Spontaneous Rib Fractures After Breast Cancer Treatment Based on Bone Scans: Comparison Of Conventional Versus Hypofractionated Radiotherapy

Background Spontaneous rib fractures (SRFs) are defined as fractures without apparent blunt force trauma. This study evaluated the incidence and risk factors of SRFs after treatment of patients with breast cancer based on bone scans. In addition, we analyzed radiation-associated SRFs and identified radiotherapy (RT) factors related to SRF. Patients and Methods We retrospectively reviewed 1265 patients with breast cancer who underwent surgery in 2015 at our institution, and were followed-up with at least 3 bone scans. Bone scans were conducted approximately every 12 months after breast cancer treatment. The endpoint was SRF detected by bone scan. In this study, 754 (60%) patients were treated with chemotherapy, 867 (69%) with RT, and 946 (75%) with anti-hormone therapy. Results The median follow-up duration was 37.5 months. A total of 209 (16.5%) patients experienced SRFs during follow-up. The incidence of SRFs increased sharply during the 3-year follow-up period after completion of treatment. In multivariate analyses, abnormal bone density, chemotherapy, and RT were significant risk factors for SRFs. In patients treated with RT (n = 867), 159 (18%) rib fractures occurred: 127 (80%) in the ipsilateral breast and 32 (20%) in the contralateral breast. Among the patients with ipsilateral SRFs who received tumor bed boost (n = 84), the SRF occurred inside the boost field in 80 (95%) cases. Multivariate analysis of RT subgroups showed that hypofractionated RT increased the rate of SRFs (P = .002). Conclusions Most of the rib fractures that occurred after treatment were spontaneous. Hypofractionated RT increased the risk of ipsilateral rib fractures in RT-treated patients.