Dosimetric Analysis of Proximal Bronchial Tree Subsegments to Assess The Risk of Severe Toxicity After Stereotactic Body Radiation Therapy of Ultra-central Lung Tumors

Background and purpose: Stereotactic body radiotherapy (SBRT) of ultra-central lung tumors (UCLT) is associated with an increased risk of severe toxicity. The aim of this study was to perform a detailed dosimetric analysis of the proximal bronchial tree (PBT) anatomical sub-segments to evaluate the safety of risk-adapted SBRT and to evaluate potential differences in radiation tolerance between PBT sub-segments.Material and methods: Fifty-seven patients treated with SBRT for UCLT between 2014 and 2021 were included. UCLT were defined as tumor abutting or overlapping with the trachea, PBT, or esophagus. This study analyzed overall survival, local control, progression-free survival, and grade >= 3 toxicity events. Bayesian inference was used to build a dose-response model with upper limits for toxicity. Results: Twenty-seven (47.4%) of the irradiated lesions were primary or locoregionally recurrent NSCLC and 30 (52.6%) oligometastases. All patients were treated with risk-adapted SBRT of median 45.0 Gy (range: 30.0-60.0 Gy) in 8 or 10 fractions. Grade >= 3 radiation pneumonitis was observed in two patients (3.5%), while no bronchial stenosis, hemorrhage or fistula were observed. The dose-response model predicted a grade >= 3 toxicity (stenosis, hemorrhage or fistula) limited to 4.9% (0 -11.4%) when delivering EQD2_3 = 100 Gy to any location of the PBT (D0.2cc). Detailed dosimetric analysis of PBT substructures showed no variation in the dose -response model between the anatomical PBT sub-segments. Conclusion: Risk-adapted SBRT regimens delivered in 8 or 10 fractions for ultra-central lung tumors resulted in high rates of local tumor control with low toxicity rates, without differences in radiation tolerance between the anatomical PBT sub-segments.