High-dose-rate brachytherapy combined with external beam radiotherapy for localized prostate cancer: Correlation between clinical and dosimetric parameters and the incidence of rectal bleeding.
213 Background: Several investigators have advocated that the alpha/beta ratio for prostate cancer is atypically low, and that hypofractionated radiotherapy or high-dose-rate brachytherapy (HDR-BT) regimens using appropriate radiation doses are expected to improve the local control rate for localized prostate cancer. However, the increase in the total biological effective dose (BED) may cause increased severity and incidence of normal tissue complications. The purpose of this study was to investigate what clinical and dosimetric factors affected the incidence of rectal bleeding after HDR-BT combined with external beam radiotherapy (EBRT). Methods: A total of 143 patients with localized prostate cancer underwent HDR-BT combined with EBRT. The fractionation schema for HDR-BT and EBRT was prospectively changed: 9 Gy x 2 + 2 Gy x 20 (BED1.5 = 219 Gy, BED3 = 139 Gy) in 57 patients (Group 1); and 9 Gy x 2 + 3 Gy x 13 (BED1.5 = 243 Gy, BED3= 150 Gy) in 86 patients (Group 2). Median follow-up was 59 (range, 36 – 94) months. The toxicities were graded based on the National Cancer Institute-Common Terminology Criteria for Adverse Events v3.0. Results: Sixteen (11.2 %) and one patients developed Grade 2 and 3 rectal bleedings, respectively. There were no significant differences between Group 1 and Group 2 in the incidence of Grade 2 and 3 rectal bleedings (12.3% and 11.6%, respectively). Grade 2 and 3 rectal bleedings occurred much more in the patients receiving the antiplatelet therapy (AT) (19.4%) than those without a history of AT (9.8%) and in the patients with diabetes mellitus (DM) (37.5%) than those without DM (10.4%). However, neither AT nor DM were risk factors in univariate analysis. Regarding dosimetric factors, V75 > 2cc, V90 > 0.2cc, D2 > 7 Gy, and D1 > 7.4 Gy were statistically-significant risk factors. In multivariate analysis, DM was the only statistically-significant risk factor. Conclusions: BED escalation could be performed without severe rectal bleeding in HDR-BT combined with EBRT. V75 > 2cc, V90 > 0.2cc, D2 > 7 Gy, D1 > 7.4 Gy, and DM were risk factors for the incidence of rectal bleeding in HDR-BT combined with EBRT. Clinical trial information: 18-9.