Int. J. Radiation Oncology Biol. Phys., Vol. 74, No. 2, pp. 583–592, 2009Copyright � 2009 Elsevier Inc.

Printed in the USA. All rights reserved0360-3016/09/$–see front matter

doi:10.1016/j.ijrobp.2008.12.034

PHYSICS CONTRIBUTION

DOSE RECALCULATION AND THE DOSE-GUIDED RADIATION THERAPY (DGRT)PROCESS USING MEGAVOLTAGE CONE-BEAM CT

JOEY CHEUNG, B.A.,* JEAN-FRANCOIS AUBRY, M.S.,*y SUE S. YOM, M.D., PH.D.,*

ALEXANDER R. GOTTSCHALK, M.D., PH.D.,* JUAN CARLOS CELI, PH.D.,xyy AND JEAN POULIOT, PH.D.*

*Department of Radiation Oncology, University of California San Francisco, Comprehensive Cancer Center, San Francisco, CA;yDepartement de physique, genie physique et d’optique, Universite Laval, Quebec, Quebec, Canada; xOncology Care Systems Group,

Siemens Medical Solutions USA, Inc., Erlangen, Germany; yycurrently at IBA Dosimetry, Schwarzenbruck, Germany

Purpose: At the University of California San Francisco, daily or weekly three-dimensional images of patients intreatment position are acquired for image-guided radiation therapy. These images can be used for calculatingthe actual dose delivered to the patient during treatment. In this article, we present the process of performingdose recalculation on megavoltage cone-beam computed tomography images and discuss possible strategies fordose-guided radiation therapy (DGRT).Materials and Methods: A dedicated workstation has been developed to incorporate the necessary elements ofDGRT. Patient image correction (cupping, missing data artifacts), calibration, completion, recontouring, anddose recalculation are all implemented in the workstation. Tools for dose comparison are also included. Examplesof image correction and dose analysis using 6 head-and-neck and 2 prostate patient datasets are presented to showpossible tracking of interfraction dosimetric endpoint variation over the course of treatment.Results: Analysis of the head-and-neck datasets shows that interfraction treatment doses vary compared with theplanning dose for the organs at risk, with the mean parotid dose and spinal cord D1 increasing by as much as 52%and 10%, respectively. Variation of the coverage to the target volumes was small, with an average D5 dose differ-ence of 1%. The prostate patient datasets revealed accurate dose coverage to the targeted prostate and varyinginterfraction dose distributions to the organs at risk.Conclusions: An effective workflow for the clinical implementation of DGRT has been established. With these tech-niques in place, future clinical developments in adaptive radiation therapy through daily or weekly dosimetricmeasurements of treatment day images are possible. � 2009 Elsevier Inc.

Dose-guided radiation therapy, Megavoltage cone-beam CT, Dose calculation, Treatment replanning, Image-guided radiation therapy.

INTRODUCTION

With recent advancements in radiation treatment planning

and delivery systems such as intensity-modulated radiation

therapy (IMRT), very sharp dose gradients can be achieved

to attain optimal dose distributions. These advancements,

however, require accurate patient positioning and setup to en-

sure the proper treatment delivery (1, 2). This problem has

been addressed through the development of in-room patient

imaging systems that can image the patient on the treatment

table, which can then be used to verify and correct for patient

misalignments or target organ movement. This technique is

5

known as image-guided radiation therapy (IGRT) and has

quickly become a popular solution for patient setup (3, 4).

At the University of California San Francisco Comprehen-

sive Cancer Center, IGRT has been implemented using

a Siemens MVision megavoltage cone-beam computed

tomography (MVCBCT) system (5–8) to correct for patient

setup errors. For head-and-neck patients, a thermoplastic pa-

tient-specific mask is used to position the patient daily, and

weekly cone-beam images are acquired to correct for setup

errors and to assess the long-term patient positioning accu-

racy. For prostate patients, daily low-dose cone-beam images

Reprint requests to: Joey Cheung, University of California SanFrancisco Dept. of Radiation Oncology, Helen Diller Family Com-prehensive Cancer Center, 1600 Divisadero St., Suite HM006, SanFrancisco, CA 94115. Tel: (415) 353-7179; E-mail: CheungJ@radonc.ucsf.edu

Partly supported by Siemens Oncology Care Systems.Acknowledgments—The authors would like to thank ChandrasekharNunna, Thomas Boettger, and Venkataramana Abbaraju for their

83

aid in the development of the workstation and Sherry Leeper forher contribution in this work. One of the authors (J.F.A.) acknowl-edges financial support from the Natural Sciences and EngineeringResearch Council of Canada (NSERC).

