Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_9_3200 - Dosimetric Impact of Inter-Fraction Variations in Bladder Filling and Rectal Emptying in Patients Undergoing Prostate SBRT

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Dosimetric Impact of Inter-Fraction Variations in Bladder Filling and Rectal Emptying in Patients Undergoing Prostate SBRT
D. Gorovets Jr, P. Lichtenwalner, C. Shi, K. L. Borofsky, P. K. Parhar, M. J. Zelefsky, and M. F. Chan; Memorial Sloan Kettering Cancer Center, New York, NY

Purpose/Objective(s): There are currently limited objective data to guide physician approval/disapproval of cone-beam CTs (CBCT) for patients undergoing prostate SBRT. This study aimed to determine the amount of inter-fraction variation in bladder and rectal volumes that are considered clinically acceptable and evaluate the dosimetric impact of this variability.

Materials/Methods: Fifty CBCT images approved by physicians for treatment of 10 consecutive prostate SBRT patients were used to measure bladder and rectal volumes for comparison to simulated/planned volumes. For simulation and each treatment, all patients were instructed to drink 16oz of water 45 minutes prior and administer an enema 3 hours prior. All patients were prescribed 40Gy in 5 fractions using VMAT plans that met institutional dose constraints for bladder (V36<10%, V20<50%) and rectum (D1cc<38.5Gy, V24<25%, V10<52%, mean<13Gy). Daily delivered dose was recalculated on the CBCTs to evaluate for violations of these constraints.

Results: CBCT bladder volumes ranged from 78% smaller to 163% larger than at the time of simulation (mean 17% smaller). CBCT rectal volumes ranged from 13% smaller to 79% larger than at the time of simulation (mean 13% larger). There were individual fractions where dose exceeded tolerance, however over the entire course of treatment for each individual patient, dose constraints were still met based on the average bladder and rectal variations. When individual fractions exceeded tolerance for bladder, the CBCT bladder volume was >50% smaller than the simulation volume. When individual fractions exceeded tolerance for rectum, the CBCT rectal volumes were >25% larger than the simulation volumes.

Conclusion: Despite bladder filling and rectal emptying instructions, there were considerable inter-fraction differences in bladder and rectal volumes that were considered acceptable for treatment. Despite this variability, bladder and rectal dose constraints for the entire course were still met for each patient. The acceptable amount of variation for each fraction that would consistently meet constraints still needs to be determined.

Author Disclosure: D. Gorovets: None. P. Lichtenwalner: None. C. Shi: None. K.L. Borofsky: None. M.J. Zelefsky: Consultant; Consultant. M.F. Chan: None.

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