Advances in our understanding of tumor biology now permit greater personalization of radiotherapy for early-stage breast cancer. De-escalation of radiotherapy is an important opportunity that exists due to improvements in breast imaging, surgery, pathology, and systemic therapy. These modern tools, in addition to molecular subtype and prediction assays, have allowed us to select certain patients with breast cancer based on favorable biology who are at sufficiently low-risk of local recurrence that they may reasonably consider the omission of radiotherapy to reduce toxicity and burden. For decades, clinical trials have been conducted to try to identify the population of patients in whom the risk of recurrence in the absence of radiotherapy is sufficiently small that omission of adjuvant radiotherapy might acceptably be considered after breast conserving surgery. Trials to date have largely included patients with estrogen receptor-positive breast cancer selected based upon clinicopathologic characteristics alone and have yet to identify a patient population for whom radiotherapy is not effective. They have, however, identified women with lower absolute risks of locoregional recurrence with radiotherapy omission, and current guidelines do recognize this patient population as candidates for radiotherapy omission after breast conserving surgery. Studies that assess not just clinicopathologic features but predictive molecular biomarkers of the underlying biology are currently ongoing. This manuscript will briefly review historical trials evaluating the role of radiotherapy after breast conserving surgery as well as discuss the exciting ongoing trials in hormone receptor-positive disease leveraging our growing appreciation of tumor biology and molecular prediction to identify a population of patients who may reasonably consider omitting radiotherapy after breast conserving surgery.

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