Spatial uncertainty of Elekta stereotactic cones in the treatment of multiple brain metastases using multiple cones

To evaluate the spatial accuracy of radiation centers (RCs) of Elekta stereotactic cones while changing the cone sizes during intra-fraction treatment of multiple brain metastases (MBM). In this study, nine cones of varying sizes (5, 10, 12.5, 15, 17.5, 20, 25, 30, and 35 mm) were used. The ball bearing (BB) was used as the target. The positions of BBs and RCs obtained using electronic portal imaging device (EPID) were transformed to a coordinate system X/Y/Z that was fixed with respect to the treatment room. The positions of BBs when using cones sized between 12.5 and 35 mm were relatively stable. The maximum deviations between BBs and RCs were as follows: 1.11 mm in negative X and 1.30 mm in positive X, 0.87 mm in negative Y and 0.69 mm in positive Y, and 0.06 mm in negative Z and 0.58 mm in positive Z. The maximum offset between BBs and RCs gradually increased as the cone size decreased in the X and Y directions. The maximum offsets were 2.30, 1.56, and 0.64 mm in the X, Y, and Z directions, respectively, for the 12.5-mm cone. The maximum distance between the RCs was 0.43 mm when using the G135T270 beam between the 17.5-mm and 12.5-mm cones. The treatment of MBM with Elekta stereotactic cones demonstrated good spatial uncertainty. However, we need to pay particular attention to treatment using two or more cones in clinical practice. More external expansion margins are required in such cases.