Background:Although various studies have been conducted on the effects of radiation therapy for prostate cancer,rectal toxicity after radiation therapy for prostate cancer,which is an important late adverse event associated with radiation therapy,has not been sufficiently examined.This study aimed to assess the associations of late rectal disorder (LRD) with dosimetric,anatomic,and clinical factors in patients with prostate cancer who underwent three-dimensional conformal radiation therapy (3D-CRT).Methods:We retrospectively analyzed 104 patients undergoing 3 D-CRT between January 2009 and October 201 l.Thirty patients were administered anticoagulation/antiplatelet (AC/AP) agents.The standard dose was 74 Gy.Uni-and multi-variate analyses were performed to identify factors predictive of LRD after 3D-CRT.Results:The median follow-up period was 66 (range:14-87) months.LRD occurred in 10.6％ (11/104) of patients.The median time from RT to LRD was 15 months (range:7-41 months).Sixty-four percent of those with LRD (7/11 patients) had been given AC/AP agents.Fifty-five (6/11) patients had severe intal iliac artery calcification.By univariate analysis,significant predictors of LRD were intal iliac artery calcification,administration of AC/AP agents,and age.Being very elderly was the significant predictor identified by multivariate analysis (P =0.0276).For patients receiving AC/AP agents and those with severe intal iliac artery calcification,the LRD incidences were 23.3％ (7/30 patients) and 23.1％ (6/26 patients),respectively,and being 75 years of age or older was a significant predictor in these subsets.Conclusions:Our results suggest advanced age,administration of AC/AP agents,and severe intal iliac artery calcification to be risk factors for LRD in patients undergoing standard RT.Therefore,it is necessary to administer radiation with particular caution in the very elderly,especially those receiving AC/AP agents and/or with severe intal iliac artery calcification.