Comparison of small bowel and bladder doses between prone and supine positions in preoperative intensity-modulated radiotherapy for rectal cancer.

Evidence supporting the benefit of the prone position (PP) over the supine position (SP) in preoperative intensity-modulated radiotherapy (IMRT) for locally advanced rectal cancer remains limited. Furthermore, criteria available at simulation for selecting patients likely to benefit from PP are lacking. This study assessed the dosimetric impact of PP versus SP on the small bowel and bladder and explored factors associated with PP-related dose sparing to identify patients likely to benefit. We retrospectively analyzed simulation CT scans in both PP and SP from consecutive rectal cancer patients treated with preoperative radiotherapy (July 2019–April 2024). Paired IMRT plans were created for each position, and dose-volume parameters for the small bowel and bladder were compared using the Wilcoxon signed-rank test. Predictors of small bowel dose reduction were assessed using univariable and multivariable regression analyses, including a novel indicator, the ratio of abdominal protrusion, defined as the protrusion at the sacral promontory level in SP divided by that in PP. Among 24 patients, mean small bowel V40Gy was lower in PP (56.2 vs. 72.6 cm3, p = 0.03) and mean bladder V25Gy–V50Gy were consistently lower in PP (all p < 0.01). A lower ratio of abdominal protrusion was associated with greater small bowel V40Gy reduction in univariable and multivariable analyses (p = 0.01 in each). PP reduced high-dose small bowel exposure and mid- to high-dose bladder exposure. CT simulation in both positions to calculate the abdominal protrusion ratio may guide individualized positioning.