S1237
Clinical - Urology
ESTRO 2026
procedural variables. These findings underscore the importance of distinguishing PSAb from true biochemical recurrence. Keywords: prostate cancer, PSA bouce, SBRT
IRCCS Sacro Cuore Don Calabria Hospital, Cancer Care Center, Negrar di Valpolicella, Italy. 3 Medical Physics Unit, IRCCS Ospedale Sacro Cuore Don Calabria, Negrar di Valpolicella, Italy. 4 University of Brescia, University of Brescia, brescia, Italy Purpose/Objective: Prostate-specific antigen bounce (PSAb), characterized by a transient elevation in PSA levels followed by a spontaneous decline, is a well-documented phenomenon in prostate cancer (PC) patients undergoing radiotherapy. While PSAb can cause diagnostic uncertainty, mimicking biochemical recurrence, its etiology and clinical implications remain poorly understood. This study aims to analyze the incidence, characteristics, and prognostic significance of PSAb in a cohort of PC patients treated with stereotactic body radiotherapy (SBRT) using a 1.5T MR- Linac platform. Material/Methods: A total of 305 patients with low-to-intermediate risk PC (stage T1-T2, Gleason score ≤ 4+3) were treated with SBRT in five fractions (35-36.25 Gy) using an adaptive workflow guided by pre-treatment MRI and real-time cine-MRI monitoring. PSAb was defined as a temporary increase in PSA levels ≥ 0.2 ng/mL above the nadir, followed by a return to or below the nadir. Statistical analyses evaluated correlations between PSAb and clinical/dosimetric variables, including tumor volume, treatment scheduling, and adverse events. The Chi-square test was used for categorical variables, while Spearman's correlation and linear regression assessed continuous variables. Results: PSAb occurred in 25% of patients, with a median time to bounce of 11 months post-treatment. The incidence of PSAb within the first 6 months was 86.4%. The median time to post-bounce nadir (PSAn) was 25 months, with a mean PSAn of 0,90 ng/mL (range: 0.05- 5.02 ng/mL). Biochemical relapse was observed in only 2,95% of patients. No significant associations were found between PSAb and tumor volume metrics or treatment scheduling. adverse events. analysis revealed predominantly mild-to-moderate acute events, including cystitis (28.2%) and proctitis (4.6%), with minimal late adverse events (cystitis: 9.2%; proctitis: 2,9%; sexual impotence: 4.3%). Univariate analysis demonstrates a significant correlation between PSAb and improved biochemical relapse-free survival (p=0.016). Conclusion: PSAb is a common and benign phenomenon in PC patients treated with SBRT, associated with favorable oncological outcomes. The absence of significant correlations with tumor volume or treatment scheduling suggests that PSAb may be influenced by individual biological factors rather than dosimetric or
Proffered Paper 2713
Acute Toxicity in the DESTINATION 1 Trial: A Prospective Prostate SBRT Dose De-escalation Feasibility Study Sian Cooper 1,2 , Sophie Alexander 1,3 , Charlotte Cherry 4 , Joan Chick 5 , Alex Dunlop 5 , Shermarke Hassan 6 , Mathijs G Dassen 6 , Trina Herbert 1 , Francesca Mason 4 , Adam Mitchell 5 , Simeon Nill 5 , Uwe Oelfke 5 , Floris Pos 6 , Murtaza Saifuddin 7 , Rosalyne Westley 2 , Uulke A van der Heide 6 , Danny Vesprini 7 , Alison Tree 1,2 1 Radiotherapy, The Royal Marsden Hospital, London, United Kingdom. 2 Radiotherapy Research, The Institute of Cancer Research, London, United Kingdom. 3 Radiotherapy Research, The Institute of Cancer Researchtitute of Cancer Research, LondonLondon, United Kingdom. 4 MRL trials unit, The Royal Marsden Hospital, London, United Kingdom. 5 The Joint Department of Physics, The Royal Marsden Hospital, The Institute of Cancer Research, London, United Kingdom. 6 Research Unit, Netherlands Cancer Institute, Amsterdam, Netherlands. 7 Research Unit, Sunnybrook Health Sciences Centre, Toronto, Canada Purpose/Objective: Stereotactic body radiotherapy (SBRT) has emerged as an effective treatment for localised prostate cancer although more men experience genitourinary side effects. Focal boost to the dominant lesion shows promise for improving disease control. [1,2] The DESTINATION study investigates a novel whole gland dose de-escalation approach, with a focal boost, using MR-guided adaptive radiotherapy, with the aim of maintaining cancer control whilst demonstrating acceptable toxicity. [3,4] Material/Methods:
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