anterior, posterior, superior, and inferior directions. To assess cavity movement from preto post-delivery of each fraction, a total of 100 preand post-fraction MR scans were manually registered based on the cavity, and resultant cavity translations in the longitudinal, vertical, and lateral planes were determined. Results: Onboard sagittal cine MR was visually monitored at the treatment machine and qualitative evaluation demonstrated no significant intrafraction motion of the lumpectomy cavity. In quantitative analysis of intrafraction motion, the mean movement of 4 tracking points placed at the superior, inferior, anterior, and posterior cavity edges was 0.34 0.07, 0.45 0.12, 0.42 0.09, and 0.28 0.07 mm in the ant-post direction; and 0.44 0.09, 0.62 0.10, 0.46 0.08, and 0.61 0.10 mm in the supinf direction, respectively. Overall, the median motion of any tracking point in any direction was < 0.5 mm. Mean translation of the lumpectomy cavity from preto post-fraction scans in the longitudinal, vertical, and lateral planes was 0.44 0.32, 0.40 0.30 and 0.41 0.32 mm, respectively. Conclusion: Analysis of intra-fraction motion of the lumpectomy cavity utilizing continuous on-board cine MR images and preand post-fraction MR imaging confirm minimal cavity motion, on the order of < 0.5 mm, in patients undergoing APBI with MR-IGRT. Given the minimal cavity motion and excellent visualization with onboard MRI, we used a 0 mm margin for setup uncertainty and intra-fraction motion, which maintained the ability to target the cavity and adjacent breast tissue at risk, while treating a smaller resultant PTV. Resultant reductions in dose to the ipsilateral normal breast tissue and adjacent normal structures such as the heart and lungs may impact late effects and cosmesis. This work, to our knowledge, is the first to describe real-time movement of lumpectomy cavities on MRI during radiation therapy. Author Disclosure: B.W. Fischer-Valuck: None. T.R. Mazur: None. S. Acharya: None. H. Wan: None. R. Kashani: None. H. Li: None. I. Zoberi: None. M.A. Thomas: Employee; Washington University.