Blake S, Vial P, Holloway L, McNamara A, Greer P, Kuncic Z
The University of Sydney, Sydney, NSW, Australia.
Liverpool and Macarthur Cancer Therapy Centres, NSW, Australia.
Med Phys. 2012 Jun;39(6Part5):3650. doi: 10.1118/1.4734826.
To investigate the sensitivity of a Monte Carlo (MC) model of a standard clinical amorphous silicon (a-Si) electron portal imaging device (EPID) to variations in optical photon transport parameters.
The Geant4 MC toolkit was used to develop a comprehensive model of an indirect-detection a-Si EPID incorporating x-ray and optical photon transport. The EPID was modeled as a series of uniform layers with properties specified by the manufacturer (PerkinElmer, Santa Clara, CA) of a research EPID at our centre. Optical processes that were modeled include bulk absorption, Rayleigh scattering, and boundary processes (reflection and refraction). Model performance was evaluated by scoring optical photons absorbed by the a-Si photodiode as a function of radial distance from a point source of x-rays on an event-by-event basis (0.025 mm resolution). Primary x-ray energies were sampled from a clinical 6 MV photon spectrum. Simulations were performed by varying optical transport parameters and the resulting point spread functions (PSFs) were compared. The optical parameters investigated include: x-ray transport cutoff thresholds; absorption path length; optical energy spectrum; refractive indices; and the 'roughness' of boundaries within phosphor screen layers.
The transport cutoffs and refractive indices studied were found to minimally affect resulting PSFs. A monoenergetic optical spectrum slightly broadened the PSF in comparison with the use of a polyenergetic spectrum. The absorption path length only significantly altered the PSF when decreased drastically. Variations in the treatment of boundaries noticeably broadened resulting PSFs.
Variation in optical transport parameters was found to affect resulting PSF calculations. Current work is focusing on repeating this analysis with a coarser resolution more typical of a commercial a-Si EPID to observe if these effects continue to alter the EPID PSF. Experimental measurement of the EPID line spread function to validate these results is also underway. Cancer Institute NSW Research Equipment Grants 10/REG/1-20 and 10/REG/1-10 Cancer Council NSW Grant, ID RG 11-06 NHMRC Project Grant, ID569211 The University of Sydney Postgraduate Research Scholarship in Medical Physics SWSCS Radiation Oncology Student Scholarship, 2012.
研究标准临床非晶硅（a-Si）电子射野影像装置（EPID）的蒙特卡罗（MC）模型对光学光子传输参数变化的敏感性。
使用Geant4 MC工具包开发一个包含X射线和光学光子传输的间接探测a-Si EPID综合模型。将EPID建模为一系列均匀层，其特性由我们中心一台研究型EPID的制造商（加利福尼亚州圣克拉拉市的珀金埃尔默公司）指定。建模的光学过程包括体吸收、瑞利散射和边界过程（反射和折射）。通过逐事件（分辨率为0.025 mm）对a-Si光电二极管吸收的光学光子进行评分，将其作为距X射线点源径向距离的函数，来评估模型性能。主要X射线能量从临床6 MV光子能谱中采样。通过改变光学传输参数进行模拟，并比较所得的点扩散函数（PSF）。研究的光学参数包括：X射线传输截止阈值；吸收路径长度；光学能谱；折射率；以及荧光屏层内边界的“粗糙度”。
发现所研究的传输截止值和折射率对所得PSF的影响最小。与使用多能谱相比，单能光学能谱使PSF略有展宽。仅当吸收路径长度大幅减小时，才会显著改变PSF。边界处理方式的变化会使所得PSF明显展宽。
发现光学传输参数的变化会影响所得PSF的计算结果。目前的工作重点是使用更接近商业a-Si EPID的较粗分辨率重复此分析，以观察这些效应是否继续改变EPID的PSF。同时也正在对EPID线扩散函数进行实验测量以验证这些结果。新南威尔士州癌症研究所研究设备资助项目10/REG/1 - 20和10/REG/1 - 10；新南威尔士州癌症理事会资助项目，编号RG 11 - 06；澳大利亚国家卫生与医学研究理事会项目资助，编号569211；悉尼大学医学物理研究生研究奖学金；南威尔士州西南部癌症协会放射肿瘤学学生奖学金，2012年。
