To evaluate the incidence of radiation induced rib fracture after lung stereotactic ablative radiotherapy (SABR) and identify risk factors for the development of rib fracture.
We investigated the patients treated with SABR for primary or metastatic lung tumors using Cyberknife® between July 2008 and October 2014. Of the total 44 patients, patients with less than 6 months of follow-up were excluded and final 39 patients with 53 lesions were analyzed in this study. The prescription dose ranged from 45 Gy to 60 Gy in 3-5 fractions depending on the risks. To determine the risk factors associated with rib fractures, patient characteristics, treatment factors and variables obtained from dose-volume histograms (DVHs) were analyzed. To account for different fraction size, equivalent dose in 2 Gy fractions (EQD2) was calculated and used as a common metric.
Overall median survival time was 33.0 months in all patients, 26.7 in patients with metastatic lung tumors and 38.7 in those with primary lung tumors. Two-year local control rate was 75.2%. After follow-up of median 26.7 months (range: 8.4–80.0), 8 patients experienced rib fractures. Among the patients with rib fracture, four were asymptomatic, three complained of chest wall pain and the remaining one patient had initial chest wall pain before SABR. Median time to rib fracture was 13.4 months (range: 8.0–38.5) and the 2-year actuarial risk of rib fracture was 14.1%. Dose to the 0.5cc of the ribs (D0.5cc), rib volume received 150 Gy or more (V150Gy), maximum dose to the ribs (Dmax) and distance between tumor and rib were significant predictor for rib fracture. Multivariate analysis showed that D0.5cc >270 Gy and V150Gy >15cc were statistically significant risk factors for development of rib fracture.
Parameters from DVH are useful in predicting the risk of rib fractures after SABR for lung tumors. Efforts should be made to reduce the risk of the rib fracture after lung SABR.
Clinical trial identification
All authors have declared no conflicts of interest.