Abstract
Synchronous bilateral breast cancer (sBBC) is rare, more common in younger patients, and associated with median overall survival of about 5 years. sBBC presents a challenge for radiotherapy (RT) planning due to the large and complex target volumes. Traditional tangential techniques result in hot spots due to field overlap, low coverage of lymph nodes, and high dose to cardiopulmonary structures. Although intensity-modulated radiation therapy (IMRT) dosimetry has improved, IMRT doses to heart and lungs in sBBC consistently exceed standard dose constraints. We hypothesize intensity-modulated proton therapy (IMPT) may offer dosimetric advantages over IMRT plans that are dramatic enough to position IMPT as the RT standard of care for synchronous bilateral breast cancer.
We retrospectively reviewed the radiation plans of all 17 consecutive sBBC patients treated at our institution with IMPT from 2020 to 2023. IMPT prescriptions were for 45-50 Gray (Gy) in 25-28 fractions with 95% of the target volumes covered by 95% of the prescription dose and dose constraints to OARs per published prospective trials. IMRT comparison plans were designed for 11 patients.
Treatment was delivered to the bilateral reconstructed chest wall and regional nodes (n = 8), bilateral intact breasts and regional nodes (n = 5) or to the bilateral intact breasts without nodes (n = 4). Of all the proton plans, heart mean dose was 0.7 Gy, left anterior descending artery (LAD) mean dose was 4.7 Gy, lung mean dose was 5.3 Gy, lung V20 was 8.3%, and lung V5 was 28.8%. Among the comparison plans using the same anatomy (Table 1), IMPT significantly decreased the average mean heart dose from 6.5 Gy to 0.67 Gy and LAD dose from 11 Gy to 3 Gy, as compared to IMRT. IMPT also decreased the bilateral lung V5 from 83% to 28% and V20 from 22% to 7%, as compared to IMRT.
In the challenging case of sBBC, IMPT allows for conformal treatment of bilateral breast cancer while dramatically sparing the heart and lungs. In contrast, IMRT provides unacceptable plans that can lead to critical lung and heart toxicity- unable to meet standard heart mean, lung mean, or lung V5 constraints. 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.