Central Lung Tumors Eligible for Stereotactic Ablative Radiotherapy (SABR) Trials: Analysis of a Cohort Treated Using MR-Guided SABR

Abstract

SABR for central and ultracentral lung tumors results in increased toxicity. We implemented MR-guided SABR in patients meeting the IASLC classification for central tumors, namely a Gross Tumor Volume (GTV) ≤2cm from trachea, main bronchi, esophagus, heart, brachial plexus, main mediastinal vessels, phrenic or laryngeal nerves and spinal cord. As patient inclusion criteria for ongoing clinical trials varies, we studied the eligibility of MR-SABR patients for inclusion into 3 clinical trials for central tumors. An Ethics approved institutional database was accessed for details of patients treated using multifractionated lung MR-SABR between 2016 and 2022. GTV and Planning Target Volumes (PTV) had been delineated on a breath-hold planning MRI and CT scans. For each tumor meeting the IASLC classification, patient eligibility for HILUS (NCT04917224), STRICT/STARLUNG (NCT05354596) and SUNSET (NCT03306680) trials were examined. Multifraction MR-SABR was performed in 97 central lung lesions meeting IASLC criteria. Of these, 73 patients (75.3%) fulfilled inclusion criteria for at least 1 trial, but inclusion in all 3 trials was possible in just 18.6% of treated lesions. Inclusion in the HILUS, STRICT/STARLUNG and SUNSET trials were possible in 33.0%, 73.2% and 25.8% of MR-SABR patients, respectively. For HILUS-eligible patients, 11 met the HILUS group A classification (GTV ≤1cm from the trachea and main bronchi), and 21 in group B (GTV >1cm from the trachea and main bronchi but ≤1cm from the intermediate and lobar bronchi). Use of STRICT/STARLUNG trial criteria would have defined 13 tumors as ultracentral (GTV ≤0.5cm from the trachea, main bronchi or intermediary bronchus or esophagus), and 58 as central (GTV 0.5cm-2.5cm from previously described structures or ≤0.5cm from the heart, aorta, spinal cord or lobar bronchi). For the SUNSET study, 25 tumors fulfilled criteria for ultracentral location (PTV touching or overlapping the central bronchial tree, esophagus, pulmonary vein or pulmonary artery). Ongoing clinical trials in central or ultracentral lung SABR enroll considerably different patient populations than those selected using the IASLC definition of central tumors. Due to steep dose-fall off in SABR plans, detailed descriptions of the location of tumor and organs-at-risk, as well as dose-distributions achieved, are necessary in order to allow study results to be uniformly extrapolated to clinical practice.