Proctitis after conventional external radiation therapy for prostate cancer: importance of minimizing posterior rectal dose.

PURPOSE: To identify factors associated with the prevalence of radiation proctitis in patients with prostate cancer treated with external radiation therapy and to determine if a dose-response relationship exists. MATERIALS AND METHODS: The study included 101 prostate cancer patients treated at the University of Minnesota Hospital and Clinics between 1970 and 1987. Radiation therapy at a daily dose of 175-200 cGy was given for 5 consecutive days per week for 7 weeks with a 10-24-MV linear accelerator. Mean dose to the prostate was 6,795 cGy (range, 5,579-7,417 cGy). Simulation radiographs obtained with rectal barium were analyzed in 51 patients. Doses were calculated at specific points in the boost field. RESULTS: Radiation proctitis developed in 19 patients. No pretreatment or treatment factors were associated with a higher prevalence of proctitis. Statistically significant correlation (P < .042) was found between the development of proctitis and dose to the posterior rectum. At chi 2 analysis, proctitis was statistically significantly (P < .005) associated with posterior rectal doses of > 5,000 cGy. CONCLUSION: A high posterior rectal dose (> 5,000 cGy) is associated with increased prevalence of proctitis after radiation therapy.