A total of 133 lung SBRT patients were preselected for direct soft-tissue tracking based on manufacturer recommendations (peripherally located tumors ¡Ý1.5 cm with a dense appearance) and staff experience. Patients underwent a tumor visualization test to verify adequate detection by the tracking system (orthogonal radiographs). An analysis of potential predictors of successful tumor tracking was conducted looking at: tumor stage, size, histology, tumor projection on the vertebral column or mediastinum, distance to the diaphragm, lung-to-soft tissue ratio, and patient body mass index.
Tumor visualization was satisfactory for 88 patients (66 % ) and unsatisfactory for 45 patients (34 % ). Median time to treatment start was 6 days in the success group (range, 2-18 days) and 15 days (range, 3-63 days) in the failure group. A stage T2 (P=.04), larger tumor size (volume of 15.3 cm3 vs 6.5 cm3 in success and failure group, respectively) (P<.0001), and higher tumor density (0.86 g/cm3 vs 0.79 g/cm3) were predictive of adequate detection. There was a 63 % decrease in failure risk with every 1-cm increase in maximum tumor dimension (relative risk for failure = 0.37, CI=0.23-0.60, P=.001). A diameter of 3.6 cm predicted a success probability of 80 % . Histology, lung-to-soft tissue ratio, distance to diaphragm, patient's body mass index, and tumor projection on vertebral column and mediastinum were not found to be predictive of success.
Tumor size, volume, and density were the most predictive factors of a successful XSight Lung tumor tracking. Tumors >3.5 cm have ¡Ý80 % chance of being adequately visualized and therefore should all be considered for direct tumor tracking.