A Method to Determine the Coincidence of MRI-Guided Linac Radiation and Magnetic Isocenters

Abstract

To assure accurate treatment delivery on any image-guided radiotherapy system, the relative positions and walkout of the imaging and radiation isocenters must be periodically verified and kept within specified tolerances. In this work, we first validated the multiaxis ion chamber array as a tool for finding the radiation isocenter position of a magnetic resonance–guided linear accelerator. The treatment couch with the array on it was shifted in 0.2-mm increments and the reported beam center position was plotted against that shift and fitted to a straight line, in both X and Y directions. From the goodness-of-fit and intercepts of the regression lines, the accuracy and precision were conservatively estimated at 0.2 and 0.1 mm, respectively. This holds true whether the array is irradiated from the front or from the back, which allows efficient collecting the data from the 4 cardinal gantry angles with just 2 array positions. The average isocenter position agreed to within at most 0.4 mm along any cardinal axis with the linac vendor’s film-based procedure, and the maximum walkout radii were 0.32 mm and 0.53 mm, respectively. The magnetic resonance imaging isocenter walkout as a function of gantry angle was studied with 2 different phantoms, one employing a single fiducial at the center and another extracting the rigid displacement values from the distortion map fit of 523 fiducials dispersed over a large volume. The results were close between the 2 phantoms and demonstrated variation in the magnetic resonance imaging isocenter location as high as 1.3 mm along a single axis in the transverse plane. Verification of the magnetic resonance imaging isocenter location versus the gantry angle should be a part of quality assurance for magnetic resonance-guided linear accelerators.

Evaluation of cine imaging during multileaf collimator and gantry motion for real‐time magnetic resonance guided radiation therapy

“…The magnitude of the longitudinal (Y) isocenter shift did not exceed 0.11 mm and exhibited no strong gantry‐position‐dependent behavior. The static gantry‐position‐dependent isocenter displacement of the ViewRay 3D imaging was previously investigated by Latifi et al 14 In their paper, the minima and maximum of the vertical isocenter displacements occurred at the same positions that we report, and the horizontal isocenter exhibited a similarly minimal dependence on gantry position.…”

“…The 3D data were used to establish the position of the phantom so that the geometric distortion in cine images could be evaluated. Because the magnetic susceptibility of the gantry is not uniformly distributed, the gantry position affects the homogeneity of the static magnetic field and can therefore produce geometric distortions 2,14 . For this reason, the static data were taken at multiple gantry positions in order to discriminate between geometric distortions arising from different static gantry positions and those arising from gantry motion.…”

On the MLC leaves alignment in the direction orthogonal to movement

“…Also included in this group of MLC-related checks were the gantry star shot with radiochromic film and a complementary gantry isocentricity test with an ion chamber array. 6 The latter two are used to define the radiation isocenter position at the time of installation.…”

“…It has been shown previously that the Y-position readout correlated tightly with the radiation beam center measurement. 6…”

Visual Isocenter Position Enhanced Review (VIPER): a Cherenkov imaging‐based solution for MR‐linac daily QA

“…4 There have been procedures in the literature describing methods to perform this measurement, based on film, polymer gel, spatial localization of individual ion chambers, or MR-compatible ion chamber arrays. [5][6][7] These methods can be somewhat cumbersome and time-consuming either with setup or post-exposure processing or rely on reduced information to infer 3D isocenter coincidence.…”