Anatomical and clinical predictors of acute bowel toxicity in whole pelvis irradiation for prostate cancer with Tomotherapy.

PURPOSE Assessing predictors of acute bowel toxicity after whole-pelvis irradiation (WPRT) Image-guided Tomotherapy with simultaneous integrated boost on prostate/prostate bed. METHODS AND MATERIALS In the period March 2005-April 2009, 178 patients were treated with radical or adjuvant/salvage intent with WPRT Tomotherapy. Median dose to the pelvic nodes was 51.8 Gy/28 fractions while concomitantly delivering 65.5-74.2 Gy to prostate/prostatic bed. The impact of many anatomical and clinical parameters on ≥ Grade 2 acute bowel toxicity was investigated by logistic analyses. RESULTS Only 15/178 patients (8.4%) experienced Grade 2 toxicity (none Grade 3). Main predictors at univariate analysis were nodal CTV (CTVN ≥ 380 cc; OR: 3.7, p=0.017), treatment duration (< 40 days; OR: 6.2, p=0.006) and Grade 2 acute rectal toxicity (OR: 6.5, p=0.015). A multivariate analysis including only pre-treatment variables revealed an independent role of CTVN and age; if including treatment-related factors the best predictors were age, treatment duration and Grade 2 rectal toxicity. This last was correlated with the overlap between PTVN and loops (OVPN ≥ 51 cc; OR: 14.4, p=0.0003) that is representative of the volume of loops receiving the prescribed dose (51.8 Gy, 1.85 Gy/fr). CONCLUSIONS Acute bowel toxicity after WPRT Tomotherapy is mild, relatively rare and associated to larger CTVN and older age. While efforts to further reduce it do not appear to be relevant, the pre-treatment assessment of "high-risk" patients may help physicians in better managing symptoms. A prospective validation would be very important in confirming these results and in better refining dose-volume bowel effects including symptoms milder that the ones here investigated and retrospectively assessed.