Delineation of treatment targets and organs-at-risk (OAR) for treatment planning is the standard of care across all disease sites for curative radiation therapy. However, this approach has been variably adopted for breast cancer with significant variation amongst experts regarding contouring breast cancer target volumes. In order to create a framework for the contouring of breast cancer cases, particularly within the context of clinical trials, the Radiation Therapy Oncology Group (RTOG) Breast Atlas was created in 2009. Since that time, numerous other atlases have been developed and several retrospective studies since 2015 have called into question the adequacy of clinical target volume delineation as defined by the RTOG atlas and other atlases based on patterns of failure.  Here, we will review the existing atlases, highlighting the differences between them and potential opportunities for improvement based on these patterns of failure studies. Ultimately, the results of recently completed and ongoing prospective clinical trials, which included contouring guidelines based on current atlases, will provide important information regarding patterns of failure and toxicity in patients treated with modern radiotherapy using a systematic target volume delineation and OAR-driven approach to treatment planning.

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