Background: We retrospectively examined the clinical outcome of irradiated versus non-irradiated groups of Japanese breast cancer patients according to their clinical and histopathological characteristics following breast-conserving therapy. Methods: We retrospectively evaluated a total of 1197 Japanese female breast cancer patients (598 irradiated and 599 non-irradiated) who received breast-conserving therapy. The median age of the patients was 54 years (range: 24-99 years). We retrospectively examined the local recurrence-free survival rates in those with or without post-operative irradiation according to age, surgical margin status and histopathological characteristics including histological grade, estrogen receptor expression and HER2 status. Results: Local recurrence-free survival rates in the irradiated group were significantly higher than those in the non-irradiated group, especially in surgical margin-positive [hazard ratio (HR): 0.334, 95% confidence interval: 0.14-0.79, P = 0.001], estrogen receptor-positive (HR: 0.249, 95% confidence interval: 0.11-0.54, P < 0.001), HER2-negative (HR: 0.382, 95% confidence interval: 0.21-0.69, P = 0.001) and non-triple-negative (HR: 0.382, 95% confidence interval: 0.21-0.69, P = 0.001) breast cancer patients. Conclusion: The results indicated that irradiation after breast-conserving therapy is strongly recommended in Japanese breast cancer patients, especially those with surgically positive margins, estrogen receptor-positive tumors and HER2-negative invasive breast cancers.
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