Dose Conformality and Acute Toxicity Analysis in Patients With Prostate Adenocarcinoma Treated With Volumetric Modulated Arc Therapy (VMAT) Versus Conventional Intensity Modulated Radiation Therapy (IMRT)

Poster Viewing Abstracts S401

Volume 84  Number 3S  Supplement 2012 Author Disclosure: T.M. Niazi: None. B. Bahoric: None. N. Tomic: None. T. Vuong: None.

2484 EBRT of Prostate Cancer: Does Rectal Spacer Application Show Influence on Inter- and Intrafraction Prostate Mobility? F. Sedlmayer,1 H. Deutschmann,1 G. Kametriser,1 A. Vaszi,1 H. Weichenberger,1 A. Koch,1 S. Hruby,2 C. Gaisberger,1 D. Kellner,3 and R. Forstner4; 1Department of Radio-Oncology, Paracelsus Medical University, Salzburg, Austria, 2Department of Urology, Paracelsus Medical University, Salzburg, Austria, 3radART Institute, Paracelsus Medical University, Salzburg, Austria, 4Department of Radiology, Paracelsus Medical University, Salzburg, Austria Purpose/Objective(s): In EBRT of prostate cancer, safety margins can be reduced by the means of interfractional correction following IGRT, hence reducing doses to organs at risk. An innovative method for further rectal dose reduction was successfully introduced by spacer materials injected between prostate and rectum. Aim of the study was the investigation of the spacer’s influence on inter- and intrafractional movements of the prostate. Materials/Methods: In a prospective matched-pair comparison, eight patients were investigated up to now. In all of them, four gold marker fiducials were implanted perineally into the prostate under endorectal sonographic guidance. Four of the patients additionally received a rectal spacer consisting of 10 mL polyethylene glycol (PEG) hydrogel. Fiducials were used to determine translations and rotations of the prostate. During 7 field IMRT, we captured MV portal images on the fly and registered them by means of a panel flex correction procedure. Two-dimensional marker positions were automatically detected by means of a marker kernel convolution algorithm. Three-dimensional marker positions were reconstructed from two oblique projections. Daily analysis of first pair of MV images (gantry angles 220 and 265 ) was done to determine interfractional movements. Intrafractional movements were derived from shifted marker positions in the last pair of MV portal images (gantry angles 95 and 140 ). The time gap in between the two reconstructions was 4.4 min on average. Results: Eight hundred eighty-four images gained during 221 fractions (supine, flat couch, knee support, comfortably full bladder, empty rectum, no intraprostatic marker migrations >2 mm of more than one marker) were analyzed: 95 fractions in the spacer group and 126 fractions in the control group, respectively. Interfractional 3D vector translations and L-R rotations were found to be 9.4 +/- 4.5 mm / 5.5 +/- 6.4 with spacer, 7.5 +/3.5 mm / 2.2 +/- 2.9 without spacer, respectively.(mean of means). Intrafractional movements were 1.9 +/- 2.1 mm / 2.3 +/- 2.7 in the spacer cohort, 1.9 +/- 1.1 mm / 2.0 +/- 2.8 in the control cohort. Differences were statistically not significant. Our findings compare to previously published 9.3 +/- 4.4 mm / 5.3 +/- 4.9 inter- and 3.0 +/- 3.7 mm / 2.5 +/2.3 intrafractional movements of 39 patients / 833(1013) fractions without spacer. Distensions of the anterior rectal wall could successfully be provided in all spacer patients. Spacers so far did not show a stabilizing effect on motion degree and direction between and during treatment fractions. Both cohorts will be completed to a sample size of 20 each. Conclusions: Whereas spacer application leads to a significant dose reduction in the anterior rectal wall, inter- and intrafractional mobility of the prostate was not affected, which has to be taken into account in safety margin design. Author Disclosure: F. Sedlmayer: E. Research Grant; investigated specimen provided for free. H. Deutschmann: None. G. Kametriser: None. A. Vaszi: None. H. Weichenberger: None. A. Koch: None. S. Hruby: None. C. Gaisberger: None. D. Kellner: None. R. Forstner: None.

