Predicting acute small-bowel toxicity in patients with rectal cancer treated with neoadjuvant chemoradiation: A comparison of peritoneal space versus small-bowel loop contouring techniques.

Abstract

673 Background: In lieu of contouring individual small bowel loops, the peritoneal space (PS) has been suggested as a possible surrogate volume for predicting small bowel toxicity. A dose-volume relationship for the PS has not been firmly established. The aim of this study was to determine whether contours of the PS better predict significant acute small bowel toxicity in neoadjuvant rectal cancer patients when compared with contours of individual small bowel loops. Methods: A standardized contouring method was developed for the PS and retrospectively applied to the radiation treatment plans of sixty-seven patients treated with neoadjuvant chemoradiotherapy for rectal cancer. All patients had locally advanced disease, no evidence of metastases, and received concurrent radiation and infusional 5-Fluorourocil chemotherapy. Dose-volume histogram (DVH) data was extracted and analyzed against patient toxicity. Receiver operating characteristic analysis and logistic regression was carried out for both contouring methods. Results: Grade ≥ 3 small bowel toxicity occurred in 16% (11/67) of patients. Volumes of the contoured small bowel loops correlated with grade ≥ 3 toxicity at each 5 Gray (Gy) dose level from 5 to 45 Gy, with the greatest area under the curve (AUC) measuring .964 (p=.000) associated with the volume receiving at least 25 Gy (V25). Volumes of the contoured PS predicted toxicity from 5 to 40 Gy, with the greatest AUC also at the V25 and measuring .896 (p=.000). Logistic regression analysis demonstrated a less than 15% risk of acute grade ≥ 3 toxicity was associated with a V25 of 215 cc for the small bowel and 710 cc for the PS. Conclusions: DVH analysis of peritoneal space volumes predicts grade ≥ 3 small bowel toxicity in neoadjuvant rectal cancer patients, suggesting the peritoneal space is a reasonable surrogate for contouring individual small bowel loops. However, contouring individual small bowel loops is a more sensitive method for predicting toxicity at each dose increment. For both contouring methods, the greatest sensitivity for predicting toxicity was associated with the volume receiving at least 25 Gy (V25).