Received Sept 17, 2008, and in revised form Nov 26, 2008.Accepted for publication Dec 19, 2008.

584 I. J. Radiation Oncology d Biology d Physics Volume 74, Number 2, 2009

Fig. 1. Overview of the dose-guided radiation therapy (DGRT) workflow and its integration into the image-guided radi-ation therapy (IGRT) workflow. Dashed lines show possible feedback loops into the clinical process.

are acquired and the patients are realigned based on the posi-

tion of the prostate determined by implanted gold seeds (9,

10). For some prostate patients, weekly cone-beams of

slightly higher dose are also acquired to validate alignment

with soft tissue information or for research purposes.

Because these images are taken directly before treatment,

it has been proposed that they can be used for daily assess-

ment of the actual delivered dose to the patient, providing

feedback to the radiation physician and therapists on the ac-

curacy of the treatment delivery. As a result, deviations from

the plan that are not taken into account by IGRT, such as tu-

mor shrinkage, weight loss, and other internal anatomical

changes, can be assessed. Furthermore, the verification of

the actual treatment dose can help the physician track the evo-

lution of the treatment and determine if and when a replan or

plan adaptation is needed (11, 12).

Recently, our group has worked to develop a workstation

that integrates all of the necessary elements of dose-guided

radiation therapy (DGRT) in an attempt to streamline and in-

troduce the process into the clinical workflow (13, 14). The

workstation is currently built on top of a Syngo-based soft-

ware platform and is codeveloped by the Siemens Oncology

Care Systems innovation group and members of the Univer-

sity of California San Francisco research staff. This article

will go through the steps required to perform dose recalcula-

tion on MVCBCT treatment images and the possible strate-

gies that can be developed using DGRT. In addition,

several examples are chosen to illustrate how this process

can be performed on patient images. Case studies for tracking

daily delivered patient dose are explored.

MATERIALS AND METHODS

Acquisition protocolsSix head-and-neck and 2 prostate patient datasets were used to

illustrate the possibilities of the dedicated DGRT workstation. These

patients received MVCBCT imaging with low-dose image

Fig. 2. The conversion process for converting cone-bean (CB) num-bers in CB images to computed tomography (CT) numbers. The cal-ibration curves are obtained from measurements of phantoms withinserts of known densities. The CB numbers are converted to CTnumbers by matching the calibrated physical densities betweenboth systems.

DGRT process using MVCBCT d J. CHEUNG et al. 585

Table 1. Tumor location, diagnosis, and treatment for head-and-neck and prostate cases

# Age/Gender Location (Primary Site) Stage Prescription (cGy) No. Fraction Total Dose (Gy) Replan? Chemo?

1 55/m Nasopharynx T1N1M0 IIB 212 33 69.96 ConcurrentUndifferentiated carcinoma

2 69/m Oral cavity T1N2bM0 IVA 200 33 66 ConcurrentSquamous cell carcinoma

3 48/m Left tonsillar pillar T3N2b 212 33 69.96 ConcurrentSquamous cell carcinoma

4 48/m Unknown primary with metastasis to leftneck

TXN2bM0 IV 212 33 69.96 No

Squamous cell carcinoma5 59/m Base of tongue T2N3M0 212 33 69.96 Yes Concurrent

Squamous cell carcinoma6 64/m Melanoma of right ear pT1N0 IA 600 5 30 No

Malignant melanoma

# Age/gender Location (primary site) Stage Prescription (cGy)No.

FractionTotal Dose

(Gy) Postradiotherapy CyberKnife Boost

7 70/M Prostate adenocarcinoma cT1c 180 25 45 2 fractions 9.5 GyGleason 4+4 per fractionPSA 4.27

8 57/M Prostate adenocarcinoma cT2a/uT3a 180 25 45 2 fractions 9.5 GyGleason 4+5 per fractionPSA 4.3

acquisition protocols under institutional review board approval.

