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Bladder Size Variations During Radiotherapy of Prostate Cancer and the Clinical Impact On Treatment Quality

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M Moteabbed

M Moteabbed*, A Trofimov , G Sharp , A Zietman , J Efstathiou , H Lu , Massachusetts General Hospital, Boston, MA


SU-H3-GePD-J(A)-3 (Sunday, July 30, 2017) 4:00 PM - 4:30 PM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: To investigate the clinical impact of bladder volume variations on delivered treatment quality for proton therapy and intensity-modulated radiotherapy of prostate cancer.

Methods: Twenty patients who underwent weekly verification CT and Cone beam CT (CBCT) scans were included. The bladder was outlined on all images. Weekly images were rigidly aligned with planning CT using 3 fiducial markers per clinical setup protocols, and the planned dose (i.e. 79.2Gy to prostate and 50.4Gy to prostate plus 5-15mm of proximal seminal vesicles in 44 fractions) was recomputed on each image. Planning CTs were deformably registered to intensity-corrected CBCTs to represent the weekly anatomy for IMRT dose calculation. Recalculated doses were scaled and summed via deformable dose accumulation representing total delivered dose. Interfractional variations in bladder volume were correlated with the dose received by target and organs at risk (OARs).

Results: Despite specific instructions aimed to maintain consistently full bladder volume at the time of simulation and treatment, variations as large as 670 cc were observed. A clear trend associating reduction in bladder size and increased bladder mean-dose was found. Also bladder size inconsistencies, especially systematic reductions, could lead to loss of target coverage due to induced prostate rotations/deformations affecting setup. Patient-specific correlations between bladder changes and PTV dose differences were not strong (R2<0.7). All clinically relevant bladder dose-volume indices remained within tolerance after dose accumulation. The accumulated target volume indices were above minimum tolerance for all cases except one, with systematic bladder inconsistency leading to 1.9/10.7% underdose to CTV/PTV. Bladder variations did not predict rectal dose inconsistencies in the presence of rectal balloons. No modality dependence was found.

Conclusion: Bladder size variations can be large but have small effect on target and OAR dose in the presence of robust protocols. Patients should be encouraged to keep bladder filling consistent rather than maximized.

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