Objectives:

The proximal bronchial tree (PBT) is a critical organ at risk (OAR) during delivery of stereotactic body radiation therapy (SBRT) for centrally located thoracic tumors. While motion management techniques are utilized to account for tumor motion, PBT movement is often not considered during treatment planning. We examined the magnitude of PBT movement across all phases of the respiratory cycle for patients with central non-small cell lung cancer (NSCLC) tumors undergoing thoracic SBRT.

Methods:

A single-institutional retrospective review of patients with primary NSCLC undergoing definitive SBRT for centrally located tumors from February 2013 to November 2021 was performed. Central tumors were defined as within 2 cm of the proximal airway, mediastinum, great vessels, or spinal cord whereas ultracentral tumors were directly abutting any of the above structures. All patients underwent five-fraction SBRT to a total dose of 50 to 60 Gy and utilized phased-based motion management during treatment delivery. The PBT was contoured in all 10 phases of the respiratory cycle. The total deviation of the PBT center of mass across all respiratory phases was calculated along the x-axis, y-axis, and z-axis.

Results:

Among 10 included patients, the median age was 68 years (range,47-80), and 80% patients were female. The median PBT volume was 27.1 cc. The median PBT deviation in the x-axis was 0.3 cm (range, 0.1-0.4), in the y-axis was 0.2 (range, 0-0.4), and in the z-axis was 0.6 (range, 0.2-0.7).

Conclusion(s):

Significant motion of the PBT is observed during SBRT delivery, which can have important implications for mitigating toxicity. Deviations of the PBT from respiratory motion may be used to inform planning organ at risk volume (PRV) expansions and selection of treatment phases based on both tumor and OAR motion.