Abstract: The dose of the rotation setup deviation in single-isocenter VMAT for multiple brain lesions was evaluated by simulating rotations of known degrees (0.5°, 1°, 1.5°, and 2°) on the treatment planning system with varying tumor volume size (1 cm, 2 cm, and 3 cm) and distance to isocenter (up to 6 cm). A quantitative analysis of reducing the effect of the rotation setup deviation by increasing the tumor volume margin was done. With the increase of the rotation angle, the dose coverage of PTV decreased significantly, which was less for GTV. As the distance from the isocenter increased, the dose coverage rate decreased, and the smaller the target volume, the more significant it was. When the GTV was marginned by 2 mm, the decreasing trend of GTV dose coverage was significantly improved, all within -5%. It is recommended to use a six-dimensional couch top to correct the rotation setup deviation when using single-isocenter VMAT to treat patients with multiple brain metastases. For the organizations without a six-dimensional couch top, the appropriate margin should be considered according to the size of the target volume, the distance from the isocenter, and the clinical rotation setup deviation and tolerance.

Key words: rotation setup deviation, single-isocenter, multiple brain metastases, six-dimensional couch top