Late radiologic changes after stereotactic ablative radiotherapy for early stage lung cancer: A comparison of fixed-beam versus arc delivery techniques

Background and purpose: To characterize the radiologic changes occurring following arc stereotactic ablative radiotherapy (SABR) for early-stage non-small cell lung cancer relative to those following fixed-beam SABR. Methods: Twenty-nine patients treated with arc SABR without local recurrence and more than two years follow-up were retrospectively evaluated using a published scoring system. The late morphologic patterns, timing and severity of radiologic change were assessed and compared to 54 patients treated with fixed-beam SABR that we previously assessed using the same system. Results: The baseline characteristics and follow-up of both cohorts were well matched and SABR technique was not associated with morphologic differences before 6 months (p = 0.23). Thereafter the predicted probabilities of a modified-conventional pattern following arc and fixed-beam SABR were 96.3% vs. 68.9%, respectively (OR 11.7, 95% CI 3.38–40.8, p < 0.001). In addition, at 1 year follow-up the predicted probabilities of arc and fixed-beam SABR patients having expected or pronounced radiologic changes were 64.9% and 22.1%, respectively (OR = 6.56, 95% CI: 3.13–13,7, p < 0.001). Conclusions: Post-SABR radiologic changes differ with delivery technique, which has important implications during follow-up. Confirmation in larger studies is required and etiologic factors remain to be determined