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Background: To analyze in a prospective study the long-term safety and efficacy of 3-dimensional conformal radiotherapy (3D-CRT) to deliver accelerated partial breast irradiation (APBI) for Japanese women with early breast cancer., Methods: Breast cancer patients with pathological tumor size ≤ 3 cm, age ≥ 20 years, lumpectomy with at least a 5 mm margin, and ≤ 3 positive axillary nodes were eligible. APBI was delivered by 3D-CRT at a dose of 38.5 Gy in 10 fractions over 10 days. The primary endpoints were the frequency and severity of acute and late radiation toxicities, and secondary endpoints were local control, survival, and cosmesis. The sample size was determined based on the incidence of ≥ grade 3 acute and late radiation toxicities, which required 71 enrollments., Results: Between 2008 and 2010, 73 patients enrolled in this trial. Twelve patients (16%) had 1-3 lymph node metastases. At a median follow-up of 12.6 years (range: 2.7-13.9 years), there were no cases of grade ≥ 3 acute or late toxicity. There were 4 ipsilateral breast tumor recurrence (IBTR) events: 12-year IBTR incidence was 4.4%. The difference in the incidence of IBTR between node-negative and node-positive patients was marginal (1.9% vs. 16.7%, p = 0.055). The majority of patients (94.4% at 2 years, 89.3% at 10 years after enrollment) had excellent/good cosmesis., Conclusions: APBI delivered with 3D-CRT is a feasible treatment option for Asian females, but it was indicated that node-positive status might increase IBTR risk., Competing Interests: Declarations. Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Informed consent: Informed consent was obtained from all participants included in the study. Research involving human participants and/or animals: All procedures performed in studies involving human participants were conducted in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards., (© 2024. The Author(s).)