PurposeTo determine the rib fracture rate in a cohort of patients with breast cancer treated with proton therapy.Patient and MethodsFrom a prospective database, we identified 225 patients treated with proton therapy between 2012 and 2020 (223 women; 2 men). Clinical and dosimetric data were extracted, the cumulative incidence method assessed rib fracture rate, and Fine-Gray tests assessed prognostic significance of select variables. In-field rib fracture was defined as a fracture that occurred in a rib located within the 10% isodose line. Out-of-field rib fracture was defined as a fracture occurring in a rib location outside of the 10% isodose line.ResultsOf the patients, 74% had left-sided breast cancer; 5%, bilateral; and 21%, right-sided. Dual-energy x-ray absorptiometry scans showed normality in 20%, osteopenia in 34%, and osteoporosis in 6% (test not performed in 40%). Additionally, 57% received an aromatase inhibitor. Target volumes were breast ± internal mammary nodes (IMNs) (16%), breast and comprehensive regional lymphatics (32%), chest wall ± IMNs (1%), and chest wall/comprehensive regional lymphatics (51%). Passive-scattered proton therapy was used for 41% of patients, 58% underwent pencil-beam scanning (PBS), and 1% underwent a combination (passive scattering/PBS), with 85% of patients receiving a boost. Median follow-up was 3.1 years, with 97% having >12-month follow-up. The 3-year cumulative in-field rib fracture incidence was 3.7%. Eight patients developed in-field rib fractures (1 symptomatic, 7 imaging identified) for a 0.4% symptomatic rib fracture rate. Median time from radiation completion to rib fracture identification was 1.8 years (fractures were identified within 2.2 years for 7 of 8 patients). No variables were associated with rib fracture on univariate analysis. Three fractures developed outside the radiation field (0.9% cumulative incidence of out-of-field rib fracture).ConclusionIn this series of patients with breast cancer treated with proton therapy, the 3-year rib fracture rates remain low (in-field 3.7%; symptomatic 0.4%). As in photon therapy, the asymptomatic rate may be underestimated owing to a lack of routine surveillance imaging. However, patients experiencing symptomatic rib fractures after proton therapy for breast cancer are rare.