Characterization of rib fracture development following liver directed stereotactic body radiation therapy

Purpose: Rib fractures are a well-described complication following thoracic stereotactic body radiation therapy (SBRT). However, there are limited data in the setting of liver-directed SBRT. Methods: Patients who underwent liver SBRT from 2014 to 2019 were analyzed. Logistic regression models were used to identify the demographic, clinical, and dosimetric factors associated with the development of rib fractures. Results: Three hundred and forty-three consecutive patients were reviewed with median followup of 9.3 months (interquartile range [IQR]: 4.7-17.4 months); 81% of patients had primary liver tumors and 19% had liver metastases. Twenty-one patients (6.2%) developed rib fractures with a median time to diagnosis of 7 months following SBRT (IQR: 5-19 months). Of those patients, 11 experienced concomitant chest wall pain, while 10 patients had an incidental finding of a rib fracture on imaging. On univariate analysis, female gender (odds ratio [OR]: 2.29; p = 0.05), V30 Gy (OR: 1.02; p < 0.001), V40 Gy (OR: 1.08; p < 0.001), maximum chest wall dose (OR: 1.1; p < 0.001), and chest wall D30 cm(3) (OR: 1.09; p < 0.001) were associated with an increased probability of developing a rib fracture. On multivariate analysis, maximum chest wall dose (OR: 1.1; p < 0.001) was associated with developing a rib fracture. Receipt of more than one course of SBRT (p = 0.34), left versus right sided lesion (p = 0.69), osteoporosis (p = 0.54), age (p = 0.82), and PTV volume (p = 0.55) were not significant. Conclusions: Rib fractures following liver SBRT were observed in 6.2% of patients with the majority being asymptomatic. To mitigate this risk, clinicians should minimize dose delivery to the chest wall. Female patients may be at increased risk.