Identification of a rectal subregion highly predictive of rectal bleeding in prostate cancer IMRT

Background and purpose: To identify rectal subregions at risks (SRR) highly predictive of 3-year rectal bleeding (RB) in prostate cancer IMRT. Materials and methods: Overall, 173 prostate cancer patients treated with IMRT/IGRT were prospectively analyzed, divided into ``training'' (n =118) and ``validation'' cohorts (n = 53). Dose-volume histograms (DVHs) were calculated in three types of rectal subregions: ``geometric'', intuitively defined (hemi-rectum,...); ``personalized'', obtained by non-rigid registration followed by voxel-wise statistical analysis (SRRp); ``generic'', mapped from SRRps, located within 8 x 8 rectal subsections (SRRg). DVHs from patients with and without RB were compared and used for toxicity prediction. Results:- Training cohort SRRps were primarily within the inferior anterior hemi-rectum and upper anal canal, with 3.8 Gy mean dose increase for Grade >= 1 RB patients. The SRRg, representing 15% of the absolute rectal volume, was located in 10 inferior-anterior rectal subsections. V18-V70 for SRRps and V58-V65 for SRRg were significantly higher for RB patients than non-RB. Maximum areas under the curve (AUCs) for SRRp and SRRg RB prediction were 71% and 64%, respectively. The validation cohort confirmed the predictive value of SRRg for Grade >= 1 RB. The total cohort confirmed the predictive value of SRRg for Grade >= 2 RB. Geometrical subregions were not RB predictors. Conclusion: The inferior-anterior hemi anorectum was highly predictive of RB. (C) 2016 Elsevier Ireland Ltd. All rights reserved.