Purpose/Objective(s): Mature toxicity outcomes from the first phase I trial using stereotactic body radiotherapy (SBRT) to the prostate bed for recurrent prostate cancer have been recently reported. From that study, we herein report inter-and intra-fractional positional changes in the clinical target volume (CTV) and rectum with the use of an endorectal balloon. Materials/Methods: Prostate bed SBRT was delivered over 5 fractions in escalating doses of 35, 40, and 45 Gy. Three fiducial markers were planted in the CTV volume. A subset of 10 patients on the trial received both preand post-treatment CBCT for each fraction and were the subject of this analysis. An endorectal balloon was placed during both simulation and daily treatments. Patients were instructed to maintain a full bladder. On pre-treatment CBCT we localized to the anterior surface of the balloon to align the anterior rectal wall and CTV. Inter-fractional variation was measured using registrations based on pelvic anatomy. Intra-fractional changes in balloon position were assessed with soft-tissue registration. The t-test was used to evaluate the significance of correlation coefficient with significance level set at 0.05. Results: The average daily air-filled balloon volume (AEstandard deviation) was 77.0AE5.0 cc (range: 60.6 cc e 88.5 cc), and the average pre-treatment bladder volume was 235.0AE109.7 cc. Compared to the planning CT, the pretreatment bladder volume changed by-35.0AE87.9 cc (range:-227.1 e 123.1 cc). Relative to the planning positions, the anterior rectal wall had an average inter-fractional shift of-1.6AE5.6 mm (range:-15.2 e 7.5 mm) in the anteroposterior (AP) direction at the mid-section of the balloon (negative value: shifts posteriorly), with ! 3mm shift in 66% of total fractions. Correlation between daily bladder volume variations and anterior rectum position variations in the AP direction was statistically significant (Pearson's correlation coefficient Z 0.76, p Z 0.02). The average time between the pre-and posttreatment CBCT scans was 9.8AE4.0 minutes (range: 6 e 29 minutes). The post-treatment bladder volume increased significantly (p < 0.01) by an average of 58.9AE51.6 cc. The balloon volume changed by <5% in each treatment. The center-of-mass of the balloon had average intra-fractional variation of 0.3AE0.8 mm in the left-right (LR) direction (range:-1.3 e 2.5 mm),-0.9AE2.4 mm in the AP direction (range:-11.0 e 3.0 mm), and-1.7AE2.2 mm in the superoinferior (SI) direction (range:-10.9 e 0.7 mm) (negative value: shift anteriorly). No correlation was seen between increased bladder volume and intrafractional AP shifts, since the majority of greater than 3 mm shifts were attributed to one patient. Conclusion: Although not statistically significant, we observed higher than expected intrafractional AP and SI shifts. Our future phase II trial will incorporate larger margins to account for this finding.

significantly associated with improved local control (HR 0.34 [95% CI, 0.13-0.85], pZ0.02), whereas concurrent CHT had no impact (HR 1.33 [95% CI, 0.48-3.59], pZ0.58). Freedom from pelvic nodal relapse at 3 and 5 yrs was 92% and 91%, respectively. RT covering the regional pelvic lymph nodes was not associated with pelvic recurrence (HR 0.32 [95% CI, 0.07-1.41], pZ0.13). Freedom from distant failure at 3 and 5 yrs was 74% and 67%, respectively. Acute grade 2 urinary (GU) and gastrointestinal (GI) toxicity was seen in 38% and 26% of patients, respectively. Incidence of late grade 2 GU and GI toxicity was 33% and 5%, respectively. No patients died from treatment-related causes. Median OS was 23 months. OS at 3 and 5 yrs was 42% and 25%, respectively. OS was significantly better in patients that were medically fit to undergo RC (HR 0.47 [95% CI, 0.27-0.80], p<0.01) and those receiving concurrent CHT (HR 0.49 [95% CI, 0.29-0.86], pZ0.01). Conclusion: Definitive RT +/-CHT is a safe, effective, and well-tolerated treatment strategy for elderly patients with MIBC.