Purpose/Objective(s):
Stereotactic body radiotherapy (SBRT) is the primary curative therapy for patients with inoperable stage I lung tumors. Despite advances in SBRT delivery, existing SBRT airway constraints do not capture subsegmental dose–response relationships or radiotherapy (RT)-induced airway remodeling that may lead to stenosis or collapse. Quantifying imaging-based airway changes after SBRT may help clarify regional RT sensitivity, refine dose–response modeling, and improve prediction of clinically meaningful airway toxicity. This study aimed to quantify early changes in the dimensions of irradiated airways and compare them to those of non-irradiated airways within the first-year following SBRT. We hypothesized that irradiated airways would demonstrate greater markers of early airway injury, such as wall thickening and luminal narrowing.
Materials/Methods:
We retrospectively evaluated 22 patients who received a single course of SBRT for central (planning tumor volume [PTV] ≤2 cm from the bronchial tree) or ultracentral (PTV abutting the bronchial tree) thoracic tumors. CT scans from simulation and at 4, 8, and 14 (±2) months post-treatment were used to segment the airway tree and quantify lumen area (LA) and wall thickness (WT). Irradiated airways were defined as ≥5 Gy and non‑irradiated as <1 Gy; the 5 Gy cutoff is a conservative threshold informed by clinical evidence that airway injury emerges at doses of several Gy and is used to flag small‑volume subsegments at risk. Dose mapping was performed by registering the segmented airways to the delivered RT plan. Within-patient differences in the change from pre-RT between irradiated and non-irradiated airways at each follow-up timepoint were evaluated using delta regression analyses, accounting for repeated measures and CT lung volume.
Results:
Of the 22 patients (mean age: 70±9 years) evaluated, 82% and 18% were treated for central and ultracentral tumors, with SBRT doses 46-50 Gy delivered in 4-5 fractions. Median imaging follow-up was 6.4 months (IQR 3.8-8.6). The adjusted difference in change in LA from pre-RT to 4, 8, and 14 months post-RT between the irradiated and non-irradiated airways was -1.1 mm2 (95% CI -5.6 to 3.4 mm2 [P=0.6]), 4.7 mm2 (95% CI -1.5 to 10.9 mm2 [P=0.1]), and 0.2 mm2 (95% CI -5.4 to 5.8 mm2 [P=0.9]). The adjusted difference in change in WT from pre-RT to 4, 8, and 14 months post-RT was 0.4 mm (95% CI 0.03 to 0.8 mm [P=0.03]), 0.3 mm (95% CI 0.03 to 0.7 mm [P=0.03]), and 0.3 mm (95% CI -0.08 to 0.7 mm [P=0.1]).
Conclusion:
This study is the first to quantitatively report the impact of central and ultracentral SBRT on airway lumen area and wall thickness in the first-year post-SBRT. The findings reveal that irradiated airways exhibit significant wall thickening at both 4 and 8 months post-SBRT compared to non-irradiated airways. Although the sample size is small, these results demonstrate the feasibility of evaluating early SBRT-induced airway toxicity.