A total of 31 head-and-neck cone-beams and 8 weekly prostate

cone-beams were used. In the following discussion, the necessary

procedures to perform dose recalculation are outlined in detail.

A proposed clinical workflow diagram of the DGRT process is

shown in Fig. 1.

Image processing and reconstructionBefore reconstruction, a diffusion filter is applied to each cone-

beam projection image using a built-in function in MeVisLab (Me-

Vis Medical Solutions) to improve the contrast-to-noise ratio (15,

16). The cone-beam images are then reconstructed offline through

a modified Feldkamp-Davis-Kress back-projection reconstruction

algorithm and imported to the DGRT workstation (17). Because

of the geometry of the MVCBCT system setup, the center of the im-

age is the isocenter (7).

Image correction and calibration methodsAs with most cone-beam systems, a typical cupping artifact,

resulting from scatter and beam hardening, is present in the images

obtained from our MVCBCT system (18). In addition, a missing

data artifact also appears if the section of the patient being imaged

is larger than the field-of-view of the system. For the dose to be ac-

curately calculated using these images, these artifacts need to be cor-

rected and the images need to be calibrated to the correct electron

densities.

Several methods have been developed to correct for these artifacts

in head-and-neck cases (19, 20). The workstation uses the method

developed by Aubry et al. (20) in which a reference conventional

kVCT image is used to create a set of patient-specific correction

factors for the MVCBCT images. The slice-by-slice correction is ap-

plied to the cone-beam image based on the CT number values of the

blurred and filtered kVCT image. After applying the correction

factors, the corrected image is already calibrated to the correct CT

numbers because the planning kVCT image was used as a reference.

These CT numbers can then be calibrated in the dose calculation

engine to the correct physical and electron densities through mea-

surements of phantoms with inserts of known densities.

For the prostate patients, a different method developed by Aubry

et al. (21) was used that utilizes a set of correction factors calibrated

using custom-built pelvic-shaped water phantoms of various sizes.

The correction factors are determined by interpolating between the

values obtained from the water phantoms based on measured aver-

age radiological thicknesses of the patients. Unlike the head-and-

neck correction, however, the resulting image is not calibrated to

the correct CT numbers because of the nonlinear response of the

kVCT and MVCBCT systems to the density of imaged materials

and further calibration is needed for image correction. This calibra-

tion is done in the process of MVCBCT completion.

MVCBCT completionThe maximum field of view of our MVCBCT system is 27 cm in

diameter in the axial plane and 27 cm in length along the craniocau-

dal axis. Because the field of view of our MVCBCT system is

smaller than most patients, the images exclude part of the patient’s

anatomy. The resulting treatment beam attenuation for dose calcula-

tion will be incorrect if this missing information is not accounted for.

To correct for this, the cone-beam image is completed using the

planning kVCT image as a reference. The cone-beam images are

registered to the kVCT image based on bony alignment in the

area of interest. A new image is created called a cone-beam+

(cone-beam plus) image that takes the kVCT image and replaces

the overlapping region of the two images with the cone-beam image.

In creating the cone-beam+, the machine isocenter from the original

cone-beam is preserved.

For the prostate patients, the calibration curves of both imaging

systems are used to convert the CT numbers from the MVCBCT im-

age (CB numbers) to CT numbers of the planning kVCT image. This

process is shown in Fig. 2.

RecontouringFor this study, two radiation oncologists specialized in head-and-

neck and prostate radiation therapy recontoured several of the

586 I. J. Radiation Oncology d Biology d Physics Volume 74, Number 2, 2009

Fig. 3. Images of a head-and-neck cone-beam (a) uncorrected, (b) corrected, and (c) completed using a kVCT image.

organs at risk on the cone-beam+ images. For head and neck, the

contours used were: brain stem, spinal cord, parotid glands, tempo-

romandibular joints (TMJ), mandible, and larynx. For the prostate

patients, they were: prostate, rectum, bladder, and seminal vesicles.

However, because these images were initially used only for patient

positioning and not for dose recalculation, some of the images were

taken with a reduced field of view to protect critical organs from ad-

ditional exposure. Thus, in some cases, a few of the organs of inter-

est were outside the field of view of the cone-beam and were

therefore excluded from this study.

