ESTRO 2025 - Abstract Book

S369

Brachytherapy - Urology

ESTRO 2025

140

Proffered Paper 20-year outcomes of Low-Dose-Rate versus High-Dose-Rate brachytherapy boost in prostate cancer: A comparative study Alai Goñi Ramirez 1 , Macarena Sevilla 2 , Maider Alberich 2 , Mikel Larruskain 2 , Darya Chyzhyk 2 , Alfonso Gomez Iturriaga 3 , Mikel Egiguren 1 , Vicent Pastor 4 , Albert Bartres 4 , Eva María Sáenz de Urturi 1 , Daniel Alberto Roura 1 , Maria Pagola 1 , Amaia Sanchez 1 , Usoa Iceta 1 , Ane Mugica 1 , Nuria Bultó 1 , Maider Campo 1 , Leyre Gonzalez 1 , Intza Uranga 1 , Ane Otaegui 1 , Beraldo Martinez 1 , Xabier Gurutzeaga 1 , Ane Dehesa 1 , Julian Minguez 1 , Arrate Querejeta 1 1 Radiation Oncology, Onkologikoa - UGC Oncología Gipuzkoa, San Sebastian, Spain. 2 Data Science, NARU Intelligence, San Sebastian, Spain. 3 Radiation Oncology, Hospital Universitario Cruces, Baracaldo, Spain. 4 Medical Physics, Onkologikoa - UGC Oncología Gipuzkoa, San Sebastian, Spain Purpose/Objective: Previous studies from the same institution, evaluating up to 10-year outcomes and toxicities in patients treated with high-dose-rate (HDR) and low-dose-rate (LDR) brachytherapy (BT) boost plus external beam radiotherapy (EBRT), demonstrated excellent survival rates with no significant differences between groups, except for higher genitourinary (GU) toxicity in the LDR-BT group. This study aims to extend the follow-up to 20 years, validate prior findings, and identify risk factors associated with long-term outcomes. Material/Methods: Between 2001 and 2015, 934 patients were treated at a single institution with either HDR-BT+EBRT (n=378) or LDR BT+EBRT (n=556). Mean follow-up was 12 years, with a maximum of 20 years. Key covariates included PSA at diagnosis, tumor stage, prostate volume, androgen deprivation therapy, and Gleason score. These were adjusted using Inverse Probability of Treatment Weighting (IPTW), with extreme weights trimmed per Stürmer’s strategy. Survival outcomes for biochemical recurrence-free (BFS), metastasis-free (MFS), cancer-specific (CSFS), GU toxicity free (GUTFS), and gastrointestinal toxicity-free (GITFS) were analyzed using Kaplan-Meier and IPTW-corrected Cox proportional hazards regression. Results: Median age was 70.6 years (Q1 65.4; Q3 74.4). The most common Gleason score was 7 (46.8%), median PSA at diagnosis was 10.3 (Q1 7; Q3 15.5), and the most frequent tumor stage was T2c (25.5%). After IPTW correction, 536 patients (158 HDR, 378 LDR) were included, with balanced covariates (effect sizes < 0.1). No significant differences were found in 20-year BFS, MFS, CSFS, or GITFS between the groups. HDR patients had 20 year BFS, MFS, CSFS, and GITFS rates of 82.3%, 91%, 94.1%, and 95%, respectively, while LDR patients had rates of 76.2%, 85%, 96.3%, and 87.6%. Grade 3 GU toxicity was significantly worse in the LDR group (p < 0.01), with 20-year GUTFS of 59.8% for HDR vs. 57.9% for LDR. At 15 years, GUTFS was 80.2% for HDR and 69.5% for LDR. Multivariate regression revealed Gleason score >7 was significantly associated with higher cancer-specific mortality (HR = 11.5), biochemical recurrence (HR = 2.08), and metastasis (HR = 2.59). Higher PSA at diagnosis was also associated with worse outcomes, including cancer-specific mortality (HR = 1.035), BFS (HR = 1.033), and metastasis (HR = 1.035). LDR brachytherapy significantly increased the risk of grade 3 GU toxicity (HR = 2.29) compared to HDR.