TH‐C‐350‐07: Impacts of Dose Distribution Variations in Proton Therapy According to Gastro‐Intestinal Tract Air Filling and Breathing

Purpose: For upper abdominal cancers, the simulation CT represents a snapshot of the possible air distribution and radiological thickness of a patient, both of which impact proton dose distribution. The purpose of this dosimetric study is to analyze the effect of gastro‐intestinal tract air filling and breathing on the dose delivered to a superior abdomen target using passive scattering proton therapy. Method and Materials: We used free‐breathing CT (FBCT) and 4D‐CT data sets showing the same 3‐cm pancreatic tumor. Air distribution was reasonable. We reproduced a situation where proton beam parameters would be calculated on a normal breathing patient with gas (no air override: O⁻FBCT), and we analyzed the dosimetric impacts if air was replaced by stools (air override: O⁺FBCT) during treatments and vice‐versa. Target coverage for 50.4 CGE in 28 fractions and dose to critical structures were evaluated for different air‐filling and breathing‐phase scenarios. Four‐field plans including 3 incidences going through some gas were compared to 2‐posterior‐field plans. Results: For the 4‐field plans, beam parameter calculations from the O⁺FBCT resulted in adequate target coverage when the same beam parameters were applied to the O⁻FBCT. The converse situation resulted in the need for an additional 1‐cm SOBP width to achieve adequate coverage for the antero‐posterior and right‐lateral beams. The 4‐field plans provided insufficient distal target coverage to account for planning uncertainties in the expiration phase, a problem avoided with the 2‐field plan. Conclusion: Our choice of planning margins on O⁺FBCT for upper abdominal cancer proton therapy is safe for target coverage, even if air filling variations occur during treatments. Planning on an O⁻FBCT, using a passive scattering technique, results in less conformal plans. The use of posterior incidences for upper abdominal cancers prevents uncertainties, especially in specific clinical situations like ileus or breathing disorders.