To investigate the dosimetric impact of changes in the large bowel content during proton therapy (PT) with simultaneous integrated boost (SIB) for locally advanced pancreatic cancer (LAPC).
Fifteen patients with LAPC were included in this study. The SIB method was performed using five fields according to our standard protocol. A total dose of 67.5 Gy(relative biological effectiveness [RBE]) was prescribed in 25 fractions using the SIB method. A dose of 45 Gy(RBE) was prescribed for the entire planning target volume (PTV) by using four main fields. The remaining 22.5 Gy(RBE) was prescribed to the PTV excluding for the gastrointestinal tract using one subfield. Five simulated doses were obtained by the forward dose calculations with the Hounsfield units (HU) override to the large bowel to 50, 0, -100, -500, and -1000, respectively. The dose-volume indices in each plan were compared using the 50 HU plan as a reference.
At D of the clinical target volume (CTV) and spinal cord-D , when the density of the large bowel was close to that of gas, there were significant differences compared to the reference plan (p < 0.05). By contrast, no significant difference was observed in stomach-D duodenum-D , small bowel-D , kidneys-V , and liver-D under any of the conditions. There were no cases in which the dose constraint of organs at risk, specified by our institution, was exceeded.
Density change in the large bowel was revealed to significantly affect the doses of the CTV and spinal cord during PT with SIB for LAPC. For beam arrangement, it is important to select a gantry angle that prevents the large bowel from passing as much as possible. If this is unavoidable, it is important to carefully observe the gas image on the beam path during daily image guidance and to provide adaptive re-planning as needed.

© 2021 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, LLC on behalf of The American Association of Physicists in Medicine.