ESTRO 2021 Abstract Book

S529

ESTRO 2021

Onkologikoa - UGC Oncología Gipuzkoa, Radiation Oncology, San Sebastián, Spain; 3 Hospital Universitario Donostia, Urology, San Sebastian, Spain; 4 V, Radiation Oncology, San Sebastian, Spain; 5 Fundación Onkologikoa - UGC Oncología Gipuzkoa, Medical Physics, San Sebastian, Spain; 6 Fundación Onkologikoa - UGC Oncología Gipuzkoa, Medical physics, San Sebastian, Spain Purpose or Objective To determinate and compare long-term biochemical control rates, survival and toxicity outcomes between high-dose-rate brachytherapy (HDR-BT) boost schemes (single 15Gy fraction versus double 9.5Gy fraction) in treatment of high-risk prostate cancer patients. Materials and Methods Between january-2007 and december-2017, 361 patients with high-risk prostate cancer were treated in our institution with external beam radiotherapy (EBRT) plus CT-based iridium192 HDR-BT boost; either with 15Gy single fraction (15Gy-BT) boost or with double 9.5Gy fraction (2x9.5Gy-BT) boost administered 6 hours apart in a single implant. All patients received EBRT 46Gy in 23 fractions of pelvic irradiation encompassing the prostate and seminal vesicles after HDR-BT boost. Those with less than 3-year follow-up and less than 3 post treatment prostate-specific antigen (PSA) were excluded. Clinical outcomes and toxicity were evaluated and compared in these BT boost schemes. Toxicity was evaluated using Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. Results About 325 from 361 patients satisfied inclusion criteria; 180 treated with 15Gy-BT boost and 145 treated with 2x9.5Gy-BT boost. The median follow-up was 7 years for the 15Gy-BT cohort and 11 years for the 2x9.5Gy-BT cohort. Median age was 70 years (range: 47-83) for both cohorts and 91.8% of patients received androgen deprivation therapy (ADT). Freedom from biochemical failure (FFBF) defined by Phoenix-criteria was 87.8% at 5 years and 82.2% at 10 years in 15Gy-BT group, and 88.5% and 80% at 5 and 10 years in 2x9.5Gy-BT group, respectively. Similarly, there was no statistical significant difference in metastasis free survival (MFS) at 5 and 10 years; 93.9% and 91.1% for 15Gy-BT group and 94.5% and 87.6% for 2x9.5Gy-BT group, respectively. There was no difference in overall survival (OS) at 10 years between schemes; 84.6% and 66.9% for the 15Gy-BT group and the 2x9.5Gy-BT group, respectively. Six months after treatment, genitourinary (GU) grade 3 or higher toxicity was 2.2% in the 15Gy-BT group and 4.1% in the 2x9.5Gy-BT group, with no grade 3 or higher acute gastrointestinal (GI) toxicity in both groups. GU grade 3 or higher late toxicity was statistically unfavorable for 2x9.5Gy-BT cohort: 9.4% and 11.7% at 5 and 10 years in 15Gy-BT group, and 18,6% and 27,6% at 5 and 10 years in 2x9.5Gy-BT group, respectively. No significant difference was found in GI grade 3 or higher late toxicity: 1.7% for both 5 and 10 years in the 15Gy-BT group and 0.7% at 5 years and 2.1% at 10 years in the 2x9.5Gy-BT group, respectively.