TH-A-137-07: Local Control Differences for SBRT Lung Patients Planned with Pencil Beam Vs. Collapsed Cone Convolution Algorithms

Abstract

Purpose: There are significant differences between pencil beam and convolution algorithms when used in the thorax. However, these differences have not been previously directly correlated with lung SBRT outcomes. Methods: Data from 237 patients treated with SBRT for NSCLC or metastatic disease from other sites were retrospectively analyzed. Of those, 138 were planned using Brainlab treatment planning software (TPS) with the pencil beam (PB) algorithm and treated on Novalis with fiducial marker-based, dual planar radiograph image guidance. Ninety-nine were planned using Pinnacle TPS with collapsed cone convolution (CCC) algorithm (considered a more accurate model) and treated on Varian linacs with CBCT image guidance. Target delineation/expansion guidelines, nominal prescription values, and dose restrictions were identical between the two groups. Median follow-up was 28 and 17 months for the PB and CCC groups, respectively. Kaplan-Meier survival curves were used to determine the statistical differences in local/marginal control between the two groups. Forty-nine plans done in Brainlab (24 with and 25 without local recurrence) were reproduced in Pinnacle with the same MUs. D99 (PTV, GITV), V90 (PTV, GITV), D95 (PTV) and the isocenter dose (D_ISO) were compared. Results: Twenty-seven patients (19.6%) planned with the PB and 5 patients (5.1%) planned with the CCC algorithms developed local recurrence. Mantel-Cox and Wilcoxon tests show significant differences between the two groups (P< 0.02). Differences between the two algorithms for target coverage were: D99_GITV=13.9%, D99_PTV=20.8%, V90_GITV=13.7%, V90_PTV=37.5%, D95_PTV=18.9% and D_ISO=6%. Conclusion: The outcome differences between the patients planned with the PB vs CCC algorithms were statistically significant. Even after controlling for potential variables between the two groups, there appears to be a loss of local control when the delivered dose is about 20% lower than the prescription. Conversely, potential benefit with dose escalation cannot be excluded.