Breast conservation therapy (BCT) is a multi-modality alternative to mastectomy to treat early (stage I or II) breast cancer. In current practice, most conventional BCT includes breast-conserving surgical excision of the tumor (lumpectomy, segmentectomy, or quadrantectomy) and whole-breast radiotherapy (WBRT), delivered 5 days/week over 5-7 weeks using external beam radiation (EBR). For those at higher risk of recurrence (based on age younger than 50 years, tumor size greater than 2-3 cm, nodal involvement, inadequate tumor-free margins, etc.), "boost" radiotherapy narrowly directed to the tumor bed is included in WBRT. In randomized trials, WBRT reduced local (i.e., in-breast) recurrence, and meta-analysis showed it also improved survival compared with breast-conserving surgery alone. Other trials and meta- analysis showed that efficacy of BCT with WBRT is equivalent to mastectomy. The radiation is hypothesized to eliminate residual cancer near the surgical site and treat any undetected multicentric disease. Radiation alone (i.e., without resection) is not recommended in current guidelines to manage early breast cancer.