ESR1 and breast cancer: In addition, the National Comprehensive Cancer Network (NCCN) guideline indicates that deimplementation of radiotherapy (RT) in this group of patients may be feasible.21 Strong evidence generated by the Cancer and Leukemia Group B (CALGB) 9343 trial22 with long-term follow-up23 show that adjuvant therapy with tamoxifen citrate alone without the addition of RT is a feasible treatment strategy for older patients with early-stage, estrogen receptor (ER)–positive breast cancer.