3469 - Prognostic Impact of Endobronchial Involvement in Ultracentral NSCLC Treated with Stereotactic Ablative Radiotherapy

Purpose/Objective(s): Ultracentral (UC) NSCLC, defined as tumor with a planning target volume (PTV) overlapping central airways or major mediastinal structures, poses significant treatment challenges. Stereotactic ablative radiotherapy (SABR) is a standard therapy for medically inoperable early-stage NSCLC. Treatment of UC tumors has been associated with a higher risk of bronchopulmonary hemorrhage and airway injury. The effect of endobronchial involvement on such risks is unclear. This study evaluates the impact of EB involvement on clinical outcomes in a cohort of patients with UC lung tumors treated with SABR.

Materials/Methods: We conducted a retrospective analysis of 33 patients with UC NSCLC treated with SABR from 2008-2020. Patients were stratified into three groups based on pre-treatment imaging (n=33) with or without EBUS (n=11): no EB involvement (n=19), EB abutment (n=9), and EB invasion (n=5). Kaplan-Meier estimates were calculated for local, regional (nodal), and distant recurrence-free survival (RFS) and overall survival. Toxicity outcomes were also recorded. Statistical comparisons between patients with no EB involvement and those with EB abutment or EB invasion were performed using the log-rank test. Significance was assumed at p=0.05.

Results:

33 patients received a median dose of 50 Gy (range, 48–70 Gy) in a median of 5 fractions (range, 4–8), all achieving a BED >100 Gy with the dose normalized so that GTV and PTV received 1005 and 95% of the prescription dose, respectively. Median follow-up was 28 months. Local failure was not significantly different between groups with a local RFS of 9.1%. Regional nodal and ipsilateral lobar failure were also not significantly different between groups with an overall RFS of 18.2% and 24.2%, respectively. Differences in distant failure approached significance between the three groups with 15.8%, 44.4%, and 40% for no EB involvement, abutment, and invasion respectively (p=0.069). When any EB involvement was grouped, distant RFS was 42.9% and was significantly higher than the no involvement group (p=0.036). OS was not significantly different between groups with a median OS of 31 months. The only grade =3 toxicities reported (both lobar collapse) were in 2 patients in the no EB involvement group with no patients of either EB involvement cohort developing any significant airway toxicity.

Conclusion: In this cohort of patients with UC NSCLC, EB abutment and invasion were not associated with inferior local, regional nodal, ipsilateral lobar control, or survival following SABR. However, EB involvement was associated with a higher risk of distant metastases which may warrant further investigation. No significant increase in treatment-related toxicity was observed in patients with EB invasion, supporting the safety of UHF-RT in this subset when appropriately fractionated without significant dose heterogeneity. These findings suggest that the presence of EB disease should not automatically preclude SABR.