Visceral adipose ratio as a novel predictor for acute bowel toxicity in patients receiving pelvic radiation: Preliminary dosimetric analysis and clinical validation.

Abstract

During radiation treatment planning, the small bowel (SB) is often contoured as a 'bowel bag' encompassing the entire peritoneal space that may be occupied by SB. This method incorporates large volumes of visceral adipose, potentially resulting in misleading estimates of radiation dose to the SB. We evaluated the relative volume of adipose within the peritoneal space and applied this as a correction factor to standard bowel bag dosimetric measures, hypothesizing that corrected SB measures would better correlate with acute toxicity. Eighteen consecutive patients receiving pelvic radiation for gynaecologic cancers over a 1-year period at an academic medical centre were included. Bowel function was assessed with the Expanded Prostate Index Composite (EPIC) questionnaire. Bowel bags were contoured on simulation computed tomography (CT) scans. Adipose was auto-contoured using previously published Hounsfield Unit criteria and used to create an adipose correction factor (ACF). The ACF was applied to V45cc and V40% volumes to create adipose corrected measures (AC-V45cc and AC-V40%). Correlations between EPIC scores and dosimetric measures were assessed using Spearman coefficients. V45cc and V40% did not correlate with overall EPIC bowel domain score; however, AC-V40% did show a significant correlation (P=0.02). Correlations of V45cc and V40% with the bowel bother subdomain of EPIC were both significantly improved by applying the ACF (P=0.02 for AC-V45cc; P<0.01 for AC-V40%). Adipose corrected bowel bag dosimetric constraints correlate better with acute bowel toxicity than current standard practice. Longer follow-up is needed to determine if similar findings are seen with late toxicity.