Background: Peer review is an important quality assurance (QA) tool in cancer care. For radiotherapy (XRT), peer review is challenging because of the complexity of XRT targets and treatment planning and significant patient numbers. During review target and contour, volumes of adjacent normal tissues are evaluated. The XRT dose to targets/normal tissues are assessed for coverage and safety. Timing of review is important, ideally occurring prior to start of XRT or before 25% of XRT dose is delivered to allow for change (CPQR, 2015). Cancer Care Ontario (CCO) has yearly targets for review of curative/adjuvant courses: 2015, 50%; 2016, 60%; 2017, 75%; and 2018, 80%. In the past 2 years, CCO has focused on introducing peer review for complex cases of palliative XRT. The Ottawa Hospital (TOH) is one of 14 Ontario Regional Ca Centers. TOH services 1.3 million people, and 4,500 patients receive XRT per year. Methods: We reviewed peer review rates for curative and palliative XRT at TOH to assess improvements in QA over the last 4 years. We evaluated if TOH met CCO targets. Timing for peer review was assessed. We evaluated if peer review rates varied by tumour site. Rates of plan modification were recorded by year and tumor site. Reasons for plan modification were evaluated. Results: 3,694 curative/adjuvant courses were completed in 2018. Peer review increased over 4 years: 2015, 62%; 2016, 64%; 2017, 80%, 2018, 84 %. Optimal timing of peer review improved over 4 years (% completed prior to 1st XRT): 2015, 43%; 2016, 44%; 2017, 58%; 2018, 64%. For palliative QA, 21 % were peer reviewed (improvement of 14% over 2017). In 2018, peer review varied by tumor site (range: 61% to 97%); however, for most sites, rates improved over 4 years. Following peer review, a change was recommended in 8% (2% major, 6% minor). The rate did not differ over 4 years (8.2% 2015 vs 8.0% 2018). Recommended changes varied by tumor site (range, 0%–18%). Conclusions: Peer review is an important tool ensuring safe quality XRT. At TOH there has been a significant improvement in peer review over the last 4 years. TOH exceeds provincial targets for curative courses completed, and more peer review activities are being completed prior to the start of the treatment. Rates of peer review and recommended changes vary dramatically amongst tumor sites. Reasons for these variations will be explored in future research. Extension of peer review to palliative XRT has recently been implemented to further improve quality cancer care.