基于EPID采用参数化梯度法测定光子束射野边界

【摘 要】 目的:探究参数化梯度方法（PGM）测量电子射野影像系统（EPID）光子束射野大小的可行性。 方法:PGM通过一个修改的双曲正切函数拟合Profile半影区。瓦里安EDGE机载aS1200采集6 MV和10 MV FF及FFF射束EPID数据，TrueBeam机载aS1000采集6 MV FF射束EPID数据。γ分析1 mm/1%标准量化PGM拟合Profile半影区与EPID测量半影区一致性。比较半高宽方法与PGM测量的FF射束射野大小，比较最大斜率方法与PGM测量的FFF射束射野大小；比较PGM在不同射束能量、不同EPID探测器类型和引入铅门位置误差后测量射野边界的稳定性和扩展性。 结果:半影区PGM拟合与EPID实测数据Pearson相关系数大于0.999，γ值小于0.2。FF射束，半高宽方法测定射野均大于PGM，且随着射野增大而增大，Profile本影去除后，两种方法测量差值显著减小；FFF射束，最大斜率方法与PGM测定射野大小差值在0.1 mm以内。PGM能够稳定测量不同能量、不同模态、不同EPID探测器类型射野边界，能够准确识别铅门1 mm位置变动。 结论:PGM可作为一种鲁棒通用的方法适用于EPID光子束射野质量保障。

EPID-based parameterized gradient method for photon beam field size determination

Abstract: Objective To investigate the feasibility of using parameterized gradient method (PGM) to determine photon beam field size based on electronic portal image dosimetry (EPID). Methods Profile penumbra region was fitted by PGM via a modified hyperbolic tangent function. Varian EDGE aS1200 detector was used to collect the EPID data of 6 MV and 10 MV flattening-filter (FF) and flattening-filter-free (FFF) photon beams, while TrueBeam aS1000 detector was used to obtain the EPID data of 6 MV FF photon beams. Gamma analysis with 1 mm/1% criterion was used to quantitatively assess the consistency between Profile penumbra region fitted by PGM and that measured by EPID. The FF photon beam field sizes calculated by full-width at half-maximum method and those determined by PGM were compared, and the FFF photon beam field sizes obtained by maximum slope method and those obtained by PGM were also compared. By varying beam energies and types of EPID detectors and introducing Jaw position errors, the robustness and scalability of PGM for photon beam field size determination were evaluated. Results Pearson correlation coefficient between penumbra region fitted by PGM and data measured by EPID was greater than 0.999 and γ was less than 0.2. For FF beams, the field size calculated with full-width at half-maximum method were larger than that obtained with PGM, and even larger as the field size increasing. However, after subtracting the umbra region from profile, the differences in measurements between two methods were significantly reduced. For FFF beams, the differences between the field sizes calculated with maximum slope method and those obtained with PGM were within 0.1 mm. PGM achieved a stable measurement of field size at different beam energies, modalities, independent of EPID detector characteristics, and is effective in detecting Jaw errors which was within ±1 mm. Conclusion PGM can be universally applied to EPID photon beam of all modalities for quality assurance (QA).