LAUSR

PURPOSE To elucidate potential risk factors important for the appearance of late rectal toxicity (LRT) after high-dose-rate boost treatment (HDRBT) of prostate cancer and to validate the predictive value of the minimal dose to the most exposed 2 cc of rectum received with HDRBT (D2ccrect). METHODS AND MATERIALS The study of LRT, defined as relative deterioration of defecation problems (RDDP) (stool frequency, pain, rectal bleeding, fecal urgency, and incontinence) during follow-up period, was carried out on 88 patients, consecutively treated from October 2006 through April 2011 with HDRBT of 3 × 6-7 Gy to 50-50.4 Gy of EBRT. The impact of patients and treatment characteristics on third year prevalence of RDDP was analyzed by using binary logistic regression method. RESULTS At third year of follow-up, RDDP was evidenced in 30 of 77 (39.0%) patients. More important as D2ccrect (OR, 1.15; 95% CI, 0.99-1.34; p = .059) was minimal dose to the most exposed 1 cc of the rectum (D1ccrect; OR, 1.15; 95% CI, 1.01-1.31; p = .032), whereas the sum of D1ccrect and EBRT mean rectal dose (EDmeanrect) was the only significant parameter in multivariate analysis (OR, 1.12; 95% CI, 1.04-1.22; p = .004). Based on a multivariate model, the safe compound 2 Gy equivalent dose was estimated at 44.4 Gy with the average ratio of D1ccrect:EDmeanrect = 1:3.1 (95% CI ± 1.8) and negative predictive value of 0.828. CONCLUSIONS The study confirms the value of composite dose parameter and the importance of rectal high-dose and low-dose regions for LRT. Taking account of suggested dose constraints and CT/MRI-based HDRBT, the incidence of LRT can be reduced by a half.