In addition to the organs at risk, the target volumes for the head-

and-neck datasets were also recontoured. At the University of

California San Francisco, the technique for replanning patients in

the middle of treatment involves maintaining the original gross

tumor volume (GTV) in the second plan while removing any vol-

umes that extend beyond the skin. Similar attempts are made for

the clinical tumor volumes (CTV). This ensures that adequate

dose is given to the high-risk volumes to eliminate the cancer and

that original planned coverage to the targets are not compromised

(13). The same process was performed on the cone-beam+ images

in this study for the GTV and the CTV59.4 (CTV receiving a pre-

scription dose of 59.4 Gy).

Image registration and dose calculationThe DGRT workstation uses a pencil beam dose calculation en-

gine (22–25) based on the KonRad Inverse Planning Software (Sie-

mens Medical) commissioned to a 6MV Siemens ONCOR linear

accelerator. The original treatment plan for each patient was created

using the Pinnacle3 Radiation Therapy Planning System (Philips

Medical Systems), which uses a collapse cone algorithm for dose

calculation. To accommodate for this difference in dose calculation

DGRT process using MVCBCT d J. CHEUNG et al. 587

Fig. 4. Comparisons of the dose different maps for Patient V on the first week of treatment and the third week overlaid onthe planning kVCT image. The colors are windowed to show PDDiffs of greater than 5% (red), less than –5% (blue), andwithin �5% (green). Although visual verification of local changes to parotid dose may not be accurate, overall dosechanges to the spinal cord, larynx, brain stem, and mandible can be useful.

engines, a new plan for the kVCT image is created on the worksta-

tion that preserves the beam shapes, angles, and energies of the orig-

inal plan but renormalizes all the beam weights by the same

multiplicative factor in order to give the same dose to the isocenter

as the original plan. The dose distribution on the planning kVCT is

then recalculated using this renormalized plan so that differences in

treatment dose will be due to anatomical and positioning changes

rather than differences in dose calculation algorithms. This new

plan is copied from the kVCT to the cone-beam+ treatment image

based on the actual treatment registration to ensure that the treatment

isocenter for dose recalculation purposes is the same as it was on the

day of treatment. Note that the renormalization process is only per-

formed once per patient on the planning kVCT image and not on the

cone-beam images.

Dose comparisonSeveral methods of comparing the planned dose to the treatment

dose have been integrated into the DGRT workstation. A simple

side-by-side comparison of the two dose distributions using the

same isodose markers allows for a visual overview of the difference

in doses between the two images. The percentage dose difference

(PDDiff) of the local treatment dose to the reference planning

dose can also be calculated and viewed as an overlay on top of

the cone-beam+ image with the contoured anatomy. Although

voxel-by-voxel dose differences may not accurately represent

dose changes on the cellular level, the PDDiff does allow the user

to see the local difference in overall dose to different parts of the pa-

tient anatomy if the anatomy has had a minimal shift in location. The

dose difference is normalized to the reference plan dose so that

underdoses and overdoses can be easily visualized and understood.

The dose and PDDiff information with user-defined limiting param-

eters for the contoured anatomy can be extracted as a text file and

analyzed. Furthermore, dose–volume histograms (DVHs) can be

plotted and viewed on the workstation.

RESULTS

Head-and-neck casesThe six head-and-neck patient datasets used for this study

were all newly diagnosed with cancers in different primary

tumor sites. Table 1 lists the treatment information along

with the location and stages of the tumors.

Figure 3 shows an example of the application of the correc-

tion method and image completion on a head-and-neck pa-

tient image. For this particular dataset, the cone-beam was

taken on the first week of treatment. The cupping and missing

data artifacts are clearly seen on the images on the left (a).

After correction (b), the images are uniform and the density

values are calibrated to the correct CT numbers needed for

dose calculation. The image can then be completed (c) using

the original planning kVCT image to complement the miss-

ing data.

Figure 4 shows the evolution of the dose differences map of

one head-and-neck patient (Patient V) throughout treatment. The

color overlay is windowed to show PDDiffs greater than 5%

(red), less than –5% (blue), and within�5% (green) of the plan-

ning dose. A number of factors can result in larger overdosed

(red) regions including patient misalignments, tumor or organ re-

gression, more global anatomical changes, and weight loss.

