Background: Peer review (PR) is an essential component of quality assurance in radiation oncology practice, endorsed by Canadian cancer agencies as a national standard of quality care. However, reported patterns of care studies indicate that PR practice varies considerably, including which contours (e.g., target volumes, organs at risk (OARs)), which dose parameters (e.g., OAR constraints, homogeneity criteria), which decision making points (e.g., decision to treat, selection of anatomical regions), and which treatment imaging parameters are included in the PR process. Both qualitative and quantitative evidence support the need for guidance on PR best practices. Here we report processes for the establishment of best-practice guidelines/minimal standards for PR. Methods: A comprehensive literature review was done to quantify PR findings for each of 3 cancer streams: head/neck, lung, and breast. This evidence summarized which aspects of radiotherapy plans were most frequently "flagged" at PR. A modified Delphi process was used to develop cancer site-specific PR guidance documents: appropriate stakeholders were first identified; a pre-meeting survey determined opinions on which plan elements are essential/important to review; a face-to-face meeting considered the evidence base and survey results to reach consensus. Parallel processes were conducted separately for each stream. Results: The literature findings were compiled to summarize the reported proportion of plans flagged by PR, and the nature of the flag (i.e. volumes, dose, OARs, other) for each cancer stream. Stakeholders included radiation oncologists, physicists, radiation therapists, and patients, specific to each stream. Draft guidance documents were created on the basis of stakeholder consensus regarding which components of PR were deemed essential; other important or “second tier” elements were also identified. Conclusions: Guidance documents including minimum standards for PR can be successfully created based on structured consensus development informed by available evidence. This guidance promises to reduce variation in PR thereby optimizing the efficiencies of PR processes, and increasing the quality of radiation treatment plans.