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Background This prospective cohort study aimed to assess whether postoperative radiotherapy could safely be omitted in women aged 65 years and older with low-risk, estrogen receptor-positive T1N0 breast cancer treated with breast-conserving surgery and adjuvant endocrine therapy.

Methods Eligible patients were women aged 65 years and older with unifocal, nonlobular, grade 1 or 2, estrogen receptor-positive, pT1N0 breast cancer treated with breast-conserving surgery and endocrine therapy for 5 years. Patients were followed up with mammography at least annually for 10 years. The primary endpoint was local recurrence. Secondary endpoints were contralateral breast cancer, recurrence-free survival, and overall survival.

Results The final study cohort included 601 patients with a median age of 71 years (range = 65-90 years) and a median tumor size of 11 mm (range = 3-20 mm). Median follow-up time was 119 months (interquartile range = 103-121 months). The cumulative incidence of local recurrence was 1.5% (95% confidence interval [CI] = 0.8% to 2.8%) and 5.5% (95% CI = 3.8% to 7.6%) at 5 and 10 years, respectively. The cumulative incidence of contralateral breast cancer was 1.7% (95% CI = 0.9% to 3.0%) at 5 years and 4.5% (95% CI = 3.0% to 6.6%) at 10 years. The overall survival rate at 10 years was 83.1% (95% CI = 80.8% to 85.4%). In total, 3 (0.5%) patients died because of breast cancer.

Conclusion Our results support the possibility to omit radiotherapy after breast-conserving surgery in a well-defined subgroup of women aged 65 years and older with low-risk, estrogen receptor-positive, pT1N0 breast cancer receiving adjuvant endocrine therapy.