The American Society for Radiation Oncology (ASTRO) and the Society of Surgical Oncology (SSO) published a consensus guideline on margins for breast-conserving surgery with whole-breast irradiation in stages I and II invasive breast cancer. The guideline document represents an intensive collaboration among experts in the radiation oncology and surgical oncology fields, led by Meena S. Moran, M.D., associate professor of the department of therapeutic radiology at Yale School of Medicine in New Haven, Conn., on behalf of ASTRO, and Monica Morrow, M.D., SSO immediate past president, breast cancer surgeon and chief of breast surgery at Memorial Sloan-Kettering Cancer Center in New York, co-chairs of the Margin Consensus Panel (MCP). In addition to determining the ideal margin width that minimizes the risk of ipsilateral breast tumor recurrence (IBTR), the guideline outlines an evidence-based surgical treatment path that could reduce unnecessary surgery for patients.