Sensitivity of volumetric modulated arc therapy plan to setup errors in radiotherapy for brain metastases of various sizes
Objective To explore the sensitivity of volumetric modulated arc therapy(VMAT)plan to setup errors in radiotherapy for brain metastases of various sizes.Methods Five spheres were set to simulate multiple brain metastases in the CT image,with the volumes of 0.5,1.7,4.0,8.1,and 14.0 cm3,respectively.A single isocenter 5-arc non-coplanar VMAT plan was generated,and the distance from isocenter to each target was 5 cm.The rotational setup errorθ around the Z axis was simulated by rotating the treatment couch(clockwise from-3.0°to 3.0°,with a step length of 0.5°);and the translational setup error(from-3.0 mm to 3.0 mm,with a step length of 0.5 mm)was simulated by changing the position of the isocenter on the three-dimensional axis(X/Y/Z).The dose distribution was recalculated in the optimization conditions remaining unchanged.The target coverage after rotation and translation was recorded and normalized to the relative coverage without setup error.The correlation between the target coverage of brain metastases of various sizes and the setup error was analyzed with linear regression method.Results The relative coverage decreased gradually as rotational setup error increased.With the same rotational setup error,the relative coverage was in linear correlation with target volume(θ=0.5°,P=0.006;θ=1.0°,P=0.024;θ=1.5°,P=0.028;θ=2.0°,P=0.019;θ=2.5°,P=0.014;θ=3.0°,P=0.007),and the relative coverage increased with increasing target volume.When the rotational setup error was less than 0.5°,the relative coverage was less than 2%regardless of whether the target area was large(14.0 cm3)or small(0.5 cm3).The relative coverage decreased gradually as translational setup error increased.Compared with large target area,small target area was more sensitive to translational setup error.When the translational setup error was less than 0.5 mm,the relative coverage of the large target area varied less than 2%,while the relative coverage of the small target area(0.5 cm3)decreased by nearly 5%.Conclusion When the rotational setup error is constant,the relative coverage of the target has linear relationship with target volume.Small target area is more sensitive to translational setup error than large target area.The rotational setup error within 0.5°and the translational setup error within 0.5 mm are recommended when implementing SRS VMAT plan for small target area.
万方数据
维普
