Gating latency of mridian system and its dosimetric effects

Purpose: MRIdian (ViewRay, Inc., Cleveland, OH, USA) is the first MRI-guided radiotherapy system that is able to image while treating cancer patients. By using real-time images during treatment, it enables tracking of the tumor, and gating directly on the tumor. Time delay which is the time between when a tumor moves into a gated region and beam turns on (beam-on time delay) or the time between when a tumor moves out from gated region and beam turns off (beam-off time delay) is an important issue for patient treatment accuracy. In this study, we aim to quantify the gating latency of the MRIdian system and assess the effects of time delay by using an MRI-compatible motion phantom and a radiochromic film. We have performed tests to determine the gating performance using static and gated deliveries of the plan. Methods: The MRI-compatible phantom has a computer controlled movable platform on which an insert containing a small sphere was positioned. The small sphere was used as a moving target and placed on that moving platform together with a radiochromic film. A sinusoidal motion (actual motion of phantom) with an amplitude of 15 mm and a period of 3.14 second was used to simulate the motion of moving tumor. A lateral rectangular field delivering a dose of 3 Gy was planned to the isocenter and delivered for the stationary mode and the dynamic mode, with gating. The gating was based on 5 mm gating window, which was defined around PTV. A MATLAB™ script was written to analyze the dosimetric deviation between the modelled and measured dose profile and to quantify the latency. Results: For the system investigated, the latency of 127 ms was estimated as a gating latency of the MRIdian system, excluding the effects of imaging sampling rate of 4 fps. Conclusion: The latency of 127 ms is influenced by the source movement mechanism, and the time needed by the software system to detect whether or not the target is in or out of the boundary. However, the 127 ms latency exceeds the AAPM TG-142 report's limit of 100ms, on top of this, the latency will be even more because of the imaging frequency. An extensive assessment is required to determine the geographic miss and/or treatment inaccuracies caused by latency. A detailed analysis would be valuable to determine the effects using realistic motion patterns and system parameters. Key Words: gated radiotherapy, gating latency, time delay, MRI-guided radiotherapy