SU‐E‐J‐81: Interplay Effect in Non‐Gated Dynamic Treatment Delivery of a Lung Phantom with Simulated Respiratory Motion

Abstract

Purpose:To quantify the interplay effect in non‐gated VMAT external beam delivery using realistic, clinically relevant 3D motion in an anthropomorphic lung phantom, and to determine if adding margins is sufficient to account for motion or if gating is required in all cases.Methods:A 4D motion stage was used to move a Virtual Water (VW) lung target containing a piece of radiochromic EBT3 film in an anthropomorphic chest phantom. A five‐arc stereotactic body radiation therapy (SBRT) treatment was planned using a CT scan of the phantom in its stationary position, using planning parameters chosen to push the optimizer to achieve a highly‐modulated plan. Two scenarios were delivered using a Varian TrueBeam: the first was delivered with the phantom and target both stationary and the second was delivered with the phantom stationary but the target moving in a realistic, irregular 3D elliptical pattern. A single piece of 4×4 cm2 film was used per fraction, located in the central coronal plane of the target. Film was calibrated on a 6 MV beam with dose values from 0.20 to 20 Gy.Results:Preliminary test films were analyzed in ImageJ and MatLab software. Dose maps were calculated on a central region of interest (ROI) delineated on both the motion‐induced and stationary films. Both static and dynamic film dose maps agreed with planning values within acceptable uncertainty.Conclusion:Including a large number of arcs in a clinically realistic SBRT treatment could reduce the effect of motion interplay due to averaging. Because all clinics do not employ multiple arcs for SBRT lung treatments, it is still important to consider the effects of motion on treatment delivery. Further analysis on the treatment films, as well as a broader investigation other planning parameters, will be conducted.