over ve months is shown in Fig. 4. As there were no major
upgrades in the beamline components, there was no noticeable change in the IVDT curve system
move. However, a slight discrepancy for high density values is evident.
B. Dosimetric impact of variation in IVDT
The DVHs for the recalculated plans using IVDT curve before and after target change were
analyzed and compared. These are illustrated in Figs. 5 and 6, respectively. A summary of the
differences for the selected DVH points (D
50
, D
5
) for OARs is shown in Table 2 for the prostate
and pelvic cases, respectively. In Table 3, pertinent dosimetric end points (HI, TC, D
99
and D
2
)
for the PTV are presented. The selected DVH points for PTVs and OARs were higher in all of
the plans when calculated with IVDT curve obtained after the target change. A dose difference
of 3% in D
99
and 2% in D
2
for the PTV was noted in the case of prostate. For the pelvic case,
the results show that D
99
and D
5
for the PTV were 0.6% and 3% higher when curve for IVDT
post-target change was used. The largest effect of the target degradation on recalculated plans
was seen for the pelvis plan where PTV is in close proximity to the femoral heads.
The target coverage using IVDT after target change in the case of prostate was approximately
2.5% higher than that calculated using pre-target change IVDT. A difference of 1% was observed
in target coverage for the pelvic case. The mean homogeneity index was not signicantly
different in either of the cases under study.
Fi g . 4. IVDT curve for average CT number taken over ve months, after target changed and TomoTherapy machine moved.
11 Yadav et al.: MVCT images and adaptive TomoTherapy 11
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
Fi g . 5. Comparison of prostate dose volume histogram before and after target change.
Fi g . 6. Comparison of pelvic dose volume histogram before and after target change.
12 Yadav et al.: MVCT images and adaptive TomoTherapy 12
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
Ta b l e 2. Dose received by 50% (D50) and 5% (D5) of organ at risk, before and after target change for prostate
and pelvic.
Organ at Risk
Before Target Change After Target Change % Difference
D
50
(Gy) D
5
(Gy) D
50
(Gy) D
5
(Gy) D
50
D
5
Rectum 43.68 70.28 43.96 71.4 0.6 1.6
Prostate
Bladder 35.84 70.84 35.98 72.8 0.4 2.7
Rt Femoral Head 14.01 17.36 14.12 17.43 0.8 0.4
Lt Femoral Head 14.84 21.06 14.92 21.34 0.5 1.3
Pelvic
Rectum 15.25 43.20 15.26 43.25 0.07 0.12
Femoral Heads 31.75 44.17 31.77 45.52 0.06 3.10
Ta b l e 3. Target coverage (TC) and Homogeneity Index (HI) for PTV in prostate and pelvic.
TC and HI Before Target Change After Target Change Difference
D
98
% (Gy) 70.00 72.24 3.1%
D
2
% (Gy) 70.56 71.96 2.0%
Prostate-PTV D
median
(Gy) 70.28 71.12 1.2%
HI 0.008 0.016 0.008
TC 0.77 0.79 2.5%
Pelvic-PTV D
98
% (Gy) 43.70 43.98 0.6%
D
2
% (Gy) 44.75 45.35 1.3%
D
median
(Gy) 43.85 44.50 1.5%
HI 0.02 0.03 0.01
TC 0.98 0.99 1.0%
The effect of change in IVDT calculated before and after system move is represented in
DVH as shown in Fig. 7. The DVH curves agree with each other since no signicant differ-
ences were observed in the IVDT.
Fi g . 7. Comparison of pelvic dose volume histogram before and after TomoTherapy system move.
13 Yadav et al.: MVCT images and adaptive TomoTherapy 13
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
C. MVCT image parameters
The variation in noise and contrast to noise ratio are displayed in Fig. 8. The blue line is the
baseline generated by taking the average of noise over a week. The baseline for CNR and noise
was set to 34.5 and 64.5, respectively. The accepted deviation was set to 10% for both CNR
and noise, based on the Varian linac study
(17)
and PIPSpro manual. The green area is within
acceptable parameters range; yellow area indicates caution level, and pink area indicates re-
jection areas, respectively. It can be noted that the noise and CNR lies in the acceptable range
over the period four months.
IV. DISCUSSION
Overall, the dose differences due to target change were all less than 5% when compared to the
original plans, and exhibit the worst-case situation just before target failure. A dose difference
of less than 0.6% was observed before and after the TomoTherapy machine was moved. For
treatment schedules of 30–40 fractions, a dose difference of 4%–5% for a few fractions would
be considered borderline. However, if this deviation occurred during a hypo-fractionated treat-
ment schedule of only a few fractions, it could have the potential for a much large impact on the
intended treatment. Our results indicate discrepancy, primarily in dose calculated for structures
in proximity of high-density structures. It is likely to impact specialized treatment such as the
spine SBRT or total scalp treatment with whole brain avoidance. In such cases, it becomes
necessary to verify the IVDT curve if MVCTs are used for adaptive planning.
