Incidence And Dosimetric Predictive Factors Of Late Rectal Toxicity After Low-Dose-Rate Brachytherapy Combined With Volumetric Modulated Arc Therapy In High-Risk Prostate Cancer At A Single Institution: Retrospective Study

PURPOSE: The purpose of this study is to investigate the incidence of rectal toxicity and to identify the associated dosimetric predictive parameters after I-125 seed low-dose-rate brachytherapy (LDR-BT) combined with volumetric modulated arc therapy (VMAT) and dose constraints.METHODS AND MATERIALS: In total, 110 patients with high-risk prostate cancer received 110 Gy LDR-BT, followed by 45 Gy VMAT. Rectal toxicity was recorded according to Common Terminology Criteria for Adverse Events v.4.03. The dosimetric factors associated with LDR-BT and VMAT were analyzed to determine their relationship with rectal toxicity. Receiver operating characteristic (ROC) curve analysis was performed for $ grade 2 (G2) rectal toxicity prediction.RESULTS: The follow-up duration was 10.1-115.2 months (median 60.5 months). Seven patients had G2 rectal hemorrhage, and none of the patients had grade 3 rectal hemorrhage. In the univariate analysis, the rectal volume receiving 100% of the prescribed dose (rV100) (p ! 0.001), the dose covering 2 cc of the rectum (rD2cc) during LDR-BT (p = 0.002), and the combined rD2cc during LDR-BT and VMAT (p = 0.001) were identified as predictors of G2 rectal hemorrhage. In the ROC curve analysis, the cutoff value was 0.46 cc for rV100, 74.0 Gy for rD2cc, and 86.8 GyEQD2 for combined rD2cc.CONCLUSION: Predictors of late $ G2 rectal hemorrhage are rV100, rD2cc, and combined rD2cc. The incidence of rectal toxicity is low and acceptable in this setting and is highly dependent on the rectal dose of LDR-BT. The use of higher-quality LDR-BT and VMAT dose constraints may further reduce the rate of rectal hemorrhage. (c) 2020 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.