Prostate casesBoth prostate cases were treated with concurrent hormone

therapy. The treatment information can be found in Table 1.

Figure 5 shows the application of the correction method and

image completion on a pelvic patient. As with head-and-neck

cases, the cone-beam correction and calibration (B) and com-

pletion (C) on pelvic images result in a final image that is den-

sity accurate.

Figure 6 shows a side-by-side dose grid comparison of

a prostate planning kVCT image with a treatment cone-

beam+ image for Patient VII. Analysis shows that the volumes

of the prostate, rectum, seminal vesicles, and bladder in this

image changed by –4%, –4%, –44%, and 36%, respectively.

Dosimetric end point trackingFigure 7 shows the time evolution of the percent difference

in mean dose compared to planning dose to the left and right

588 I. J. Radiation Oncology d Biology d Physics Volume 74, Number 2, 2009

Fig. 5. Images of a pelvic cone-beam (a) uncorrected (b) corrected and (c) completed using a kVCT image.

parotids for three of the 6 patients (Patients I, II, and V) and

the difference in D1 dose (Dx dose is the minimum dose re-

ceived by the highest dosed x-percent of the volume) to the

spinal cord for all 6 patients. The D1 dose was used instead

of the maximum dose because of the high sensitivity and var-

iation of the maximum dose. For the parotids, 3 head-and-

neck patients were omitted because their cone-beam images

were taken with a limited field of view that resulted in the

parotids being incompletely imaged or not imaged at all.

An average net increase in dose to both parotids is observed

for all 3 patients. In addition, all 6 patients also received an

average net increase in dose to the spinal cord over the course

of treatment. A summary of the percent dose differences to

the target and various recontoured organs at risk averaged

across all 6 patients is shown in Table 2.

For the prostate patients, DVHs were plotted to observe the

differences in dose to the various recontoured organs. Figure 8

shows the DVHs of the bladder, prostate, seminal vesicles,

and rectum for Patient VII (a) and Patient VIII (b). The dotted

lines show the original planned DVHs to the regions of inter-

est. Figure 9 shows the percentage volume changes from the

planning kVCT volume for the bladder, prostate, rectum, and

seminal vesicles for the 2 prostate patients in our study.

DISCUSSION

Of the different head-and-neck structures that were con-

toured, the parotids tended to be the only ones that exhibited

a long-term unidirectional change in volume and geometry.

As has been shown in other studies (26–28), the irradiated pa-

rotids tended to shrink in size over the course of treatment

and migrate inward toward the center of the skull. This moves

the parotids into higher dose regions over time and a general

increase in dose to the right parotid was seen across all

DGRT process using MVCBCT d J. CHEUNG et al. 589

Fig. 6. A side-by-side comparison of the dose maps from the planning kVCT and the treatment cone-beam+ images. Bothimages use the same dose range colors and the values are the lower bounds of the ranges. The yellow contour is the bladder,purple is the prostate, green is the seminal vesicles, and brown is the rectum.

3 patients after the third week of treatment. Patient I showed

some weight loss during treatment and received a higher dose

to the right side. Patient II received a right unilateral treat-

ment, which could explain the large increase in dose to the

right parotid over the course of the treatment. Patient V ex-

hibited a large amount of weight loss (about 4 cm of soft tis-

sue on both sides of the neck) and tumor regression, which

can be seen in the increase in dose to both parotids.

Weekly variation of the spinal cord dose seemed to be

fairly random but tended to be shifted toward overdoses

(Fig. 7). These errors could be attributed to daily setup errors,

weight loss, or changes in anatomy between the date of the

planning kVCT and the start of the treatment. One notable ex-

ception in the study is Patient V, who had lost a large amount

of weight between the cone-beam images, resulting in a large

(10.7%) increase in D1 spinal cord dose.

In the study, the TMJ was too small and too close to the

edge of the cone-beam images for 2 of the patients to give ac-

curate results. Of the 4 patients left, there was an average net

decrease in dose to both TMJ (Table 2). The D1 dose to the

mandibles tended to vary randomly based on the data. This

is likely based on patient positioning rather than weight

loss. Slight movement in the patient’s jaw position was ap-

parent between the weekly cone beams and the planning

kVCT as the anatomy was recontoured.

