S241
Clinical - Breast
ESTRO 2026
We aimed to evaluate long-term clinical outcomes of Hypo-WBI following BCS in Japanese patients. Material/Methods: Women with invasive breast cancer or ductal carcinoma in situ who underwent BCS followed by Hypo-WBI (42.4 Gy in 16 fractions, with or without a tumor-bed boost [typically 10.6 Gy in 4 fractions]) between January 2015 and December 2020 were reviewed. Patients who received supraclavicular node irradiation were excluded. All treatments were delivered with three-dimensional conformal radiotherapy. A tumor-bed boost was mainly delivered for close or positive margins or other high-risk features. Follow-up examinations were typically performed every 3–6 months for the first 5 years and annually thereafter. Clinical, pathological, and treatment data were collected from institutional electronic records. Endpoints were local recurrence- free survival (LRFS), disease-free survival (DFS), and overall survival (OS). Time-to-event outcomes were estimated using the Kaplan–Meier method. Acute and late toxicities were graded according to CTCAE v5.0. Results: In total, 264 patients were identified (median age, 70 years; range, 39–91). Pathologic stage was ypT0 in 12 (5%), pTis in 51 (19%), pT1 in 165 (63%), and pT2 in 36 (14%); 239 (91%) were node-negative, and 208 (79%) had negative margins. Estrogen receptor positivity was observed in 215 patients (81%), progesterone receptor positivity in 176 (67%), and HER2 positivity in 18 (7%). Neoadjuvant and adjuvant systemic therapy were administered in 155 (59%) and 193 (73%) patients, respectively. Boost irradiation was delivered in 56 (21%) patients. The median follow-up was 6.1 years (range, 0.2–10.8), with 5 (2%) local recurrences, 7 (3%) regional recurrences, and 8 (3%) distant metastases. The 5-year LRFS, DFS, and OS rates were 96.7%, 95.4%, and 96.7%, respectively. Acute toxicities were mainly low grade: radiation dermatitis grades 2 and 3 occurred in 28 (11%) and 1 (0.4%) patients, respectively. No other acute toxicities of grade ≥ 2 were observed. For late toxicity, radiation pneumonitis grade 2 occurred in 3 (1%) patients, with no other grade ≥ 3 events. Conclusion: Long-term outcomes of Hypo-WBI following BCS were excellent, showing durable local control and minimal severe toxicity among Asian patients. These results provide valuable long-term data from an Asian population, expanding global evidence for the safety and efficacy of Hypo-WBI in treating breast cancer. Keywords: Hypo-WBI, Asian Patients, Long-term Outcomes
incidentally on routine follow-up imaging (p < 0.05). Furthermore, patients receiving SRS had an improved overall survival compared with those receiving WBRT (median overall survival 16 vs. 5 months, p < 0.05). Conclusion: This study provides compelling evidence highlighting the substantial clinical benefit of early, asymptomatic detection of brain metastases in breast cancer patients. The strong association between incidental diagnosis, the feasibility of SRS, and superior overall survival underscores the critical role of timely intervention. Even though it was not in this study's scope to assess quality of life after treatment, it is established that SRS preserve cognitive function as compared to WBRT. All of these findings taken together advocate for integrating brain imaging into the routine follow-up of asymptomatic breast cancer survivors to facilitate early diagnosis and optimize radiotherapy treatment selection. References: Bleyer, Archie, and H. Gilbert Welch. 2013. “Effect of Three Decades of Screening Mammography on Breast- Cancer Incidence.” Obstetrical & Gynecological Survey 68 (6): 440–42.Derks, Sophie H. A. E., Astrid A. M. van der Veldt, and Marion Smits. 2022. “Brain Metastases: The Role of Clinical Imaging.” The British Journal of Radiology 95 (1130): 20210944.“Estimativa.” n.d. Instituto Nacional de Câncer - INCA. Accessed October 16, 2025. https://www.gov.br/inca/pt- br/assuntos/cancer/numeros/estimativa.Welch, H. Gilbert, Philip C. Prorok, A. James O’Malley, and Barnett S. Kramer. 2016. “Breast-Cancer Tumor Size, Overdiagnosis, and Mammography Screening Effectiveness.” The New England Journal of Medicine 375 (15): 1438–47. Keywords: Brain mets, breast cancer, Radiosurgery Long-term Outcomes of Hypofractionated Whole- Breast Irradiation After Breast-Conserving Surgery in Asian Patients Takamasa Mitsuyoshi 1 , Yuki Mochizuki 1 , Tomoaki Yano 1 , Hiroyuki Inoo 1 , Itaru Ikeda 1 , Sayaka Takebe 2 , Mariko Tokiwa 2 , Eiji Suzuki 2 , Masaki Kokubo 1 1 Department of Radiation Oncology, Kobe City Medical Center General Hospital, Kobe, Japan. 2 Department of Breast Surgery, Kobe City Medical Center General Hospital, Kobe, Japan Purpose/Objective: Hypofractionated whole-breast irradiation (Hypo-WBI) following breast-conserving surgery (BCS) is the established standard of care in Western countries, and its use is expanding across Asia. However, long-term outcome data from Asian populations remain limited. Digital Poster 1486
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