MA02.06 Dose-Volume Factors Predicting Airway Stenosis After SBRT for Ultra-Central Lung Tumors

The safety of SBRT is uncertain for ultra-central tumors (near the proximal airways or esophagus). One potential toxic effect of ultra-central SBRT is stenosis of the proximal airways, which can lead to airway obstruction and lung collapse. Predictors of such toxicity in this population are urgently needed. We therefore studied dose-volume correlates of airway stenosis after ultra-central SBRT. 88 patients with tumors abutting the proximal bronchial tree (PBT) or PTVs overlapping esophagus (n = 76 and 23; 11 met both criteria) were included. 53 (60%) had primary/locally recurrent lung cancer, and 35 had lung metastases. All had 5, 8 or 15 fractions of image-guided radiotherapy with BED ≥84Gy (α/β=10). The lobar bronchi (LB) were contoured from the takeoff from the main bronchus to the bifurcation into segmental bronchi. The primary endpoint was grade 2 or higher lobar bronchial stenosis (LBS), defined as radiographic evidence of narrowing or complete obstruction of at least one lobar bronchus (CTCAE v4). Dose-volume histograms (DVHs) using linear-quadratic equivalent doses in 2 Gy fractions were calculated for the LB with α/β = 3 Gy. Mean equivalent doses (MEDs) to the LB were tested for correlation with LBS using a Cox proportional hazards model, and the log rank test with patient data split at the median value of the MEDs to the LB. Statistical significance was defined as p <0.05. Median follow up was 14.3 months. There were 24 cases of LBS (27%). Median time to onset of LBS2+ was 8.6 months after end of treatment (range 2-19 months). LBS was significantly correlated with MED to the LB (p = 0.02). Incidence of LBS was significantly different in patients with MED to the LB < or > the median value of 35.4 Gy (p = 0.004 log-rank test), with actuarial rates of 19% and 55% respectively at 14 months, and 19% and 70% respectively at 24 months; and with raw rates of 15.9% and 38.6% respectively. We observed a high rate of lobar stenosis after ultra-central SBRT. Incidence of lobar stenosis was significantly correlated with dose to the lobar bronchi. In particular, mean equivalent dose to the lobar bronchi was significantly correlated with LBS. Our analysis suggests that limiting the mean equivalent dose to the lobar bronchi to < 35.4 Gy results in a two year actuarial incidence of LBS of <19%, and a raw incidence <16%.