V. CONCLUSIONS
In this work, we have presented the dosimetric impact on TomoTherapy treatment plans due to
variation in image value to density table (IVDT) curve as a function of target degradation. We
also have investigated the reproducibility and stability of the TomoTherapy MVCT image quality
Fi g . 8. Variation of noise and contrast to noise ratio over a period of four months in the TomoTherapy machine.
14 Yadav et al.: MVCT images and adaptive TomoTherapy 14
Journal of Applied Clinical Medical Physics, Vol. 11, No. 4, Fall 2010
over time. Multiple scans of the TomoTherapy “Cheese” phantom were performed over a period
of ve months. Based upon our study, we strongly recommend TomoTherapy users to record
IVDT for the imaging system on a monthly basis. It is also highly important to remeasure IVDT
after any major hardware repair or replacements. The IVDT curve should always be updated
after target or linac change. Based on the presented results, it is clear that noise and CNR are
uniformly distributed around the mean value with a variation of +5%. No signicant trend in
this variation was observed over the time of study. We thus recommend that measurement of
noise and CNR should be incorporated into routine imaging QA on a quarterly or semi-annual
basis to check the reproducibility and stability of the TomoTherapy machine.
REFERENCES
1. Brahme A, Roos JE, Lax I. Solution of an integral equation encountered in rotation therapy. Phys Med Biol.
1982;27(10):1221–29.
2. Kapatoes JM, Olivera GH, Balog JP, Keller H, Reckwerdt PJ, Mackie TR. On the accuracy and effectiveness of
dose reconstruction for tomotherapy. Phys Med Biol. 2001;46(4):943–66.
3. Yan D, Vicini F, Wong J, Martinez A. Adaptive radiation therapy. Phys Med Biol. 1997;42(1):123–32.
4. Wu C, Jeraj R, Olivera GH, Mackie TR. Re-optimization in adaptive radiotherapy. Phys Med Biol.
2002;47(17):3181–95.
5. Shaw RM, Mackie T. MVCT superiority over KVCT in assessment of electron density for treatment planning
[abstract]. Med Phys. 2006;33(6): 2124.
6. Meeks SL, Harmon JF Jr, Langen KM, Willoughby TR, Wagner TH, Kupelian PA. Performance characterization of
megavoltage computed tomography imaging on a helical tomotherapy unit. Med Phys. 2005;32(8):2673–81.
7. Hong TS, Welsh JS, Ritter MA, et al. Megavoltage computed tomography – an emerging tool for image-guided
radiotherapy. Am J Clin Oncol. 2007;30(6):617–23.
8. Stock M, Pasler M, Birkfellner W, Homolka P, Poetter T, Georg D. Image quality and stability of image-guided
radiotherapy (IGRT) devices: A comparative study. Radiother Oncol. 2009;93(1):1–7.
9. Langen KM, Meeks SL, Poole DO, et al. The use of megavoltage CT (MVCT) images for dose recomputations.
Phys Med Biol. 2005;50(18):4259–76.
10. Jeraj R, Mackie TR, Balog J, et al. Radiation characteristics of helical tomotherapy. Med Phys. 2004;31(2):396–404.
11. Forrest LJ, Mackie TR, Ruchala K, et al. The utility of megavoltage computed tomography images from a helical
tomotherapy system for setup verication purposes. Int J Radiat Oncol Biol Phys. 2004;60(5):1639–44.
12. Rehbinder H, Forsgren C, Löf J. Adaptive radiation therapy for compensation of errors in patient setup and
treatment delivery. Med Phys. 2004;31(12):3363–71.
13. Staton RJ, Langen KM, Kupelian PA, Meeks SL Dosimetric effects of rotational output variation and x-ray target
degradation on helical tomotherapy plans. Med Phys. 2009;36(7):2881–88.
14. Ruchala KJ, Olivera GH, Schloesser EA, Hinderer R, Mackie TR. Calibration of a tomotherapeutic MVCT
system. Phys Med Biol. 2000;45(4):N27–N36.
15. Ruchala KJ, Olivera GH, Kapatoes JM. Limited-data image registration for radiotherapy positioning and verica-
tion. Int J Radiat Oncol Biol Phys. 2002;54(2):592–605.
16. Yoon M, Park SY, Shin D, et al. A new homogeneity index based on statistical analysis of the dose-volume
histogram. J Appl Clin Med Phys. 2007;8(2):9–17.
