S153
Brachytherapy - Urology
ESTRO 2026
survival rates were 86.2%, 98.7%, and 86.7%, respectively. Patients with biochemical failure showed significantly higher PSA nadir values compared with those without failure (median 0.40 vs. 0.09 ng/ml, p < 0.001). No gastrointestinal grade 2–3 toxicities were observed. Acute genitourinary grade 2–3 toxicity occurred in 11.1% of patients, and late genitourinary grade 2–3 toxicity in 8.9%. Conclusion: HDR brachytherapy as monotherapy for localized prostate cancer provides excellent long-term biochemical control with minimal toxicity. PSA nadir significantly correlates with biochemical failure. With a median follow-up of nearly 8 years in over 200 patients, our results are consistent with published series and confirm HDR monotherapy as a safe and effective treatment option for selected low- and intermediate-risk patients. Keywords: HDR brachytherapy, monotherapy, prostate cancer Poster Discussion 539 From Seeds to SABR: Insights from LDR Brachytherapy Dose Patterns to Guide Prostate Focal Boost Strategies David Quigley 1 , Orla Houlihan 2 , Sergio Esteve 3 , Geraldine Workman 3 , Monica Byrne 3 , Eoin Napier 4 , Kevin M Prise 5 , Alan Hounsell 3 , Darren Mitchell 1 , Suneil Jain 1,5 1 Department of Clinical Oncology, Northern Ireland Cancer Centre, Belfast Health and Social Care Trust, Belfast, United Kingdom. 2 Department of Clinical Oncology, The Beacon Cancer Centre, Musgrove Park Hospital, Taunton, Somerset, United Kingdom. 3 Department of Radiotherapy Medical Physics, Northern Ireland Cancer Centre, Belfast Health and Social Care Trust, Belfast, United Kingdom. 4 Department of Radiology, Northern Ireland Cancer Centre, Belfast Health and Social Care Trust, Belfast, United Kingdom. 5 Patrick G. Johnston Centre for Cancer Research, Queen's University Belfast, Belfast, United Kingdom Purpose/Objective: With increasing adoption of MR simulation and stereotactic ablative radiotherapy (SABR), there is growing interest in focal dose escalation to the dominant intraprostatic lesion (DIL). Low-dose-rate (LDR) prostate brachytherapy is associated with much greater heterogeneity of dose than SABR (prostate doses ranging from 120Gy to 200Gy within an implant), providing a model to study the importance of dose to DIL for prostate cancer outcomes. To date, no study has examined dose to DIL and freedom from biochemical/clinical failure (BCF) in patients treated
cc.SIMULATION RESULTSA Python-based simulation of 10,000 random patient pairs (combined volume >85 cc) showed that only 215 pairs (2.15%) experienced a deficit, and in just 17 cases (0.17%) was the deficit greater than 10 seeds. Conclusion: The proposed approach achieves two objectives: (1) a reduction of approximately 10% in the number of ordered seeds, representing about 36% of treated cases, and (2) a significant decrease in the incidence of seed shortages. This strategy ensures, with 99% confidence, that subsequent patients will have surplus seeds available, improving efficiency, safety, and environmental sustainability in prostate brachytherapy. Keywords: seeds, prostate Long-term outcomes of HDR brachytherapy as monotherapy for localized prostate cancer: a single-institution experience Miren Gaztañaga 1,2 , Virginia Álvarez 3 , Zulima Aza 3 , Carmen Escribano 1 , Beatriz Montero 1 , María Inés Sevillano 1 , Manuel Gonzalo Vázquez 1 1 Radiation Oncology, Hospital Clínico San Carlos, Madrid, Spain. 2 Radiation Oncology, Genesis Care, Madrid, Spain. 3 Medical Physics, Hospital Clínico San Carlos, Madrid, Spain Poster Discussion 500 Purpose/Objective: High-dose-rate (HDR) brachytherapy as monotherapy has emerged as an effective treatment for localized prostate cancer. With TRUS guidance, it offers excellent dosimetric precision and minimal toxicity. The purpose of this study was to analyze long-term oncological outcomes and toxicity in a cohort of patients treated with HDR brachytherapy at our institution. Material/Methods: Between 2013 and 2021, a total of 225 patients with clinically localized prostate cancer were treated with HDR brachytherapy alone. The prescribed dose was 13.5 Gy per fraction, delivered in two sessions two weeks apart. Biochemical failure was defined by the Phoenix criteria, and toxicity was graded using the Common Terminology Criteria for Adverse Events
(CTCAE). Results:
Based on the NCCN risk classification, 142 patients were low-risk and 81 were favorable intermediate-risk. Median age was 72 years. Thirty-two patients received short-term androgen deprivation therapy (ADT) for a median duration of six months. With a median follow- up of93.6 months, the biochemical failure-free survival, prostate cancer–specific survival, and overall
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