Stereotactic body radiotherapy for central non-small cell lung cancer: risk analysis of radiation pneumonitis and bronchial dose constraints

The present study investigated risk factors and bronchial dose constraints for symptomatic radiation pneumonitis (RP) in stereotactic body radiotherapy (SBRT) for central early-stage non-small cell lung cancer (NSCLC). We reviewed 245 patients with early-stage NSCLC treated with SBRT, and 78 patients with a tumor within 3 cm of the main or lobar bronchus were included in this study. Dose-volume histogram data were converted to a 4-fraction equivalent using the linear-quadratic model with an α/β value of 3. To examine the independent effects of dose parameters on grade ≥ 2 RP after adjusting for clinical factors, the Fine-Gray model with death as a competing risk was used for evaluation. With a median follow-up period of 44 months, the 4-year cumulative incidence of grade ≥ 2 and ≥ 3 RP was 22.5% and 8.5%, respectively. After adjustment for clinical factors, 6 bronchial dosimetric factors were significantly associated with grade ≥ 2 RP. Lung dosimetric factors were not significantly associated with grade ≥ 2 RP. Among significant dosimetric factors of the bronchus, bronchus V35Gy had the highest hazard ratio (HR) (HR 1.24, 95% CI 1.03-1.49, P = 0.027). The optimal threshold for bronchus V35Gy based on receiver operating characteristic curve analysis was 0.04 cc. The 4-year incidence of grade ≥ 2 RP in the bronchus V35Gy ≤ 0.04 cc vs. >0.04 cc groups was 15.7% vs. 37.0% (P = 0.036). In SBRT for central early-stage NSCLC, bronchus V35Gy < 0.04 cc is the definitive indicator for preventing grade ≥ 2 RP.