SU-E-T-724: Evaluate the Impact of Bowel Gas Movement in Pancreatic Proton Therapy

Abstract

Purpose: Current pancreatic proton planning protocol overrides bowel gas' CT Hounsfield Unit (HU) to water, based on the assumption of random bowel gas movement during whole treatment course. However, no follow‐up dosimetric study has been done to investigate the impact of the gas movement of each patient in his/her treatment period. The goal of this study is to assess passive‐scattering proton therapy (PT) dose variation due to the gas movement during the patient's treatment course. Methods: Five pancreatic cancer patients who received passive‐scattering PT using double scattering and uniform scanning system were identified. All of the patients received two verification CTs during the whole treatment course to evaluate dose distribution change. Two to three beam arrangements based on individual cases were used (two‐beam: posterior, right posterior oblique; Three‐beam: posterior, right posterior oblique and left posterior oblique). The proton plans were calculated using the Eclipse Proton Convolution Superposition algorithm and delivered 50.4Gy to the initial volume. Theinitial treatment plans with all beam parameters calculated on the CT with bowel gas overridden to water (compensator, MLC, beam line settings, etc) were recalculated using the same beam parameters to the verification CTs. Results: This study shows that the bowel mean dose varies from −135cGy to +342cGy due to the bowel gas movement compared to the initial proton plan. A large kidney dose variations were also observed due to inter‐fraction organ motions. Bowel gas movement doesn't affect target coverage with posterior proton beams proton plan. All plan's PTV D95 and iCTV D95 varies within 1% throughout all verification CTs. Conclusion: This evaluation method using verification CT data sets during the treatment course should also be useful in assessing dose distribution in other anatomic sites. Our quantitative results will help evaluate the effects of transient bowel gas movement in pancreatic proton therapy treatment course.