Brain stem variability appeared to be the smallest of all the

anatomy studied. This is likely due to the relative position

and size of the brain stem. Because the base of the skull is

fairly easy to align for IGRT, the brain stem is likely to be po-

sitioned accurately during treatment and receive close to the

planned dose. The larynx, on the other hand, showed high

variability for all 6 patients, most likely from the non-rigidity

of the organ and its close location to the skin surface where

high dose variability can occur.

Dose coverage to the target volumes remained fairly consis-

tent for all of the cone-beam images. The D95 dose to the GTV

changed by an average (standard deviation) of 0.1% (0.8%)

with a maximum decrease of 0%, showing a consistent overall

coverage to the GTV. High-dose coverage to the GTV also re-

mained fairly consistent, with a mean dose increase of 1.0%

(1.7%) and a maximum decrease of –4.0% for the D5 dose. Sim-

ilar results were found for the CTV59.4 with values for the D95

and D5 with a maximum absolute change of 3.0% and 5.1% and

an average change of 0.1% and 0.7%, respectively.

590 I. J. Radiation Oncology d Biology d Physics Volume 74, Number 2, 2009

Fig. 7. Time evolution of the percentage mean dose difference to the parotids (a) and the D1 percentage dose difference tothe spinal cord (b).

For the prostate patients, the variation of dose to the blad-

der and rectum are likely the result of volume changes. The

current procedure used in our institution aligns the patients

based on daily prostate images. Therefore, although the pros-

tate may be well aligned for each fraction, the bladder, rec-

tum, and other organs may not be well aligned, leading to

varying fractional dose distributions to these regions. It is

also important to consider the accuracy of the recontoured

anatomy on the lower contrast treatment day images when

evaluating the DVH curves. The same oncologist who pre-

pared the treatment plan recontoured the images used in

this study. Figure 9 examines the changes in volume for

the bladder, prostate, rectum, and seminal vesicles. These

Table 2. Statistics for head-and-neck targets and organs atrisk percentage dose differences

Mean Standard Minimum Maximum

Spinal cord D1* 2.2 2.9 –3.3 10.7Left parotid mean dose 5.8 9.5 –4.2 37.9Right parotid mean dose 18.0 18.0 –2.1 51.8Mandible D1* 1.8 4.1 –6.1 12.2Brain stem D1* 1.1 2.7 –4.8 10.1Left TMJ D1* –1.6 8.0 –16.0 10.0Right TMJ D1* –0.5 5.1 –12.3 5.2Larynx mean dose 6.7 9.3 –11.5 25.4GTV D95* 0.1 0.8 0.0 2.1GTV D5* 1.0 1.7 –4.0 5.8CTV59.4 D95* 0.1 1.1 –3.0 2.3CTV59.4 D5* 0.7 1.3 –1.2 5.1

Abbreviations: TMJ = temporomandibular joint; GTV = grosstumor volume; CTV = clinical tumor volume.

* D1 and D5 are the minimum doses received by at least 1% and5% of the volume that received the largest dose, respectively. Thiswas used instead of the maximum dose due to the high sensitivityand variation of the maximum dose. D95 is the maximum dose re-ceived by at least 95% of the volume. These values were chosenbased on the planning dosimetric considerations for the organs atrisk. All values are given as percent increases from the planningdose.

variations in volume will likely affect the dose received by

these organs as reflected by the variation in DVHs in

Fig. 8. However, because these volume changes occur in

three dimensions, the actual dosimetric consequences of

these volume changes are much more complicated and

must be evaluated on a case-by-case basis.

The dose to the prostate and seminal vesicles tended to be

fairly accurate when compared with the original planned

dose. On one of the cone-beam days, Patient VIII received

3.3% less dose to 95% of the prostate volume (D95). How-

ever, on closer observation of the images themselves, the

prostate was not aligned properly on that day of treatment,

which explains the diminished prostate dose coverage. Be-

cause the patients are aligned daily based on prostate location

using gold seed implants, the DGRT process provides a good

check that the primary tumor volume is getting the prescribed

dose throughout treatment.

