Previous studies on PBSI demonstrate that it is a safe and effective alternative form of radiation for appropriately selected patients after lumpectomy. However, those results have been obtained mainly from a single institution, with only 4 patients treated in another center. Further research is still needed to evaluate its safety in a multi-center setting. The purpose of this study is to ensure the appropriate training of clinicians who will be performing this procedure and to capture long term outcomes and rare complications if any.