ESTRO 2025 - Abstract Book

S459

Clinical - Breast

ESTRO 2025

873

Digital Poster Operable breast cancer patients who refuse surgery can be cured by definitive radiotherapy with or without radiosensitization strategy Yuta Shibamoto 1,2 , Seiya Takano 3 , Shinya Takemoto 2 , Minori Niwa 2 1 Proton Therapy, Narita Memorial Proton Center, Toyohashi, Japan. 2 Radiology, Narita Memorial Hospital, Toyohashi, Japan. 3 Radiology, Nagoya City University Hospital, Nagoya, Japan Purpose/Objective: We aimed to develop a novel definitive treatment employing modern sophisticated radiation techniques (stereotactic irradiation and intensity-modulated radiation therapy, IMRT) for operable breast cancer patients who refuse any type of surgery. Preliminary results have been published (1,2), and herein we show updated results with 54 tumors in 51 patients. Material/Methods: Fifty-four operable breast cancers in 51 patients were treated by fractionated whole-breast radiotherapy in combination with stereotactic radiotherapy (for primary tumor) or IMRT (for primary tumor with/without regional lymph nodes) boost. Three patients received bilateral treatment for simultaneous or metachronous bilateral breast cancers. Until 2020, standard radiation doses were 50 Gy/25 fractions for the whole breast, 21 Gy (range: 19.5 25.5)/3 fractions for stereotactic boost, and 20 Gy/8 fractions for IMRT boost. Since 2021, a more hypofractionated regimen was used; the doses were 44.8 Gy/16 fractions for the whole breast, 21 Gy/3 fractions for the SBRT boost, and 19.6 Gy/7 fractions for the IMRT boost. Disease stages were 0 (ductal carcinoma in situ) in 8, I in 19, II in 23, and III in 4. In 35 patients, intratumoral hydrogen peroxide injection (Kochi Oxydol Radiation Therapy for Unresectable Carcinomas [KORTUC] treatment or hyperthermia with oral tegafur-gimeracil-oteracil potassium (S-1) was also used to sensitize the tumors to radiation. Hormonal and standard systemic therapy were administered in 37 and 14 patients, respectively. Results: Complete and partial responses were observed in 25 and 29 tumors, respectively; in patients with partial response, no further regrowth of the residual mass was observed, except for three patients who developed local recurrence. During a follow-up of 8-190 months (median, 60 months for living patients), 3, 1, and 2 patients developed local relapse, subclavicular node metastasis, and distant metastasis, respectively. The 5-year rates for overall, progression-free, and local relapse-free survival were 97, 90, and 93 %, respectively. Nineteen patients developed Grade 3 radiation dermatitis but all recovered after treatment. The affected breast tended to become better-rounded, and in 43%, the nipple of the irradiated breast became higher by >1 cm than the contralateral nipple. The Harvard scale of breast cosmesis was excellent in 24, good in 26, and fair in 1. Conclusion: The results obtained so far do not seem to differ from those obtained by surgery. Cosmetic outcomes were satisfactory in most patients. Our method might be a good treatment option for operable breast cancer patients who do not wish to undergo surgery.

Keywords: definitive radiotherapy, radio-sensitization

References: 1. Shibamoto Y, et al. Definitive radiotherapy with SBRT or IMRT boost for breast cancer: excellent local control and cosmetic outcome. Technol Cancer Res Treat 2018, 17: doi: 10.1177/1533033818799355 2. Shibamoto Y, et al. Definitive radiotherapy with stereotactic or IMRT boost with or without radiosensitization strategy for operable breast cancer patients who refuse surgery. J Radiat Res 2022, 63: 849-855, doi: 10.1093/jrr/rrac047