Predictors of chest wall toxicity after stereotactic ablative radiotherapy using real-time tumor tracking for lung tumors

Background: To evaluate the incidence of chest wall toxicity after lung stereotactic ablative radiotherapy (SABR) and identify risk factors for the development of rib fracture. Methods: Thirty-nine patients with 49 lesions underwent SABR for primary or metastatic lung tumors using Cyberknife r with tumor tracking systems. Patient characteristics, treatment factors and variables obtained from dose-volume histograms (DVHs) were analyzed to find the association with chest wall toxicity. Four-dimensional (4D) dose calculations were done to investigate the effect of respiratory motion on dose to the ribs. Results: After follow-up of median 26.7 months (range: 8.4-80.0), 8 patients (20.5%) experienced rib fractures and among these patients, three (37.5%) had chest wall pain at 2-3 months after 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 12.2%. Dose to the 4.6 cc of the ribs (D-4.6cc) and rib volume received 160 Gy or more (V-160) were significant predictor for rib fracture. No significant differences between three-dimensional (3D) and 4D dose calculations were found. Conclusions: Parameters from DVH are useful in predicting the risk of chest wall toxicity after SABR for lung tumors. Efforts should be made to reduce the risk of the rib fracture after lung SABR.