ESTRO 2025 - Abstract Book

S1997

Clinical - Urology

ESTRO 2025

Conclusion: MRL permits to safely delivery a second SBRT course in patients with exclusive local relapse after a first course of RT. Toxicity was acceptable using online daily plan adaptation and local relapse were uncommon in short-term follow-up. Further studies including treatment margin reduction are ongoing at our institution.

Keywords: MRgRT, reirradiation, prostate cancer

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Poster Discussion Development and internal validation of a nomogram predicting ADT-free survival in oligorecurrent prostate cancer treated with metastasis-directed SBRT Timo F.W. Soeterik 1 , Marcin Miszczyk 2,3 , Max Peters 4 , Mateusz Bilski 5,6,7 , Giulio Francolini 8 , Pietro Garlatti 8 , Maris Mezeckis 9 , Justyna Kociolek 10 , Carlo Greco 10 , Tomasz Techmanski 11 , Eline Huele 12 , Freek Teunissen 13 , Paulien Westhoff 13 , Petra Kroon 1 , Wessel Vis 4 , Dorien Haverkort 4 , Jochem R.N. Van der Voort van Zyp 1 , Martijn Intven 1 , Joanne Van der Velden 1 , Wietse S.C. Eppinga 1 1 Department of Radiation Oncology, University Medical Center Utrecht, Utrecht, Netherlands. 2 Department of Urology, Comprehensive Cancer Center, Medical University of Vienna, Vienna, Austria. 3 Collegium Medicum - Faculty of Medicine, WSB University, Dąbrowa Górnicza, Poland. 4 Department of Radiation Oncology, Radiotherapiegroep, Deventer, Netherlands. 5 Brachytherapy Department, Saint John’s Cancer Center, Lublin, Poland. 6 Radiotherapy Department, Medical University of Lublin, Lublin, Poland. 7 Radiotherapy Department, Saint John’s Cancer Center, Lublin, Poland. 8 Department of Radiation Oncology, Azienda Ospedaliero-Universitaria Careggi, University of Florence, Florence, Italy. 9 University of Latvia, Sigulda Hospital Centre of Stereotactic Radiosurgery, Sigulda, Latvia. 10 Department of Radiation Oncology, The Champalimaud Centre for the Unknown, Lisbon, Portugal. 11 3rd Department of Radiotherapy and Chemotherapy, Maria Skłodowska -Curie National Research Institute of Oncology, Gliwice, Poland. 12 Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, Netherlands. 13 Department of Radiation Oncology, Radboud University Medical Center, Nijmegen, Netherlands Purpose/Objective: Metastasis-directed therapy (MDT) offers a strategy to delay or even avoid androgen deprivation therapy (ADT) and its morbidities in hormone-sensitive oligorecurrent prostate cancer (PCa). To identify patients most likely to benefit, we developed and internally validated a prognostic tool to predict ADT-free survival using a large, multicenter cohort of PSMA PET/CT-staged oligorecurrent PCa patients. Material/Methods: A multicenter cohort of 561 patients with oligorecurrent, hormone- sensitive PCa (≤5 pelvic or distant nodal, bone, or visceral lesions) treated with SBRT to all metastatic sites (without systemic therapy) was used for model development and internal validation using bootstrapping. ADT-free survival was predicted using multivariable Cox proportional hazards regression. Model performance was evaluated with the C-index and calibrated at different time points, resulting in a novel risk classification score with three prognostic groups. Results: A total of 561 patients were analyzed, which had a median ADT-free survival of 26 months (IQR 16 – 43). During the study period, ADT was initiated in 266 of the patients (47%). ADT-free survival rates were 85% at 1 year and 69% at 2 years. Factors significantly associated with ADT initiation included PSA before first SBRT (HR 1.06, 95% CI 1.04 – 1.09, p<0.001), PSA doubling time (months) (HR 0.97, 95% CI 0.95 – 0.996, p=0.02), time to onset of oligometastatic disease (years) (HR 0.95, 95% CI 0.92 – 0.99, p=0.02), total number of lesions (HR 1.33, 95% CI 1.14 – 1.54, p<0.001), and presence of distant metastasis (vs. pelvic nodal disease) (HR 1.58, 95% CI 1.22 – 2.05, p=0.001). A nomogram incorporating these predictors achieved an uncorrected C-index of 0.68 and 0.66 after internal validation (Figure 1). The model demonstrated good calibration. A nomogram-based risk score was developed to stratify patients into