The DGRT process presented can be used for verification

of the accuracy of the treatment delivery and for an assess-

ment of the daily trend of dose differences over the course

of treatment. Dose accumulation is not currently imple-

mented in the process. Although non-rigid deformation algo-

rithms could be used as an approximation for organ

movement, it is still insufficient in providing dose accumula-

tion on the cellular level. The DGRT process as presented

does not implement non-rigid deformation in order to reduce

calculation times for clinical use, but the process can be easily

integrated if desired and is an avenue that we are currently ex-

ploring for future clinical use. However, even without the

ability to perform dose accumulation, DGRT can provide

valuable feedback about the dose delivered during each treat-

ment fraction. Trends in dose differences can reveal impor-

tant consequences of anatomy change or setup errors.

CONCLUSION

All of the functionalities required to provide an effective

workflow for the clinical implementation of DGRT have

DGRT process using MVCBCT d J. CHEUNG et al. 591

been integrated in our workstation, enabling the regular com-

parison of the dose of the day with the plan. In the examples pre-

sented, it was found that although dose coverage to the target

volumes have remained fairly consistent for head-and-neck

cases, doses to various organs at risk, including the spinal

cord, parotids, mandible, brain stem, TMJ, and larynx, have

been observed to vary between fractions. For the prostate cases,

we have shown that accurate prostate positioning using IGRT

has allowed for accurate dose coverage to the targeted prostate.

However, doses to the organs at risk including the bladder, rec-

tum, and seminal vesicles are observed to vary. With DGRT,

this variation can be monitored to ensure that the dose to the

organs at risk do not surpass critical limits.

Regular monitoring of the daily patient dose based on im-

ages acquired of the patient on the treatment table can be

a powerful tool for tracking the progress and accuracy of

the treatment. In addition, with IGRT implemented in

many facilities, DGRT can be easily integrated into the

clinical workflow. Access to information about the dose-

of-the-day will open the doors to many areas of research

and clinical improvements in treatment verification and

dose accumulation. A systematic approach can also be de-

veloped to determine when a patient requires replanning

based on dosimetric considerations for either the organs at

risk or the target tumor volume. Furthermore, dose–

response relations can be developed based on long-term pa-

tient follow-up studies. The studies presented in this article

are only for a limited set of patients and serve as an exam-

ple of the many possible case studies that can be performed

using the proposed DGRT process.

Fig. 8. Dose–volume histograms of the contoured anatomy for the prostate patients: (a) Patient VII, (b) Patient VIII. Thedotted line shows the original planning dose–volume histogram.

Fig. 9. Plots of the percentage volume change from planning of the bladder, prostate, rectum, and seminal vesicles for the 2pelvic patients.

592 I. J. Radiation Oncology d Biology d Physics Volume 74, Number 2, 2009

REFERENCES

1. O’Daniel JC, Garden AS, Schwartz DL, et al. Parotid glanddose in intensity-modulated radiotherapy for head and neck can-cer: is what you plan what you get? Int J Radiat Oncol Biol Phys2007;69:1290–1296.

2. Webb S. Intensity-modulated radiation therapy. Bristol, UK:Institute of Physics Publishing; 2001.

3. Ma CM, Paskalev K. In-room CT techniques for image-guidedradiation therapy. Med Dosim 2006;31:30–39.

4. Verellen D, Ridder MD, Storme G. A (short) history of image-guided radiotherapy. Radiother Oncol 2008;86:4–13.

5. Pouliot J, Bani-Hashemi A, Chen J, et al. Low-dose megavolt-age cone-beam CT for radiation therapy. Int J Radiat Oncol BiolPhys 2005;61:552–560.

6. Sillanpaa J, Chang J, Mageras G, et al. Developments in mega-voltage cone beam CT with an amorphous silicon EPID: reduc-tion of exposure and synchronization with respiratory gating.Med Phys 2005;32:819–829.

7. Morin O, Gillis A, Chen J, et al. Megavoltage cone-beam CT:System description and clinical applications. Med Dosim2006;31:51–61.

8. Morin O, Gillis A, Descovich M, et al. Patient dose consider-ations for routine megavoltage cone-beam CT imaging. MedPhys 2007;34:1819–1827.

