不同体积脑转移瘤对容积旋转调强计划摆位误差的敏感性

Sensitivity of volumetric modulated arc therapy plan to setup errors in radiotherapy for brain metastases of various sizes

目的:探讨不同体积脑转移瘤对容积旋转调强(VMAT)计划摆位误差的敏感性。方法:在头部CT图像上设置5个球形模拟多发脑转移瘤,体积依次为0.5 cm^(3)(V1)、1.7 cm^(3)(V2)、4.0 cm^(3)(V3)、8.1 cm^(3)(V4)和14.0 cm^(3)(V5)。制定单等中心5弧非共面VMAT计划,等中心点到各靶区的距离为5 cm。通过旋转治疗床模拟绕Z轴旋转的摆位误差θ(-3.0°~3.0°,步长为0.5°),通过移动等中心在三维轴向(X/Y/Z)上的位置模拟平移误差(-3.0 mm~3.0 mm,步长为0.5 mm),不改变优化条件的情况下,重新计算剂量分布。记录旋转、平移后靶区的覆盖率,并归一到未发生摆位误差下的相对覆盖率。运用线性回归法,分析不同体积脑转移瘤靶区覆盖率与体积的相关性。结果:随着旋转误差的增加,靶区相对覆盖率逐渐下降。恒定旋转误差情况下,靶区相对覆盖率与体积呈线性相关(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),随着靶区体积增加,靶区相对覆盖率逐渐上升。当旋转误差θ<0.5°时,无论大体积靶区(14.0 cm^(3))还是小体积靶区(0.5 cm^(3)),其靶区相对覆盖率变化均小于2%。随着平移误差的增大,靶区相对覆盖率逐渐下降。相对于大体积靶区,小体积靶区对平移误差更为敏感。当平移误差<0.5 mm时,大体积靶区相对覆盖率变化<2%,而小体积靶区(0.5 cm^(3))的相对覆盖率下降接近5%。结论:旋转误差恒定情况下,靶区相对覆盖率与体积呈线性关系。相对于大体积靶区,小体积靶区对平移误差更为敏感。建议对于小体积靶区在实施立体定向放射外科VMAT计划时,旋转误差应控制在0.5°以内,平移误差应控制在0.5 mm以内。

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 cm^(3),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 errorθaround 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 cm^(3))or small(0.5 cm^(3)).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 cm^(3))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.