Dosimetry Impact of Bladder Volume Changes and Rectum Filling/Emptying at Proton Therapy of Prostate Cancer; A Simulation Study

Purpose: In the recent decade, proton therapy facilities are increasing worldwide. This study aimed to analyze the influence of volumetric changes in bladder and rectum filling on the dose received by normal surrounding tissues at prostate cancer proton therapy. In this work, an anthropomorphic phantom dedicated to the prostate organs and nearby tissues has been developed using the FLUKA simulation code. Materials and Methods: The geometry of the prostate and normal nearby tissues, bladder volumetric changes, and rectum filling/emptying status were simulated according to a database of real patients to mimic actual treatment, assuming the prostate as the target receives the prescribed dose uniformly with no over- and under dosage at each treatment session. Furthermore, the dosimetric effect of air- and water-filled balloons as prostate fixation tools was considered on the rectum, during our simulation process. Results: Final analyzed results showed that the overall dose received by normal nearby organs with be decreased at proton therapy of prostate cancer if the bladder is full, although this dose reduction is not remarkable. Moreover, rectum filling/emptying and also implementation of balloons with different matters have no significant effect on the amount of dose received by this organ. Conclusion: The dosimetric impact of bladder volumetric variations onto normal nearby organs will not be a crucial issue in proton therapy of prostate cancer if the prescribed high dose is delivered on the target with proper uniformity laterally and in-depth. Based on the obtained results, a full bladder is recommended while target bombarding by a proton beam.