9. Pouliot J, Aubin M, Langden K, et al. (Non)-migration of radi-opaque markers used for on-line localization of the prostate withan electronic portal imaging device. Int J Radiat Oncol BiolPhys 2003;56:862–866.

10. Balter JM, Sandler HM, Lam K, et al. Measurement of prostatemovement over the course of routine radiotherapy using im-planted markers. Int J Radiat Oncol Biol Phys 1995;31:113–118.

11. Hansen E, Bucci MK, Quivey JM, et al. Repeat CT imaging andreplanning during the course of IMRT for head-and-neck can-cer. Int J Radiat Oncol Biol Phys 2006;64:355–362.

12. Lu W, Olivera GH, Chen Q, et al. Deformable registration of theplanning image (kVCT) and the daily images (MVCT) for adap-tive radiation therapy. Phys Med Biol 2006;51:4357–4374.

13. Pouliot J. From dose to image to dose: IGRT to DGRT. In:Mould RF, editor. Choices in advanced radiotherapy. Nucle-tron, BV: The Netherlands; 2007. p. 243–250.

14. Chen J, Morin O, Aubin M, et al. Dose-guided radiation therapywith megavoltage cone-beam CT. Br J Radiol 2006;79:S87–S98.

15. Monteil J, Beghdadi A. New adaptive nonlinear anisotropic dif-fusion for noise smoothing. Volume 3 of the IEEE International

Conference on Image Processing, page 254. Chicago, IL, USA,1998. IEEE Comp Soc, Los Alamitos, CA.

16. Perona P, Malik J. Scale-space and edge detection using aniso-tropic diffusion. IEEE Trans Pattern Anal Machine Intell 1990;12:629–639.

17. Feldkamp LA, Davis LC, Kress JW. Practical cone-beam algo-rithm. J Opt Soc Am A 1984;1:612–619.

18. Spies L, Ebert M, Groh BA, et al. Correction of scatter in mega-voltage cone-beam CT. Phys Med Biol 2001;46:821–833.

19. Morin O, Chen J, Gillis A, et al. Dose calculations using mega-voltage cone-beam CT. Int J Radiat Oncol Biol Phys 2007;67:1202–1210.

20. Aubry JF, Pouliot J, Beaulieu L. Correction of megavoltagecone-beam CT images for dose calculation in the head andneck region. Med Phys 2008;35:900–908.

21. Aubry JF, Cheung J, Gottschalk A, et al. Correction of mega-voltage cone-beam CT images of the pelvic region based onphantom measurements for dose calculation purposes. J ApplClin Med Phys 2009;10:2852.

22. Mohan R, Chui C, Lidofsky L. Differential pencil beam dosecomputation model for photons. Med Phys 1986;13:64–73.

23. Bortfeld T, Schlegel W, Rhein B. Decomposition of pencilbeam kernels for fast dose calculations in three-dimensionaltreatment planning. Med Phys 1993;20:311–318.

24. Boudreau C, Heath E, Seuntjens J, et al. IMRT head and necktreatment planning with a commercially available Monte Carlobased planning system. Phys Med Biol 2005;50:879–890.

25. Yang J, Li J, Chen L, et al. Dosimetric verification of IMRTtreatment planning using Monte Carlo simulations for prostatecancer. Phys Med Biol 2005;50:869–878.

26. Barker JL, Garden AS, Ang KK, et al. Quantification of volu-metric and geometric changes occurring during fractionated ra-diotherapy for head-and-neck cancer using an integrated CT/linear accelerator system. Int J Radiat Oncol Biol Phys 2004;59:960–970.

27. Vasquez Osorio EM, Hoogeman MS, Al-Mamgani A, et al.Local anatomic changes in parotid and submandibular glandsduring radiotherapy for oropharynx cancer and correlationwith dose, studied in detail with nonrigid registration. Int JRadiat Oncol Biol Phys 2008;70:875–882.

28. Lee C, Langen KM, Lu W, et al. Assessment of parotid glanddose changes during head and neck cancer radiotherapy usingdaily megavoltage computed tomography and deformableimage registration. Int J Radiat Oncol Biol Phys 2008;71:1563–1571.