ESTRO 2025 - Abstract Book

S2072

Clinical - Urology

ESTRO 2025

3840

Digital Poster Dosimetric and Clinical Outcomes of MRI-Linac Online Adaptive Radiotherapy for Prostate Cancer: A Single Center Experience Francesco Preziosi 1 , Francesco Catucci 1 , Althea Boschetti 1 , Martina Iezzi 1 , Alessia Re 1 , Carmela Di Dio 1 , Antonio Piras 1 , Flora Anna Mauro 1 , Michele Aquilano 1 , Sebastiano Menna 2 , Francesca Gruosso 1 , Claudio Votta 1 , Flaviovincenzo Quaranta 2 , Elisa Pilloni 2 , Luca Vellini 2 , Attilio Lo Vuolo 1 , Chiara Flore 1 , Maria Mameli 1 , Eleonora Sanna 1 , Danila Piccari 1 , Davide Cusumano 1 , Gian Carlo Mattiucci 1 1 Radiotherapy, Mater Olbia, Olbia, Italy. 2 Physics, Mater Olbia, Olbia, Italy Purpose/Objective: This study assesses the dosimetric and clinical impact of daily online adaptive stereotactic body radiotherapy (SBRT) for localized prostate cancer using MRI-linac. We retrospectively reviewed data from 32 prostate cancer patients treated between October 2022 and April 2024 according to the PACE-B trial protocol, receiving 36.25Gy in five fractions. In case of significant variation, the treatment fraction was daily adapted to accommodate variations in patient anatomy, so allowing to evaluate adaptive planning's role in terms of target coverage and organ-at-risk (OAR) sparing. Material/Methods: For each patient, we generated a predicted treatment plan at the start of the treatment session based on initial imaging and in case of significant variation in daily anatomy, subsequently adjusted this plan daily based on MRI acquired anatomical changes. A comparative dosimetric analysis was conducted between predicted (non-adaptive) and daily-adapted treatment plans. Dose-volume histograms (DVH) metrics were assessed for the planning target volume (PTV), bladder, and rectum. On a sub-cohort of patients (17 on 32 cases), the urethra was delineated and included in the planning optimisation. For PTV coverage was evaluated the percentage of volume covered by the prescription isodose (36.25Gy) and the maximum dose. For OARs, the dose constraints provided by PACE B were used. Acute and late genitourinary (GU) and gastrointestinal (GI) toxicities were registered using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0. Results: On a total of 160 treatment fractions, 101 required daily adaptation (63%). Figure 1 reports boxplot analysis for the DVH parameters evaluated: adaptive planning demonstrated a notable benefit in PTV coverage (p=0.001), with a mean 4.7% improvement observed in the adapted plans over the predicted plans. No significant dosimetric differences were observed for bladder or rectum between the predicted and adapted plans. As for the urethra, optimisation allows to significant reduce the V42Gy. In terms of clinical toxicity outcomes, results are reported in Table 1. In particular, acute GU toxicity (grade 1) occurred in 11 patients, with 7 developing chronic toxicity (1 grade 2, 6 grade 1). Three cases of grade 1 GI toxicity were reported. No grade 3 or higher toxicity was observed.