ESTRO 2025 - Abstract Book

S2081

Clinical - Urology

ESTRO 2025

3943

Digital Poster Hypofractionated reirradiation with proton therapy for prostate cancer local recurrence: early toxicity assessment Federico Mastroleo 1,2 , Giulia Marvaso 1,2 , Costantino Putzu 1,2 , Mikayel Arustamyan 3 , Mohammadjavad Majdani 2 , Gaia Piperno 1 , Annamaria Ferrari 1 , Luca Bergamaschi 1 , Giovanni Carlo Mazzola 1 , Samantha Dicuonzo 1 , Stefania Volpe 1,2 , Dario Zerini 1 , Stefania Comi 4 , Floriana Pansini 4 , Marco Liotta 4 , Federica Cattani 4 , Roberto Orecchia 5 , Daniela Alterio 1 , Barbara Alicja Jereczek-Fossa 1,2 1 Division of Radiation Oncology, IEO European Institute of Oncology IRCCS, Milan, Italy. 2 Department of Oncology and Hemato-Oncology, University of Milan, Milan, Italy. 3 Department of Radiation Oncology, Erebouni Radiotherapy Center, Yerevan, Armenia. 4 Unit of Medical Physics, IEO European Institute of Oncology IRCCS, Milan, Italy. 5 Scientific Directorate, IEO European Institute of Oncology IRCCS, Milan, Italy Purpose/Objective: Reirradiation (reRT) using proton therapy represents a valid option for locally recurrent prostate cancer (PCa), aiming to minimize toxicity to surrounding organs at risk (OARs). This study assesses dosimetric parameters and preliminary results on early toxicity profiles and oncological outcomes of patients undergoing reRT with proton therapy. Material/Methods: Inclusion criteria were as follow: (1) histologically confirmed PCa; (2) patients with one previous course of radiotherapy, including external beam or brachytherapy, at the level of the pelvis; (3) absence of > G2 toxicity from previous treatments; (4) restaging after biochemical recurrence performed by multiparametric magnetic resonance and/or Choline positron emission tomography (PET) and/or PSMA-PET with no distant metastasis. PCa patients underwent proton beam therapy (PBT) reRT between April and October 2024. Concomitant ADT administration was allowed. Patient and tumor characteristics, dosimetric data, and toxicity outcomes were collected prospectively within the POWER registry (NCT05860361). All patients underwent multiparametric MRI of the prostate before the RT treatment. Treatment was delivered using pencil-beam scanning PBT with the IBA Proteus®ONE system. CTCAE v5 scale was used for toxicity. Results: Seventeen patients were included. Patient, treatment and dosimetric characteristics are available in Table 1. The median interval between the last RT-course and reRT was 6.18 [IQR, 5.06-6.96] years. Median PSA before reRT was 2.7 [IQR, 2.39-4.2]. All treatments were administered in 5 fractions every other day, the median PBT dose at reirradiation was 35 Gy, with a clinical target volume (CTV) D98% average of 31.81 [SD 5.73] Gy and a D50% average of 35.05 [SD 3.02] Gy. The PBT plan utilized two lateral beams passing through the femurs and two anterior beams with an angle of at least 30 degrees to spare the rectum and bladder, which are more prone to variations during treatment delivery. This configuration minimizes high-LET deposition in critical OARs. All plans were developed using single-field uniform dose technique and robust optimization with a 5 mm margin to account for setup uncertainty (Fig.1). At the last follow-up, available for eleven patients, all exhibited an optimal PSA response at 3 months with a median PSA equal to 0.44 [IQR, 0.18-2.3]. Acute toxicity at last follow-up (minimum 3 months) included one patient with G1 hematuria and one with G1 frequent urination.