13 Background: Our outpatient radiation treatment centers implemented a peer review program in 2007 that required presentation of all definitive cases to a weekly peer review conference. The purpose of this review is to document compliance and determine how this program impacted care over 4 years. Methods: Between 9/2007 and 9/2011, 2,988 patients were eligible for peer review. Patient data was presented to a group of physicians, physicists, and dosimetrists via teleconferencing and the radiotherapy plan was reviewed. Details of any changes made to patient care were documented within a QA note dictated after discussion. Any changes recommended by the peer review process were categorized as changes to radiation dose, target, or major changes. Results: Breast cancer accounted for 47.9% of all cases, followed in frequency by H/N (14.8%), GI (9.9%), GU (9.3%), and thoracic (6.7%) malignancies. Of the 2,988 eligible patients, 158 (5.3%) were not presented for peer review. The number of missed presentations decreased over time; 2007-8.2%; 2008-5.7%; 2009-3.8%; and 2010-2.7% (p<0.001). The reason for a missed presentation was unknown, but varied by disease site and physician. Of the 2,830 cases presented for peer review, a change was recommended in 346 cases (12.2%) and categorized as a dose change in 28.3%, a target change in 69.1% and a major treatment change in 2.6%. When examined by year of treatment the number of changes recommended decreased over time: 2007-16.5%; 2008-11.5%; 2009-12.5%; and 2010-7.8% (p<0.001). The number of changes recommended varied by disease site and physician. H/N, GYN, and GI malignancies accounted for the majority of changes made. Conclusions: Compliance with this weekly peer review QA program was satisfactory and improved significantly over the 4 year study period. The QA program resulted in decreased treatment plan changes over time reflecting a move toward radiation treatment consensus and consistency. Weekly peer review directly improved care in all patients by creating a culture of QA where guideline adherence and discussion are part of normal practice.