SU‐E‐T‐422: Evaluation of PTV Dose Margins for Lung SBRT On Cyberknife Treatment

Abstract

Purpose: The purpose of this study is to retrospectively evaluate the PTV margins used on CyberKnife for lung SBRT treatment. Methods: The CyberKnife system uses an active tracking system that models the motion of the target based on information from orthogonal images and the skin markers. Deviations between the model predicted position and the real target position show up when patient breaths irregularly. This discrepancy can be up to 1 cm for some situations. Some previous research suggested that 5 mm or 6 mm PTV margins are needed to cover the target 96∼98% of the time. The CK lung SBRT treatment usually prescribed at 55–70% isodose line (Monte Carlo dose calculation), that means the region covered by the full prescription dose in real delivery will be larger than the planned PTV region when the tracking uncertainty is accounted. This indicates that the PTV margin could be less than 5∼6 mm when the real dose is considered. 20 previous treated lung SBRT patients were examined. The real target location relatives to the model position at that time were extracted and the dose was recalculated based on the real target position using an in‐house developed Monte Carlo program. Results: Analysis of all the CyberKnife 20 patients (94 treatments) showed that by using 2 mm physical PTV margin, the region received full prescription dose was 5.4 mm +/− 1.6mm from the GTV. The planning criteria are 100% GTV volume receives at least 100% prescription dose. By consider the tracking uncertainty, for 2 mm physical PTV margin, 99.5 +/− 0.4% GTV volume still receives at least 100% prescription dose. The maximum dose reduced by about 12% on average. Conclusion: Conclusions: Image tracking log from the CyberKnife indicate that a 5mm margin may be overkill when motion tracking is used.