17. McGarry CK, Grattan MW, Cosgrove VP. Optimization of image quality and dose for Varian aS500 electronic
portal imaging devices (EPIDs). Phys Med Biol. 2007;52(23):6865–77.
18. Rajapakshe R, Luchka K, Shalev S. A quality control test for electronic portal imaging devices. Med Phys.
1996;23(7):1237–44.
    • "Fan-beam megavoltage (MV) CT images may also be acquired with the patient in the treatment position using helical TomoTherapy sys- tems [87] . Tomotherapy MVCTs are routinely used for image guid- ance [88] and provide accurate patient electron density data that may be used for dose reconstruction [89,90]. "
    [Show abstract] [Hide abstract] ABSTRACT: Variations in the position and shape of the prostate make accurate setup and treatment challenging. Adaptive radiation therapy (ART) techniques seek to alter the treatment plan, at one or more points throughout the treatment course, in response to changes in patient anatomy observed between planning and pre-treatment images. This article reviews existing and developing ART techniques for prostate cancer along with an overview of supporting in-room imaging technologies. Challenges to the clinical implementation of adaptive radiotherapy are also discussed.
    Full-text · Article · Apr 2016
    • "Helical tomotherapy's ability to generate megavoltage (MV) computed tomography (CT) images is mainly used to provide proper daily patient positioning.[1314] Past studies have shown that although their quality is surpassed by that of kilovoltage (kV) CT images, the MVCT images produced by a helical tomotherapy unit are adequate for image guidance in radiotherapy and can be used reliably for re-contouring.[1516] This property makes it possible to create adapted plans that account for changes in anatomy. "
    [Show abstract] [Hide abstract] ABSTRACT: Helical tomotherapy's ability to provide daily megavoltage (MV) computed tomography (CT) images for patient set-up verification allows for the creation of adapted plans. As plans become more complex by introducing sharper dose gradients in an effort to spare healthy tissue, inter-fraction changes of organ position with respect to plan become a limiting factor in the correct dose delivery to the target. Tomotherapy's planned adaptive option provides the possibility to evaluate the dose distribution for each fraction and subsequently adapt the original plan to the current anatomy. In this study, 30 adapted plans were created using new contours based on the daily MVCT studies of a bladder cancer patient with considerable anatomical variations. Dose to the rectum and two planning target volumes (PTVs) were compared between the original plan, the dose that was actually delivered to the patient, and the theoretical dose from the 30 adapted plans. The adaptation simulation displayed a lower dose to 35% and 50% of the rectum compared to no adaptation at all, while maintaining an equivalent dose to the PTVs. Although online adaptation is currently too time-consuming, it has the potential to improve the effectiveness of radiotherapy.
    Article · Apr 2012
    • "Presently, adaptive planning is frequently done on MVCT and kV-CBCT images to conform the dose distribution and dose coverage to the target and OAR due to significant weight loss during the treatment, shrinkage in tumor or re-growth of the tumor volume. For any sort of the adaptive planning, it is important to make use of correct parameters like image set and CT to density table.11 Doing adaptive planning on the regular basis requires a routine check of the machine’s CT to density curve. "
    [Show abstract] [Hide abstract] ABSTRACT: We have analyzed the stability of CT to density curve of kilovoltage cone-beam computerized tomography (kV CBCT) imaging modality over the period of six months. We also, investigated the viability of using image value to density table (IVDT) generated at different time, for adaptive radiotherapy treatment planning. The consequences of target volume change and the efficacy of kV CBCT for adaptive planning issues is investigated. MATERIALS AND METHODS.: Standard electron density phantom was used to establish CT to electron density calibrations curve. The CT to density curve for the CBCT images were observed for the period of six months. The kV CBCT scans used for adaptive planning was acquired with an on-board imager system mounted on a "Trilogy" linear accelerator. kV CBCT images were acquired for daily setup registration. The effect of variations in CT to density curve was studied on two clinical cases: prostate and lung. The soft tissue contouring is superior in kV CBCT scans in comparison to mega voltage CT (MVCT) scans. The CT to density curve for the CBCT images was found steady over six months. Due to difficulty in attaining the reproducibility in daily setup for the prostate treatment, there is a day-to-day difference in dose to the rectum and bladder. There is no need for generating a new CT to density curve for the adaptive planning on the kV CBCT images. Also, it is viable to perform the adaptive planning to check the dose to target and organ at risk (OAR) without performing a new kV CT scan, which will reduce the dose to the patient.
    Full-text · Article · Sep 2011
Show more

Recommended publications

Discover more