1102 Comparison of Rectal Dose-Wall Histograms and Dose-Volume Histograms in Predicting Rectal GradeToxicity After Prostate Cancer Radiotherapy

J.L. Johnson, R. Cheung, L. Dong, S.L. Tucker, R. de Crevoisier, D. Kuban

Division of Radiation Oncology, University of Texas M. D. Anderson Cancer Center, Houston, TX

Purpose/Objective: To compare rectal dose-wall histograms and dose-volume histograms for the same patient population andtheir respective ability to predict rectal complications after prostate cancer radiotherapy.

Materials/Methods: 128 patients with American Joint Committee on Cancer (AJCC) stages T1c to T3c (Gleason scores from5 to 9, ages from 49 to 79) were treated between November 1992 and February 1999 at our institution. Patients received3-dimensional conformal radiotherapy (3D-CRT) to the prostate with treatment doses ranging from 74 to 78Gy. Of the 128cases, 8 experienced Grade 3, 27 experienced Grade 2, and 36 Grade 1 rectal toxicity. The analysis endpoint was Grade 0 orhigher late rectal bleeding within 2 years of treatment; the median follow-up period per grade was at least 2 years, (with 79%of the toxicities occurring within that time). The rectal wall for each patient was generated assuming a 3mm wall thickness fromthe contoured rectum. From the plan data, rectal dose-volume histograms (DVH) (which utilized the entire rectum and itscontents) and dose-wall histograms (DWH) were generated for each patient. The data for each pair of the DVH and DWH weregrouped according to rectal toxicity Grades 0, 1, 2 and 3. The mean and median DVH and DWH were taken for each Grade.T-tests with significance at 0.05 were completed for adjacent Grade toxicities at 10Gy dose increments.

Results: For mean rectal DVH and mean rectal DWH, pair-wise comparisons of Grade 1 to Grade 2 and Grade 2 to Grade 3were statistically significant for doses ranging from 10Gy to 80Gy. For Grade 0 to Grade 1 pair-wise comparisons, the meanrectal DVH was significant at all dose levels except 80Gy, while the mean rectal DWH was only significant at the 10, 30, 40,50 and 70Gy doses. Similar results were found for the median DVH and DWH curves.

Additionally, for specific doses 40, 60 and 75.6Gy, both the DVH and DWH curves were greatly statistically significant ina pair-wise comparison of Grade 1 to Grade 2, and Grade 2 to Grade 3. In addition, for 40Gy dose the DVH and DWH werestatistically significant in a pair-wise comparison for Grade 0 to Grade 1.

The mean and median DWH curves cross their respective DVH curves at doses near 57Gy for Grade 0, 61Gy for Grade 1,63Gy for Grade 2, and near 69Gy for Grade 3.

Graphically the mean percent volume from 30 to 50Gy gave the widest spread within the DVH and the DWH curves withrespect to late rectal bleeding, with the DVH curves above their respective DWH curves.

The amount of dose per percent volume had statistical significance in determining the rectal grade toxicity with the standarderror up to �5.4% when using either DVH or DWH. The standard deviations of the means/ standard error at levels 40, 60 and75.6Gy range from 4.3 to 10.7% volume, and dose correlation range from 0.91 for 40Gy to 0.96 for 75.6Gy.

Conclusions: The rectal Grade toxicity correlates with the DVH or DWH data information with statistical significance forGrade 1 or higher for doses ranging from 10 to 80Gy. Additionally, since Grade specific DVH and DWH curves have crossoversin the high dose region (60–70 Gy), results of clinical trials in different institutions could be directly compared using eitherDVH or DWH in these dose levels. Lastly, from 30 to 50Gy the DVH curves are consistently higher than DWH curvesindicating that the percent volume of the rectum and its contents may be larger than the percent volume of the rectal wall fora given dose. Given these results, the DVH or DWH should continue to be considered in evaluating a radiotherapy treatmentplan, and modifications made to the plan if feasible to minimize the late rectal bleeding.

1103 Characterization of Breathing Patterns of Patients undergoing Respiratory-Correlated Imaging

S. Hunjan,1 I. Rosen,1 B. Peter,1 K. Prado,1 D. Luo,1 G. Starkschall,1 H. Liu,1 C. Stevens,2 R. Mohan1

1Radiation Physics, MD Anderson Cancer Center, Houston, TX, 2Radiation Oncology, MD Anderson Cancer Center,Houston, TX

Purpose/Objective: Respiratory-induced tumor motion limits the effectiveness of radiotherapy for thoracic lesions. To accountfor respiratory-induced intrafraction motion an internal target volume (ITV) that includes tumor excursion during the respiratorycycle can be defined. We currently determine ITVs in two ways: from respiratory-correlated 4DCT images and from feed-backguided breath-hold (FGBH) images taken at end-inspiration and at end-expiration. A major assumption of the FGBH approachis that inspiration and expiration breath-hold GTVs accurately represent the limits of the free-breathing (FB) GTV motion. Weperformed a preliminary test of this assumption on a sample of 12 patients.

Materials/Methods: We currently have respiratory traces (Varian RPMTM) for over 150 patients that have undergone bothFGBH and 4DCT procedures with concurrent recordings of their respiratory motion. We created a software-based qualityassurance (QA) tool in MATLAB for statistically analyzing patient breathing patterns during respiratory-correlated imaging.From the FGBH traces we computed the mean position of the marker during inspiration breath-hold (BHinsp) and duringexpiration breath-hold (BHexp) and their standard deviations (SD). From the FB traces, we computed equivalent values: meanend-inspiration position (FBinsp) and end-expiration position (FBexp) and their SDs. We also computed the ranges of motionduring individual breath-holds and the variations in breathing cycle amplitudes to evaluate their stability. BHinsp and BHexpwere compared to FBinsp and FBexp, respectively, using a student’s t-test. The ratio of the breath-hold range of motion tofree-breathing cycle amplitude variation was also calculated as a measure of patient compliance during BH.

Results: Analysis of FB traces revealed that the standard deviation of EI values was, on average, 2.3 times larger compared to EEvalues (larger in 11 cases (92 %). Breath-hold at inspiration (BHinsp) was significantly different (p � 0.05) from the mean freebreathing position at EI in 8 of 12 patients (66 %). Breath-hold at expiration (BHexp) was significantly different (p � 0.05) from themean free breathing position at EE in 11 of 12 patients (92 %). The range of motion during BHinsp was larger (i.e. a less stable BH)than at BHexp in 11 of 12 patients (92 %) and also larger when expressed as a percentage of the mean FB amplitude.

Conclusions: We have demonstrated the use of a QA tool for rapidly analyzing respiratory traces acquired from patients undergoingrespiratory-correlated imaging for the purpose of respiratory correlated therapy. Currently it is being used for retrospective analysis,but in the future it will be used to analyze traces as they are acquired to immediately determine clinical acceptability. In this smallsampling of patients, the analysis of FB traces revealed more reproducible position at end-expiration for all patients studied. Thisindicated that end-expiration as opposed to end-inspiration is the best phase of the respiratory cycle for passive respiratory-gated

S207Proceedings of the 47th Annual ASTRO Meeting