To evaluate radiation-induced rib fractures (RIRFs) after PBT, this study from Japan included 85 stage I NSCLC patients treated with PBT to a total dose 66-70 GyRBE in 10 fractions for analysis. Rib fractures were detected by follow-up chest CT examination, independent of symptoms of thoracic pain. This study reported 116 fractured ribs in 55 patients (64.7%). The 2- and 3-year frequencies of experiencing at least one RIRF were 36.5% and 52.9%, respectively. In addition to exposure of small-rib volumes to high dose radiation, the authors suggested that multiple factors, including use of systemic corticosteroids, 1st rib, bone mineral density, and position of the maximum dose point, are associated with frequencies of RIRF.