95P Omission of postoperative radiation therapy in older patients with low-risk breast cancer treated with breast-conserving surgery: Long-term results of the KohoRT study

Postoperative radiotherapy after breast-conserving surgery in node-negative breast cancer halves the risk of local recurrence, though the absolute benefit varies depending on baseline risk of recurrence. In some subgroups, the benefit of radiotherapy is limited and might potentially justify a de-escalation approach with omission of radiotherapy. This Swedish multi-center prospective cohort study was designed to assess whether radiotherapy could be safely omitted in women ≥ 65 years with low-risk, estrogen-receptor (ER)-positive T1N0 breast cancer treated with breast-conserving surgery and adjuvant endocrine therapy for 5 years. Eligible patients were women ≥ 65 years with unifocal, non-lobular, grade 1 or 2, ER-positive, pT1N0 breast cancer treated with breast-conserving surgery. Patients were prescribed endocrine therapy for 5 years and 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. The final study cohort included 601 patients with a median age of 71 years (range: 65 to 90) and a median tumor size of 11 mm (range: 3 to 20 mm). Median follow-up time was 119 months (interquartile range: 103 to 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 cumulative incidence of regional recurrence at 10 years was 1.4% (95% CI: 0.7 to 2.7%). Distant recurrence occurred in 5 patients (0.8%) during follow-up. The overall survival rate at 10 years was 83.1% (95% CI: 80.8 to 85.4%). In total, 3 patients (0.5%) died due to breast cancer during follow-up. Our long-term study results support the emerging evidence on the possibility to omit radiotherapy after breast-conserving surgery in a well-defined subgroup of women aged 65 years or more with low-risk ER-positive pT1N0 breast cancer receiving adjuvant endocrine therapy.