PDF(Pubmed)
Abstract：
OBJECTIVE: MatriXX ionization chamber array has been widely used for the composite dose verification of IMRT/VMAT plans. However, in addition to its dose response dependence on gantry angle, there seems to be an offset between the beam axis and measured dose profile by MatriXX for oblique beam incidence at various gantry angles, leading to unnecessary quality assurance (QA) fails. In this study, we investigated the offset at various setup conditions and how to eliminate or decrease it to improve the accuracy of MatriXX for IMRT/VMAT plan verification with original gantry angles.
METHODS: We measured profiles for a narrow beam with MatriXX located at various depths in increments of 0.5 mm from the top to bottom of the sensitive volume of the array detectors and gantry angles from 0° to 360°. The optimal depth for QA measurement was determined at the depth where the measured profile had minimum offset.
RESULTS: The measured beam profile offset varies with incident gantry angle, increasing from vertical direction to lateral direction, and could be over 3 cm at vendor-recommended depth for near lateral direction beams. The offset also varies with depth, and the minimum offset (almost 0 for most oblique beams) was found to be at a depth of ∼2.5 mm below the vendor suggested depth, which was chosen as the optimal depth for all QA measurements. Using the optimal depth we determined, QA results (3%/2 mm Gamma analysis) were largely improved with an average of 99.4% gamma passing rate (no fails for 95% criteria) for 10 IMRT and VMAT plans with original gantry angles compared to 94.1% using the vendor recommended depth.
CONCLUSIONS: The improved accuracy and passing rate for QA measurement performed at the optimal depth with original gantry angles would lead to reduction in unnecessary repeated QA or plan changes due to QA system errors.
METHODS: We measured profiles for a narrow beam with MatriXX located at various depths in increments of 0.5 mm from the top to bottom of the sensitive volume of the array detectors and gantry angles from 0° to 360°. The optimal depth for QA measurement was determined at the depth where the measured profile had minimum offset.
RESULTS: The measured beam profile offset varies with incident gantry angle, increasing from vertical direction to lateral direction, and could be over 3 cm at vendor-recommended depth for near lateral direction beams. The offset also varies with depth, and the minimum offset (almost 0 for most oblique beams) was found to be at a depth of ∼2.5 mm below the vendor suggested depth, which was chosen as the optimal depth for all QA measurements. Using the optimal depth we determined, QA results (3%/2 mm Gamma analysis) were largely improved with an average of 99.4% gamma passing rate (no fails for 95% criteria) for 10 IMRT and VMAT plans with original gantry angles compared to 94.1% using the vendor recommended depth.
CONCLUSIONS: The improved accuracy and passing rate for QA measurement performed at the optimal depth with original gantry angles would lead to reduction in unnecessary repeated QA or plan changes due to QA system errors.
摘要：
目的：MatriXX电离室阵列已广泛用于IMRT/VMAT计划的复合剂量验证。然而,除了其剂量响应依赖于机架角度，对于各种机架角度的倾斜光束入射，光束轴与MatriXX测得的剂量分布之间似乎存在偏移，导致不必要的质量保证(QA)失败。在这项研究中,我们研究了在各种设置条件下的偏移，以及如何消除或减少偏移，以提高MatriXX对原始机架角度的IMRT/VMAT计划验证的准确性。
方法：我们测量了具有MatriXX的窄光束的轮廓，从阵列探测器的敏感体积的顶部到底部以0.5mm的增量位于不同深度，机架角度从0°到360°。在测量的轮廓具有最小偏移的深度处确定用于QA测量的最佳深度。
结果：测得的光束轮廓偏移随入射机架角度而变化，从垂直方向增加到横向方向，并且在供应商推荐的接近横向方向梁的深度处可能超过3厘米。偏移也随深度而变化，并且发现最小偏移（对于大多数斜梁几乎为0）在低于供应商建议深度的2.5mm处，选择所有QA测量的最佳深度。使用我们确定的最佳深度，与使用供应商推荐深度的94.1%相比,10个具有原始机架角度的IMRT和VMAT计划的QA结果(3%/2mm伽玛分析)的平均伽玛通过率为99.4%(95%标准没有失败)得到了很大改善。
结论：在具有原始机架角度的最佳深度下进行QA测量的提高的准确性和合格率将导致由于QA系统误差而导致的不必要的重复QA或计划更改的减少。
方法：我们测量了具有MatriXX的窄光束的轮廓，从阵列探测器的敏感体积的顶部到底部以0.5mm的增量位于不同深度，机架角度从0°到360°。在测量的轮廓具有最小偏移的深度处确定用于QA测量的最佳深度。
结果：测得的光束轮廓偏移随入射机架角度而变化，从垂直方向增加到横向方向，并且在供应商推荐的接近横向方向梁的深度处可能超过3厘米。偏移也随深度而变化，并且发现最小偏移（对于大多数斜梁几乎为0）在低于供应商建议深度的2.5mm处，选择所有QA测量的最佳深度。使用我们确定的最佳深度，与使用供应商推荐深度的94.1%相比,10个具有原始机架角度的IMRT和VMAT计划的QA结果(3%/2mm伽玛分析)的平均伽玛通过率为99.4%(95%标准没有失败)得到了很大改善。
结论：在具有原始机架角度的最佳深度下进行QA测量的提高的准确性和合格率将导致由于QA系统误差而导致的不必要的重复QA或计划更改的减少。
