S1163
Clinical - Urology
ESTRO 2026
(n=224) received ADT (<6 months n=19, 6 months n=201,>6 months n=4). At a median follow-up of 25.8 months (17.5months–39.7 months), four biochemical recurrences and three non-PCa related deaths were recorded. PSA trajectory modelling and clustering identified three distinct patterns: PSA-stable, PSA- steady rise, and PSA-unstable. BCR risk was significantly higher in the PSA-unstable cluster compared to PSA-stable and PSA-steady rise clusters across all groups; for the whole cohort (p < 0.01), ADT- treated subgroup (p < 0.001), and non-ADT subgroup (p < 0.01) while pre-treatment conventional prognostic factors (age, T stage, Gleason score, baseline PSA, or CPG) were not significantly associated with BCR in any group.
maximize cure potential. Keywords: Prostate cancer, Intermediate-risk, Radiotherapy.
Digital Poster 994
Early PSA trajectories as a surrogate biomarker for treatment failure in low and intermediate risk prostate cancer after SBRT and variable use of ADT. Vodathi Bamunuarachchi 1 , Zhu-Chuen Oong 1 , Ereny Saad 1 , Jane Shortall 1,2 , Hitesh Mistry 2 , Alan McWilliam 1,2 , Ruth Conroy 1 , Maria Serra 1 , Anna Tran 1 , Hanis Hanafi 1 , Martin Swinton 1 , Andrew Hudson 1 , John Logue 1 , James Wylie 1 , Peter Hoskin 1 , Yeepei Song 1 , Ananya Choudhury 1,2 1 Clinical Oncology, The Christie NHS Foundation Trust, Manchester, United Kingdom. 2 Division of Cancer Sciences, The University of Manchester, Manchester, United Kingdom Purpose/Objective: Patients with low and intermediate risk prostate cancer (PCa) treated with stereotactic body radiotherapy (SBRT) achieve excellent oncological outcomes [1,2]. A small subset recure, and early identification of these patients remains challenging. Emerging evidence suggests that prostate specific antigen (PSA) dynamics can serve as an early predictor of treatment outcomes following conventional or moderately hypo-fractionated radiotherapy [3]. We analysed PSA dynamics after SBRT for PCa, accounting for variable use of androgen depravation therapy (ADT). Material/Methods: This was a single-institution retrospective cohort study of men with PCa treated with SBRT (36.25 Gy in 5 fractions over 2 weeks). Patient demographics, tumour characteristics and post-radiotherapy PSA follow up values were collected. Log-transformed PSA follow-up values were modelled using a multi-task, population- based, Gaussian Process regression (MagmaClustR), and hierarchical clustering applied to define characteristic PSA trajectories. PSA measurements up to and including biochemical recurrence (BCR) (nadir+2ng/ml) were included. For comparison, analysis was performed for the whole cohort, and separately for ADT-treated and non-ADT treated subgroups. Kaplan–Meier analyses stratified by pre- treatment risk groups and characteristic PSA trajectory were performed to compare biochemical relapse-free
Conclusion: BCR risk was highest among patients within the PSA- unstable trajectory cluster. Early post-treatment PSA dynamics may serve as a surrogate marker for identifying patients at risk of treatment failure following SBRT for prostate cancer, irrespective of ADT use. This prognostic association appears to be independent of conventional pre-treatment risk factors and may enable earlier intervention in this high-risk subset of patients. References: 1. As et al.,2024. Phase 3 Trial of Stereotactic Body Radiotherapy in Localized Prostate Cancer. New England Journal of Medicine, 391(15), pp.1413–1425. doi:https://doi.org/10.1056/nejmoa24033652.Kishan et al.,2019. Long-term Outcomes of Stereotactic Body Radiotherapy for Low-Risk and Intermediate-Risk Prostate Cancer. JAMA Network Open, [online] 2(2), pp.e188006–e188006. doi:https://doi.org/10.1001/jamanetworkopen.2018.80 063. Shortall et al.,2025. The value of post radiotherapy prostate specific antigen dynamics for
survival. Results:
PSA follow-up data from 360 patients were analysed. Majority were Cambridge prognostic group (CPG) 2 (49.4%, n=178) and CPG 3 (47.2%, n=170). 62.2%
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