Establishing tumour tracking accuracy in free-breathing respiratory gated SBRT of lung cancer

Free-breathing respiratory gated SBRT of surgically inoperable lung cancer has been clinically commissioned. This study was to establish the tumour tracking accuracy under clinical conditions based on an implanted fiducial marker. A Visicoil (TM) marker embedded in tissue-equivalent material mounted in a phantom (ET Gating Phantom (TM) Brainlab) driven by a patient's breathing data was treated with the ExacTrac (TM) system. This one-dimensional moving marker represented a tumour motion in superior-inferior (S-I) direction measured through 4DCT study of the same patient. Both Gafchromic (TM) films and the stereoscopic kV images were used for tracking the position of the marker. For tumour motion at magnitudes of 10, 20 and 29 mm and treated with corresponding gate widths of 50%, 33% and 20% of free breathing amplitude, the implanted marker was able to be tracked with a deviation <= 1.53 mm to its planned position.