Predictors of acute bowel toxicity in patients treated with IMRT whole pelvis irradiation after prostatectomy.

PURPOSE/OBJECTIVE: Whole pelvis irradiation with IMRT (WPRT-IMRT) after prostatectomy is efficient in reducing acute toxicity: however, a number of patients still experience moderate acute bowel toxicity. MATERIALS AND METHODS: Ninety-six patients treated with WPRT-IMRT after prostatectomy with adjuvant or salvage intent were analysed. A number of parameters were individually recovered, including the DVHs of the intestinal cavity outside PTV and of the loops referred to both the WPRT phase and the whole treatment. Correlation between clinical-dosimetric parameters and acute bowel toxicity was investigated by logistic analyses. Best predictive cut-off values for continuous variables were assessed by ROC curves. RESULTS: 15/96 (15.6%) Patients experienced grade 2 toxicity (no grade 3). Best dose-volume predictors were the fraction of loops receiving more than 45, 50 and 55Gy (respectively, V45TL50cc, V50TL13cc, V55TL3cc; p-values ranging from 0.005 to 0.027). Age, GU acute toxicity, rectal acute toxicity and time between prostatectomy and IMRT were also predictors of acute bowel toxicity. Multivariate analysis showed that the most predictive independent parameters were age (OR: 1.13; 95%CI: 1.02-1.25; p=0.021) and V50TL (13cc, OR: 8.2; 95%CI: 1.7-40; p=0.009). CONCLUSIONS: The risk of moderate acute uGI toxicity during WPRT-IMRT for post-operatively treated patients increases with age; the risk is substantially reduced in patients with small overlap between PTV and loops.