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To determine whether pretreatment contrast-enhanced breast magnetic resonance imaging (MRI) alters patient selection for accelerated partial breast irradiation (APBI).Women aged 40 years or older with unifocal invasive or intraductal carcinoma ≤2.5 cm on physical examination, mammography, and ultrasound (US) were evaluated with breast MRI before enrollment on an APBI trial using single-fraction intraoperative radiotherapy (IORT) or fractionated 3-dimensional conformal radiotherapy. Abnormal MRI findings were evaluated with US-guided or MRI-guided biopsy.Between December 2002 and March 2005, 51 women (median age=61 y; range, 40 to 83 y) who met inclusion criteria underwent breast MRI before APBI. MRI demonstrated limited disease in 41 patients (80.4%): 34 received APBI using IORT (22) or 3DCRT (12), whereas 7 elected standard whole-breast radiotherapy. Ten of the 51 patients (19.6%) had indeterminate or suspicious enhancement patterns on MRI. Five of these 10 (9.8% of MRI cohort) underwent US-guided or MRI-guided biopsy revealing normal breast tissue without atypia: 3 were treated with APBI using IORT (5.9% of MRI cohort) and 2 underwent standard breast conservation therapy (3.9% of MRI cohort). The remaining 5 patients (9.8% of MRI cohort) had MRI findings revealing previously unsuspected pectoral fascia involvement (1), multifocal disease (3), or multicentric disease (1): 2 were treated with standard breast conservation therapy, whereas 3 underwent mastectomy without adjuvant radiotherapy.Pretreatment breast MRI altered patient selection for APBI by identifying additional disease in 9.8% of the candidates, all of whom fit into the “cautionary” or “unsuitable” categories as defined by the American Society for Radiation Oncology APBI consensus guidelines. The clinical significance of these findings will be clarified with the results of ongoing randomized trials of APBI that do not incorporate breast MRI as part of the selection criteria.