ESTRO 2025 - Abstract Book

S470

Clinical - Breast

ESTRO 2025

Repetti 2,1 , Luigi Cornacchia 2,1 , Mattia Zaffaroni 1 , Maria Giulia Vincini 1 , Viviana Enrica Galimberti 3 , Paolo Veronesi 3,2 , Roberto Orecchia 4 , Barbara Alicja Jereczek-Fossa* 1,2 , Daniela Alterio* 1 1 Division of Radiotherapy, European Institute of Oncology, Milan, Italy. 2 Department of Oncology and Hemato oncology, University of Milan, Milan, Italy. 3 Division of Breast Surgery, European Institute of Oncology, Milan, Italy. 4 Scientific Directorate, European Institute of Oncology, Milan, Italy Purpose/Objective: Local-regional recurrence (LRR) of breast cancer (BC) after prior adjuvant radiotherapy (RT) can present a clinical challenge. Proton beam therapy (PBT) reirradiation (reRT) may allow safer delivery of a second definitive RT course. This study reports acute toxicity after reRT for LRR of BC with PBT at the European Institute of Oncology, IRCCS, Milan, Italy. Material/Methods: Consecutive patients with recurrent BC who underwent PBT reRT and previously treated with breast or chest wall RT were considered. Patient/tumor characteristics, treatment parameters, and toxicities (end of PBT/along the follow up) were collected within a prospective registry (POWER registry NCT05860361). After a 4DCT simulation for target definition, pencil-beam scanning PBT was delivered with the IBA Proteus®ONE single-room proton therapy solution. Patients were treated in free breathing, with 2-3 anterior oblique fields equally spaced by 15°–20°, using surface guided radiotherapy in gating modality Results: The population included 12 patients with a history of prior RT and treated with PBT for LRR between April 2024 and November 2024. The median age at PBT reRT was 63 years(range 50-83years) and the median interval between RT courses was 19.3 years(range 5.4-22.5years). PBT reRT regimens included partial breast (PB) irradiation(8/12), whole breast (WB)(2/12), chest wall(CW)(1/12), and CW with regional nodes(1/12). Moderate hypofractionation(>2.5 GyRBE)was used for the majority(92%)of patients. For PB PBT reRT a total dose of 37.05 GyRBE/13 fractions was administered, while for both WB and CW PBT reRT, a total dose of 40.05 GyRBE/15 fractions was prescribed. A standard fractionation schedule(45 GyRBE/25 fractions)was used only in one case of CW plus regional nodes. Regarding the clinical target volume, the median D95% and D1% was 99%(range 95%-99%)and 106%(range 101%- 119%)of the prescription dose, respectively. An example of isodose distribution for PB PBT reRT is depicted in Fig 1 . At the end of PBT reRT skin/subcutaneous tissue disorders(CTCAE v5 scale)were as follows: dermatitis(G1:n=4;G2:n=7),pruritus(G1:n=2),pain(G1:n=2)and fibrosis(G1:n=4;G2:n=1)( Tab 1 ). A clinical evaluation after the end of PBT reRT was available for 8/12 patients(median follow-up:3.2 months, range 0.7-6 months), with only one G3 dermatitis that was recovered with topical medications in 7 days.