SU‐E‐T‐476: 4‐Dimensional Dosimetric Comparison of Stereotactic Body Radiation Therapy for Lung Tumor With Versus Without Tumor Tracking Technique

Purpose: Recently a new treatment device capable of gimbal‐based tumor tracking (Vero, BrainLAB AG, Feldkirchen, Germany) was installed at our institution. The purpose of this study is to compare dose‐volume and radiobiological parameters associated with treatment efficacy and normal tissue toxicity for lung SBRT using a conventional linac with and without tumor tracking. Methods: A digital torso phantom having the capability to simulate respiratory and cardiac motion was used for this work. Spherical targets having a diameter of 20mm were centrally located within the right lung. 12 non‐coplanar, non‐opposing conformal beams were designed for linac planning with and without tumor tracking to deliver 60Gy in 3 fractions, prescribed to cover 95% of the target. 4D dose calculations were performed using the Monte Carlo algorithm in iPlan™ using a voxel tracking technique for the moving target and surrounding tissue. Diaphragm motion was varied from 10–30 mm in the superior‐inferior direction to simulate different motion amplitudes while limiting chest wall motion to 10mm in the anterior‐posterior direction. Dose‐volume analysis was performed using parameters previously shown to correlate with normal tissue toxicity for lung SBRT. Results: All indices used to assess normal tissue toxicity were reduced for the tumor tracking model compared with non‐tracking delivery. Mean lung dose, equivalent ipsilateral lung dose delivered in 2Gy fractions, and V13 were reduced by 17%, 21%, and 22% and 27%, 30%, and 36% for small and large diaphragm motion, respectively. The maximum heart dose was reduced by 7% for small diaphragm motion and by 9% for larger diaphragm motion. Conclusion: This work validates the ability of tumor tracking techniques to increase the dose to moving targets while reducing dose to surrounding normal tissues. It is expected that SBRT dose escalation using tumor tracking techniques will improve local control while reducing rates of normal tissue toxicity.