Experimental Verification of Dosimetric Uncertainty Related to Rotational Error of Single Isocenter for Multiple Targets Technique

Abstract

Treatment of multiple brain mets using single isocenter on Linac is a more efficient technique than using multiple isocenters on robotic radiosurgery. This study investigated the uncertainties in target coverage due to the rotational set up errors with film measurements. A commercial radiosurgery head phantom built with a stack of film inserts within the brain was CT scanned at 1mm spacing. Three spherical targets with the diameter of 1cm (2) and 2cm (1) were generated at a distance of 4 ∼ 6cm from the isocenter to simulate multiple mets in the brain. A volumetric modulated arc therapy (VMAT) plan was optimized with a 6X beam to have a minimum 92% coverage for each GTV. The head phantom was aligned on 6 degrees of freedom (6DoF) couch with CBCT. Three pieces of Galchromic films were cut exactly the size of film insert and placed at the locations across each target along the Sup/Inf direction (film plane being close to the CT coronal cut). Once the positioning was confirmed by CBCT, the VMAT plan was delivered first at the planned setup (0° rotational error). A rotational error of 1° and 2° in yaw was then introduced and verified by CBCT. The same VMAT plan was then delivered again to both setups with the 1° and 2° of rotational error in yaw, respectively, with a new set of Galchromic films in place. All films were analyzed to have 1D absolute dose profiles extracted along a line of interest for comparison with the calculation. Comparison of dose profiles between the measurement and the calculation shows a good agreement for the planned delivery, i.e., no rotational errors introduced. With the rotational error of 1° and 2°, the measured dose profiles are seen an obvious shift from the planned. The max shift at the prescription dose was 0.6mm and 1.2mm for the yaw error of 1° and 2°, respectively. The deviation for each target is consistent with the magnitude of rotational errors. Table 1 summarized the max measured deviations of the prescription isodose line in distance from the planned for each GTV at a rotational error of 0°, 1° and 2°, respectively. Experiment for the delivery of a VMAT plan with single isocenter has confirmed the accuracy of the setup using 6DoF and CBCT. Distant targets are susceptible to the deviation of dose coverage due to the rotational errors. A planning margin up to 2mm should be sufficient to ensure the GTV coverage should the rotational tolerance of 6DoF is maintained less than 2°.Abstract 3813; Table 1Rotational Error in YawMax deviations (mm) relative to the planGTV1GTV2GTV30°0.20.30.31°0.60.40.42°0.91.21.0 Open table in a new tab