2485 Dose Conformality and Acute Toxicity Analysis in Patients With Prostate Adenocarcinoma Treated With Volumetric Modulated Arc Therapy (VMAT) Versus Conventional Intensity Modulated Radiation Therapy (IMRT) D. Cuthbert, C. Catton, P. Lindsay, H. Jiang, and T. Craig; Princess Margaret Hospital-University of Toronto, Toronto, ON, Canada

Purpose/Objective(s): To investigate difference in dosimetry and acute toxicity rates between volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) for treatment of low and intermediate risk prostate cancer. Materials/Methods: Dose-volume histogram data(DVH) was retrospectively generated from the planning system for consecutive patients treated with VMAT(n Z 149) or IMRT(n Z 149). All were treated to 78 Gy in 2 Gy fractions with a 10mm and 7mm posterior planning target volume(PTV). Conformality, PTV coverage and doses to organs at risk were analyzed with t-test. Weekly acute toxicity was prospectively assessed for all patients according to CTCAE v4.0 criteria with the highest score used. Acute toxicity was compared using Fishers’ exact test and CochranArmitage Trend test. Results: Mean doses to 50% and 30% of rectal wall (RW), and 50% of bladder wall(BW) were lower with VMAT compared to IMRT, with significant p values of < 0.0001, 0.006, and < 0.0001, respectively. RW mean D50 and D30 for IMRT and VMAT were 37.9 Gy(SD 6.4 Gy) vs 32.3 Gy(SD 7.5 Gy); and 61.5 Gy(SD 6.3 Gy) vs 59.5 Gy(SD 6.0 Gy) respectively. BW mean D50 was 40.9 Gy(SD 6.0 Gy) vs 37.8 Gy(SD 6.1 Gy). The mean dose to PTV D95 was significantly higher with VMAT at 75.6 Gy(SD 0.49 Gy) vs 75.4 Gy(SD 0.35 Gy)(p Z 0.0005). Skin toxicity was not significantly different between IMRT and VMAT. There were no cases of grade three or higher gastrointestinal (GI) toxicity. GI toxicity was significantly lower with VMAT compared to IMRT(p < 0.0001). Genitourinary (GU) toxicity between IMRT and VMAT was not significantly different. Trend test indicated a significant trend (p Z 0.0131) that VMAT has lower GU toxicity than IMRT. Conclusions: These results show superior conformality with VMAT for PTV coverage with lower RW and BW radiation doses. Lower acute GI toxicity and a trend to lower acute GU toxicity were identified. Additional studies are needed to evaluate late toxicity. Author Disclosure: D. Cuthbert: None. C. Catton: None. P. Lindsay: None. H. Jiang: None. T. Craig: None.

2486 Genome Wide Association Study to Identify Genetic Variants Associated With Urinary Symptoms Following Radiation Therapy for Prostate Cancer S. Kerns,1 R. Stock,1 N. Stone,1 L. Rath,1 H. Ostrer,2 and B. Rosenstein1; 1 Mount Sinai School of Medicine, New York, NY, 2Albert Einstein College of Medicine, New York, NY Purpose/Objectives: Brachytherapy and external beam radiation achieve high cure rates for prostate adenocarcinoma. Though treatment delivery has improved over time, many patients still experience some form of late urinary symptoms that significantly impact quality of life. Even after controlling for clinical factors, considerable variability in toxicity is observed suggesting a genetic component. A predictive tool including genetic factors would assist in personalizing treatment. We performed a two-stage genome wide association study (GWAS) to identify genetic factors associated with urinary morbidity following radiation therapy for prostate cancer. Methods: Prostate cancer patients treated with brachytherapy alone or brachytherapy plus external beam radiation therapy (EBRT) were assessed for urinary morbidity as measured by change in International Prostate Symptom Score (IPSS) from baseline. A total of 783 patients who had baseline IPSS available and > 1 year of follow-up were included. The change in IPSS was assessed at each 6-month follow-up interval between 1 year and 5 years post-treatment and evaluated as a quantitative trait in genetic association tests. Genotyping was done in two stages with patients split randomly into a discovery cohort (N Z 347) and a replication cohort (N Z 436). The discovery cohort was genotyped for w900,000 SNPs using arrays. The 1,480 SNPs most strongly associated with urinary morbidity were then selected for genotyping in the replication cohort using an Illumina custom array. Multivariate linear regression was used to test for association between each SNP and change in IPSS while controlling for pre-treatment IPSS, hypertension and race/ethnicity. Four different genetic inheritance models were investigated for each SNP: allelic, genotypic,