Geometric and dosimetric accuracy and imaging dose of the real-time tumour tracking system of a gimbal mounted linac

Purpose: To suggest a comprehensive testing scheme to evaluate the geometric and dosimetric accuracy and the imaging dose of the VERO dynamic tumour tracking (DTT) for its clinical implementation. Methods: Geometric accuracy was evaluated for gantry 0° and 90° in terms of prediction (EP), mechanical (EM) and tracking (ET) errors for sinusoidal patterns with 10 and 20mm amplitudes, 2-6s periods and phase shift up to 1s and for 3 patient patterns. The automatic 4D model update was investigated simulating changes in the breathing pattern during treatment.Dosimetric accuracy was evaluated with gafchromic films irradiated in static and moving phantom with and without DTT. The entrance skin dose (ESD) was assessed using a solid state detector and gafchromic films. Results: The RMS of EP, EM, and ET were up to 0.8, 0.5 and 0.9mm for all non phased-shifted motion patterns while for the phased-shifted ones, EP and ET increased to 2.2 and 2.6mm. Up to 4 updates are necessary to restore a good correlation model, according to type of change.For 100kVp and 1mAs X-ray beam, the ESD per portal due to 20s fluoroscopy was 16.6mGy, while treatment verification at a frequency of 1Hz contributed with 4.2mGy/min. Conclusions: The proposed testing scheme highlighted that the VERO DTT system tracks a moving target with high accuracy. The automatic update of the 4D model is a powerful tool to guarantee the accuracy of tracking without increasing the imaging dose.