ESTRO 2025 - Abstract Book

S1595

Clinical – Mixed sites & palliation

ESTRO 2025

Purpose/Objective: Re-irradiation (reRT) with PT represents a challenging scenario. The use of hypofractionation in this setting is rarely reported. Aim of this work was to present feasibility and early toxicity profile of patients treated with hypofractinated reRT PT at the Proton Center of the European Institue of Oncology, Milan, Italy. Material/Methods: All consecutive patients (pts) treated with hypofractionated (dose/fraction > 2 Gy RBE) PT reRT between November 2023 and October 2024 were retrieved. Toxicity has been defined at the end of the treatment using Common Terminology Criteria Adverse Event (CTCAE V4.0) scale. All data have been prospectively collected (“Power Registry” NCT0586036). Results: Among 129 pts treated with PT in the considered period 67 pts (52%) where treated with PT reRT and among these, 52 (78%) were treated with hypofractionated schedules. Mean age was 66 yrs (range 28-86 yrs) and 22 pts were female. The majority (34 pts, 65 %) of pts were treated with curative intent while 14 (27%) were treated in the postoperative setting. Two pts were submitted to a palliative therapy. The number of patients treated according the tumor site is reported in Fig.1. Median total dose was 35 Gy RBE (IQR 30-40 GyRBE) with a median number of fractions of 5 (IQR 5-14.5 Gy RBE). The treatment schedule according the tumor site has been summarized in Fig.2. Toxicity was available for 42 pts. Three (6%) pts reported a G2 toxicity while 2 pts experienced G3 toxicity. Follow-up of 3 months was available for 23 pts (mean 4 months, range 1-9 months). Among the 34 pts treated with a curative intent, complete and partial response was achieved in 11 (21%) and 2 (4%) patients, respectively while 1 patients had a stable disease. Among the 14 pts treated in the postoperative setting, none experienced a local recurrence. Three patients died for distant progression of disease.