Is proton therapy for synchronous bilateral breast cancer a medical necessity?

Summary: Synchronous bilateral breast cancer (sBBC) is rare, more common in younger patients, and associated with median overall survival of only 62 months. sBBC presents a challenge for radiotherapy (RT) planning due to large target area and complex target shape. Traditional tangential techniques result in high maxima due to field overlap, low coverage of lymph nodes, and high dose to cardiopulmonary structures. Volumetric modulated arc therapy (VMAT) dosimetry has improved on traditional techniques, but VMAT doses to heart and lungs in sBBC consistently exceed standard dose constraints in published studies. While intensity modulated proton therapy (IMPT) remains an investigational treatment for breast cancer, often involving long delays before insurance approval, IMPT physical properties appear ideal to address challenges presented by sBBC radiation planning. We hypothesized IMPT dosimetric advantages over VMAT plans are dramatic enough to position IMPT as the RT standard of care for this vulnerable patient population. We retrospectively reviewed the radiation plans of 4 consecutive sBBC patients treated at our institution with intensity-modulated proton therapy (IMPT) from September 2020 to February 2021. IMPT prescriptions were for 45-50 Gy in 25 fractions with 95% of the target volumes covered by 95% of the prescription dose and dose constraints to OARs per published prospective trials. VMAT plans were designed for the same patients with the same target coverage and OAR constraints for comparison. Three sBBC patients received treatment to the bilateral reconstructed breasts/chest wall and bilateral regional nodes and 1 patient received treatment to the bilateral intact breasts. As compared to VMAT, IMPT significantly decreased the average mean heart dose from 7 Gy to 1.3 Gy and the average V5 heart volume from 75% to 7.4%. IMPT also improved the bilateral lungs V5 from 95% to 33.9% and V20 from 27% to 12.1%. In the challenging case of sBBC, IMPT allows for conformal treatment of the bilateral breasts/chest wall with regional nodes while dramatically sparing the heart and lungs. In contrast, VMAT provides unacceptable plans that can lead to severe lung and heart toxicity. This stark difference in OAR sparing, even in a small sample, argues that IMPT should be incorporated into CMS guidelines as a medical necessity for sBBC. Cyrus Washington, MD (Presenting Author) - Jackson Memorial Hospital/University of Miami; Jessica Meshman, MD (Co-Author) - Sylvester Comprehensive Cancer Center/University of Miami