ESTRO 2025 - Abstract Book

S1873

Clinical - Urology

ESTRO 2025

9

Mini-Oral Prostate-specific antigen bounce after prostate ultra-hypofractionation: associations with 5-year outcomes in the phase 3 trial HYPO-RT-PC Astrid E Persson 1,2 , Elisabeth Kjellén 1,2 , Camilla Thellenberg-Karlsson 3 , Lars Beckman 3,4 , Adalsteinn Gunnlaugsson 1,2 , Per Nilsson 1,2 1 Department of Clinical Sciences, Lund University, Lund, Sweden. 2 Department of Haematology, Oncology and Radiation Physics, Skåne University Hospital, Lund, Sweden. 3 Department of Diagnostics and Intervention, Oncology, Umeå University, Umeå, Sweden. 4 Department of Oncology, Sundsvall Hospital, Sundsvall, Sweden Purpose/Objective: Prostate-specific antigen (PSA) bounce is a well-known phenomenon characterised by a transient increase in PSA levels following curative radiotherapy for prostate cancer. However, to our knowledge, it has not been differentiated between ultra-hypofractionation (UHF) and conventional fractionation (CF) within the setting of a randomized trial. HYPO-RT-PC was the first phase 3 trial to report 5-year outcomes after primary UHF and CF in intermediate- and high-risk localised prostate cancer, showing the non-inferiority of UHF [1]. We aimed to examine if occurrence of at least one PSA bounce was associated with the 5-year treatment outcomes, in the full per-protocol population and within each arm. Material/Methods: The per-protocol population consisting of 589 participants receiving UHF (42.7 Gy/7 fractions, 3 fractions/week) and 591 CF (78.0 Gy/39 fractions, 5 fractions/week) was included. Hormonal therapy was not allowed. In each arm, 89% had intermediate-risk and 11% high-risk localised prostate cancer. PSA bounce was defined as a level increase of ≥0.2 ng/ml above the pre -bounce nadir spontaneously dropping to or below the pre-bounce nadir, and was recorded from end of radiotherapy until the last PSA measurement or treatment failure. Associations between occurrence of at least one PSA bounce and failure-free, biochemical disease-free, prostate cancer-specific and overall survival were modelled using Cox regression, adjusted for age, baseline PSA, Gleason score and tumour stage. Interaction with radiotherapy schedule was tested in the full per-protocol population. Results: The proportions of participants experiencing a first PSA bounce were similar between the arms: UHF 31% (95% confidence interval [CI] 28 – 35) and CF 32% (95% CI 28 – 36). In the full per-protocol population, experience of PSA bounce was associated with more favourable treatment outcomes (Table). The association was also observed within each arm for failure-free (Fig.) and biochemical disease free survival, but did not reach statistical significance for overall survival (UHF hazard ratio 0.49 [95% CI 0.24 – 1.02], p=0.058; CF hazard ratio 0.51 [95% CI 0.25 – 1.04], p=0.063; unadjusted due to few events). No statistically significant interaction was observed between PSA bounce and radiotherapy schedule (p>0.082). The number of events per arm for prostate cancer-specific survival was limited.

Table. Cox regression estimates in the full per-protocol population