3209 - High Dose Rate Brachytherapy Boost in Intermediate and High Risk Localized Prostate Cancer: A Retrospective Study on Toxicity and Clinical Outcomes

12:45pm - 02:00pm PT

Hall F

Screen: 10

POSTER

Presenter(s)

Imtiaz Ahmed, MB, BS, MRCP, FRCR - Southend University Hospital NHS Foundation Trust, Essex SSO ORY, Essex

M. Choudhury¹, F. Adeoye¹, J. Amalraj¹, H. Wasee¹, K. Tun², D. Lobo³, D. Muthukumar³, D. Dawam¹, and I. Ahmed¹; ¹Southend University Hospital, Mid and South Essex NHS Foundation Trust, Southend-on-sea, United Kingdom, ²Addenbrooke's Hospital, Cambridge University Hospitals, Cambridge, United Kingdom, ³Colchester Hospital, East Suffolk and North Essex NHS Foundation Trust, Colchester, United Kingdom

Purpose/Objective(s):

This retrospective analysis evaluated long-term outcomes and toxicity profiles in intermediate and high-risk localized prostate cancer (PC) patients treated with external beam radiotherapy (EBRT) and a high dose-rate brachytherapy boost (HDR-BTB).

Materials/Methods:

We analyzed 607 patients with localized PC (T2-T4) treated between 11/2012 and 12/2022. Initial staging comprised MRI, CT pelvis, isotope bone scan, serum prostate-specific antigen (PSA) and histopathological assessment. All patients received EBRT (46 Gy/23 fx or 37.5 Gy/15 fx) with a single 15 Gy HDR-BTB. Disease was stratified using the International Society of Urological Pathology (ISUP), TNM classification, risk group and Cambridge Prognostic Group (CPG) grade. gastrointestinal (GI) and genitourinary (GI) toxicities were assessed using Common Terminology Criteria for Adverse Events (CTCAE V5.0). Primary endpoints were failure-free survival (FFS), radiographic progression-free survival (rPFS), and metastasis-free survival (MFS). Secondary endpoints included overall survival (OS), PC-related OS (PCROS), grade =3 GI and GI toxicities, impact of HDR sequencing and specialist referral patterns.

Results:

With a median follow-up of 70 months, 41.35% had CPG grade 5 disease with median baseline PSA of 12.72 ng/mL. PSA nadir averaged 0.01 ng/mL, with biochemical, radiological and distal metastatic recurrence rates of 8.07%, 7.41% and 5.27%, respectively. 82.1% of radiological relapses were managed with LHRH agonists alone. Grade 3 GI and GU events occurred in 0.33% (n=2) and 1.98% (n=12) of patients, respectively. One patient each (0.16%) experienced grade 4 GI and GU toxicity. 3.3% (n=20) and 5.2% (n=32) patients were referred to urology and gastroenterology, respectively. Treatment sequencing and prostate volume had no impact on toxicity, though an HDR-BT-first approach demonstrated increased specialist referrals (n= 42 vs 20). Five-year survival estimates were: FFS 93.78%, rPFS 94.46%, MFS 95.90%, OS 97.39%, and PCROS 98.39%. Among 56 deaths, 76.8% (n= 43) were not PC-related. Age =70 years correlated with reduced OS (98.35% vs 96.37%, p=0.0283). Higher ISUP grades were associated with poorer clinical outcomes (HR: 3.29, p<0.001) with a 5-year OS of 91.9% (ISUP Grade 5) vs. 98.5% (ISUP Grade 2). ISUP grade and TNM stage were stronger predictors of disease progression than CPG or risk grouping. Extended androgen ablation therapy (AAT) (>2 years) conferred no additional benefit.

Conclusion:

EBRT with single-fraction HDR-BTB achieves durable disease control, potentially outperforming the adoption of systemic therapies like abiraterone in the localized high-risk setting (STAMPEDE trial). The low incidence of grade =3 toxicity (<2%) confirms its safety in an older population. ISUP grade and TNM staging emerged as superior prognostic indicators, with treatment sequencing not impacting relapse and survival. Extended AAT conferred no additional benefit, supporting de-escalation after 24 months.