Background: The Radiation Treatment Program (RTP) at Cancer Care Ontario (CCO) established several Communities of Practice (CoPs), with the goal of improving radiation treatment (RT) quality and safety. The RTP identifies variation in practice and quality improvement (QI) opportunities in the 14 Regional Cancer Centres (RCCs) and facilitates the development of CoPs to share best practices and standardize care. Methods: Since 2010, the RTP has formed 7 CoPs ( > 185 members in total): 4 intra-disciplinary (Radiation Therapy, Medical Physics, Advanced Practice Radiation Therapy, Radiation Safety) and 3 inter-disciplinary (Head and Neck (HN), Gynecological (GYNE) and Lung Cancer). Members are recruited with the aim of securing engagement from all RCCs to ensure representation of regional diversity and to facilitate adoption of best practices. CoPs are supported with nominal funding and resources provided by CCO, but are led and driven by members, who identify and prioritize key quality issues and select corresponding QI projects to pursue. The RTP performs regular evaluation activities to assess initiative engagement and impact. Results: RTP CoPs have enhanced the quality and safety of RT delivery in Ontario through QI initiatives, advice documents and tools that have enabled: Improved RT safety (use of safety straps in RT delivery); Adoption of best practices (RT plan evaluation guidance); Education and knowledge transfer – (stereotactic body RT implementation and training framework); and Support for infrastructure improvements (recommendation for additional Magnetic Resonance-guided brachytherapy units) (https://www.cancercare.on.ca/ocs/clinicalprogs/radiationtreatment/). Advice documents have improved alignment with recommended practice (40% and 50% absolute increases in two HN initiatives). Evaluation surveys indicate that members believe the CoPs have enhanced inter-regional communication and collaboration (89%), knowledge transfer/exchange (91%), and professional networking between RCCs (92%). Conclusions: CoPs can be a highly effective model for improving quality of care. The establishment of CoPs should be considered for QI in other areas of the healthcare system.