TH-A-137-11: First Clinical Experience Treating Patients with the Gimbaled Linac Tumor Tracking of the Vero SBRT System

Abstract

Purpose: Reporting on the first group of patients treated with real‐time tumor tracking (RTTT) radiation therapy using the Vero gimbaled linac system and a single implanted fiducial marker. Methods: In total 8 patients diagnosed with NSCLC or oligo‐metastatic disease in lung or liver, and showing substantial breathing related tumor motion were included in this study. One single Visicoil fiducial marker was implanted inside or close to the tumor. SBRT schemes were applied with doses/fraction ranging from 5Gy up to 20Gy. Treatment planning was done using both the classic ITV approach as a reference using 5mm ITV‐PTV margin, and the RTTT planning applying a 5mm isotropic CTV‐PTV margin. RTTT was driven by a correlation model of external‐to‐internal motion and IR skin markers on the upper abdomen. The correlation model was rebuild multiple times per fraction. Data on the delivery process in terms of performance and time efficiency was accumulated. Results: The observed average peak‐to‐peak tumor motion reached up to 25mm, the average CTV volume was 9.0cc. An average PTV volume reduction was accomplished of 38%, ranging up to 52%. For this patient group, the average mean lung dose and mean liver dose were reduced from 2.6Gy to 2.0Gy, and from 11.7Gy to 9.4Gy respectively. Maximum average treatment time, including patient setup, ranged up to 35min. The correlation models were used for 6min 24 sec on average of which 2min 50sec was building time. The probability of the beams‐eye‐view tracking error to be less than 5mm averaged over all patients was exceeding 98%. Conclusion: A novel approach for real‐time tumor tracking using a gimbaled linac was successfully applied on a first group of patients. An initial evaluation was made of the possibilities of tracking to reduce the PTV volume and reestablish acceptable treatment fraction duration for respiratory correlated treatments. This collaborative work was supported by the Flemish government through the Hercules foundation, the Fonds voor Wetenschappelijk Onderzoek Vlaanderen grants G.0486.06 and G.0412.08, and corporate funding from BrainLab AG.