External beam accelerated partial breast irradiation (APBI) utilizes Planning Target Volume (PTV) expansions of 2 to 2.5 cm beyond the surgical cavity to assure treatment of target breast tissue. Decreasing the volume of irradiated breast with external beam accelerated partial breast irradiation (APBI) is a potential way to minimize dose to organs at risk and adverse cosmetic outcomes. Magnetic resonance guided radiation therapy (MRgRT) allows detailed pre-fraction imaging and continuous intrafraction monitoring which allows for external beam APBI treatments using a brachytherapy-style 1 cm surgical bed to PTV margin. We have previously shown that MRgRT APBI results in an average 52% reduction in PTV. In this study we report the outcomes of patients treated with reduced-margin APBI via MRgRT.Patients included women with early stage, node negative breast cancers treated with breast conserving surgery and adjuvant MRgRT APBI from March 2014 to July 2019. PTV was defined as the surgical cavity plus a uniform 1-cm margin of breast tissue (excluding chest wall, pectoral muscles, and 5mm from skin). The PTV received 38.5 Gy in ten fractions over five treatment days. Volumetric MR imaging was acquired prior to each fraction to localize the surgical cavity and a sagittal cine MR imaging during each treatment to monitor intrafraction motion; gating to the surgical bed was used as needed.242 patients were treated with MRgRT APBI with reduced margins. Eighty-one (33.5%) patients had pure DCIS and 161 (66.5%) patients had pT1N0 disease. Seventy-three (90.1%) patients with pure DCIS had hormone sensitive disease and 141 (87.6%) patients with invasive disease had hormone sensitive, her2/neu negative disease. Stratifying by ASTRO APBI Consensus guidelines, 36% of patients were categorized as Cautionary and one patient was Unsuitable solely due to age < 40. At a median follow up of 36 months (range, 14 - 82), two patients (0.8%) had biopsy-proven in-breast tumor recurrence (IBTR), both of whom were successfully salvaged. Per the Aaronson-modified Harris scale, providers reported 100% of patients had good/excellent cosmesis prior to APBI and at last follow-up. Nearly two thirds of patients experienced no acute skin toxicity with the remaining having small regions of grade 1 erythema. There were no observed CTCAEv4.0 grade 3+ toxicities following APBI.APBI with reduced margins using MR-IGRT is safe and has excellent cosmetic outcomes with acceptably low initial rates of IBTR.