Delta-4探测MLC细小位置偏差敏感性研究

目的 研究Delta-4对MLC细小位置误差的探测敏感性。方法 利用瓦里安Trubeam型直线加速器配置EPID对MLC模拟细小位置偏差能力进行测量。设置2.0 cm (x)×6.0 cm (y)、7.0 cm (x)×6.0 cm (y)2个射野,MLC的x₁、x₂方向同时分别外扩0.1、0.2、0.3……0.9 mm和1.0、2.0……5.0 mm,使用3 mm 3%、2.5 mm2.5%、2 mm 2%、1.5 mm1.5%和1 mm1%标准γ分析。分析Delta-4测量的相对应剂量分布与TPS计算MLC位置不变时的剂量分布间差异。结果 Trubeam型直线加速器MLC具有移动丝米级位置能力。2.0 cm (x)×6.0 cm (y)射野2.5 mm2.5%标准下γ通过率100%,在MLC的x₁、x₂方向各打开0.3 mm时γ通过率下降至95.9%,打开0.5 mm时γ通过率下降至89.4%。7.0 cm (x)×6.0 cm (y)射野1.5 mm1.5%标准下γ通过率为96.5%,x₁、x₂方向各打开0.3 mm时γ通过率下降至95%以下,在打开3.0 mm内γ通过率都>90%。结论 适当提高γ分析标准有可能会改善Delta-4探测MLC位置细小偏差的敏感性,但不能对所有MLC丝米级位置误差探测出来。对不同大小射野,建议Delta-4选择不同分析标准。

Study of sensitivity for detecting small positional deviation of MLC by 3D Detector array Delta-4

Objective To study the detective sensitivity for position of multi-leave collimators ( MLC) using Delta-4. Methods First,the small positional deviation of MLC was simulated and measured using the linac (Varian,Trubeam) equipped with EPID.Then,two beam fields 2. 0 cm (x)×6. 0 cm (y), 7. 0 cm (x)×6. 0 cm (y) were designed,the x1 and x2 of MLC were expanded 0. 1 mm,0. 2 mm,0. 3 mm... 0. 9 mm and 1. 0 mm,2. 0...5. 0 mm to external simultaneously,different parameters of 3 mm/3%,2. 5 mm/2. 5%,2 mm/2%, 1. 5 mm/1. 5% and 1 mm/1% were used in Gamma analysis to analyze the difference between dose distribution detected by Delta-4 and original dose distribution with unexpanded MLC position derived from TPS. Results For 2. 0 cm ( x) × 6. 0 cm ( y) beam field, the pass rate of original dose distribution was 100%,and that decreased to 95. 5% when x1 ,x2 of MLC were expanded 0. 3 mm to external, and decreased to 89. 4% when expanded 0. 5 mm at 2. 5 mm/2. 5% statistical standards. For 7. 0 cm ( x) × 6. 0 cm ( y) beam field,the pass rate of original dose distribution was 96. 5%,and that decreased to less than 95% when x2,x2 of MLC were expanded 0. 3 mm to external,and passing rate was above 90% when MLC expanded less than 0. 5 mm at 1. 5 mm/1. 5% statistical standards. Conclusions For MLC' s positional deviation in decimillimeter level,raise standards of Gamma analysis properly may improve the capability of Delta-4 for detecting small positional deviation,but it won' t detect all the positional deviation of MLC in decimillimeter level. For different size of beam field,it is proposed to use different analytical standards for Delta-4.