ESR1 and breast cancer: In both trials, RT omission resulted in no difference in the frequency of mastectomy for recurrence, time to distant metastasis, breast cancer-specific survival, and overall survival.6–8 Based on these results, the National Comprehensive Cancer Network guidelines were updated to include the option for RT omission for patients with ER+, human epidermal growth factor receptor 2 (HER2)-negative, node-negative breast cancer with tumors ≤3 cm who were aged ≥65 years.9