S208
Clinical - Breast
ESTRO 2026
Keywords: breast cancer
Mini-Oral 451 Carbon-ion radiotherapy as a non-surgical treatment for early-stage breast cancer: long-term follow-up of a phase I/II trial (jRCTs032180153) Noriyuki Okonogi 1,2 , Kumiko Karasawa 2,3 , Kazutoshi Murata 4 , Tokuhiko Omatsu 4 , Hiroto Murata 5 , Masaru Wakatsuki 2 , Hitoshi Ishikawa 2 1 Department of Radiation Oncology, Juntendo University Graduate School of Medicine, Tokyo, Japan. 2 QST Hospital, National Institutes for Quantum Science and Technology, Chiba, Japan. 3 Department of Radiation Oncology, Kawakita General Hospital, Tokyo, Japan. 4 QST Hospital, National Institutes for Quantum Science and Technology, Tokyo, Japan. 5 Department of Radiation Oncology, National Center for Child Health and Development, Tokyo, Japan Purpose/Objective: To assess the long-term efficacy, safety, and cosmetic outcomes of carbon-ion radiotherapy (C-ion RT) as a non-surgical treatment for patients with early-stage This single-center, prospective phase I/II trial (Breast I; jRCTs032180153), approved by the institutional review board, was the first of three ongoing prospective C-ion RT studies for BC. Eligible patients were women ≥ 60 years with stage I (cT1N0M0), ER-positive, HER2- negative invasive ductal carcinoma ≤ 2 cm. In phase I, a dose-escalation (48.0, 52.8, and 60.0 Gy [RBE] in four fractions) was tested, with excision performed 3 months post-RT for pathological evaluation. All patients subsequently received adjuvant endocrine therapy, mainly aromatase inhibitors. Based on the absence of dose-limiting toxicities and acceptable acute adverse events, 60.0 Gy (RBE) was selected for phase II. Tumor response was assessed by MRI every 3 months until complete disappearance. The primary endpoint was 5-year local control (LC); secondary endpoints included complete response (CR) rate, adverse events (AEs), cosmetic outcomes, disease-free survival (DFS), and overall survival (OS). breast cancer (BC). Material/Methods:
Results: Seven patients were enrolled in phase I. Pathological responses ≥ Grade 2b were achieved in all tumors treated with ≥ 52.8 Gy. No dose-limiting toxicities occurred, and only Grade 1 acute dermatitis was observed, supporting 60.0 Gy (RBE) as the recommended dose. In phase II, twelve patients were treated with 60.0 Gy (RBE). The CR rate was 100%, with a median time to CR of 12 months (range 4–36). At a median follow-up of 84 months for the combined phase I/II cohort, the 5-year LC and DFS rates were both 94.7%, and the OS rate was 100%. One in-field recurrence occurred in a patient with a high Ki-67 index. Acute AEs included Grade 1 dermatitis in six patients. Late toxicities were limited to Grade 1 rib fractures (n = 2) and mastitis-related pain (n = 3), managed conservatively. MRI revealed subclinical pectoral muscle inflammation in seven patients. Cosmetic outcomes were excellent in all but one patient, who underwent mastectomy for recurrence. Conclusion: C-ion RT provided excellent long-term tumor control with minimal toxicity and favorable cosmesis in selected patients with early-stage BC. As the first of three prospective trials (Breast I), it established the safety and feasibility of hypofractionated regimens up to 60.0 Gy (RBE). These results support C-ion RT as a promising non-surgical alternative, warranting validation in larger controlled studies. Keywords: early-stage breast cancer, carbon-ion radiotherapy
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