ORIGINAL ARTICLE – BREAST ONCOLOGY The Evolving Role of Partial Breast Irradiation in Early-Stage Breast Cancer Mitchel Barry, MD 1 , Alice Ho, MD 2 , and Monica Morrow, MD 1 1 Breast Service, Department of Surgery, Memorial Sloan-Kettering Cancer Center, New York, NY; 2 Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, NY ABSTRACT Whole breast irradiation (WBI) over 5–7 weeks has long been considered standard adjuvant treatment after breast-conserving surgery. Recently the concept of irradiating the whole breast has been challenged by accelerated partial breast irradiation (APBI), which exclusively targets the lumpectomy site plus margin. We review the evidence supporting APBI from modern clinical trials, the pros and cons of various APBI techniques, and the controversies regarding the applicability of APBI to specific patient groups. Whole breast irradiation (WBI) after breast-conserving surgery (BCS) has been firmly established by multiple prospective randomized trials as an important part of breast-conserving therapy. 1–4 A large meta-analysis dem- onstrated that a reduction in ipsilateral recurrence translated into a survival benefit of 5.4 % after 15 years in early-stage invasive breast cancer patients treated with BCT. 5 All of the trials included in the meta-analysis uti- lized standard fractionation, which consists of 45–50 Gy delivered to the entire breast over an interval of 5–6 weeks. Despite the proven benefits of radiation in early-stage breast cancer, 1 in 5 women in the United States did not receive radiotherapy (RT) after BCS in the late 1990s. 6,7 Cited reasons included the long duration of treatment, limited geographical access, and other issues pertaining to the inconvenience and cost of WBI treatment using stan- dard fractionation regimens. 8 To address some of these logistical issues, Whelan et al. have proposed a hypofrac- tionated regimen of breast irradiation. 9 However, this shorter treatment is unlikely to achieve 100 % compliance either. APBI techniques offer a more pragmatic solution, but are these novel techniques as effective? In this review, we assess the evolving role of APBI in the context of evidence from modern trials and describe each of the APBI techniques with its distinct advantages and disadvantages in further detail. RATIONALE AND ARGUMENTS FOR AND AGAINST APBI The rationale for APBI is based on the fact that 75–85 % of local recurrences after BCS occurred at or near the original lumpectomy site. 1,2,10 Furthermore, the develop- ment of new cancers elsewhere in the breast is equivalent to the risk of developing a contralateral breast cancer. 11 These patterns of failure suggest that APBI may offer equivalent local control to WBI, and that the primary benefit of WBI stems from eradication of disease in the region of the tumor bed. In addition, APBI techniques offer a more pragmatic approach to breast irradiation by com- pressing conventional treatment from 5 to 6 weeks to a few days or even a one-off intraoperative dose. The risk of elsewhere failures in the ipsilateral breast observed in the following studies form the basis for the argument against APBI. A pathologic study by Holland et al. evaluated mastectomy specimens from patients who fulfilled the criteria for BCT. Of the 282 invasive cancers, 43 % of patients had tumor cells [ 2 cm from the index cancer. 12 An Italian study that performed quadrantectomy (which excises a larger volume of breast tissue than is standard in the United States) in all patients demonstrated a significant reduction in ipsilateral breast tumor recurrence (IBTR) with adjuvant radiation. 2 Finally, in a randomized trial of WBI with or without boost irradiation, 29 % of the local recurrences observed were located in a quadrant separate from the index tumor, whereas 27 % were Ó Society of Surgical Oncology 2013 First Received: 3 September 2012; Published Online: 6 March 2013 M. Morrow, MD e-mail: morrowm@mskcc.org Ann Surg Oncol (2013) 20:2534–2540 DOI 10.1245/s10434-013-2923-8