Department of Medical Physics, Juravinski Cancer Centre at Hamilton Health Sciences, Hamilton, ON, Canada.
Author to whom any correspondence should be addressed.
Phys Med Biol. 2020 Dec 17;65(24):245029. doi: 10.1088/1361-6560/ab91d8.
The purpose of this work is to develop accurate computational methods to comprehensively characterize and model the clinical ExacTrac imaging system, which is used as an image guidance system for stereotactic treatment applications. The Spektr toolkit was utilized to simulate the spectral and imaging characterization of the system. Since Spektr only simulates the primary beam (ignoring scatter), a full model of ExacTrac was also developed in Monte Carlo (MC) to characterize the imaging system. To ensure proper performance of both simulation models, Spektr and MC data were compared to the measured spectral and half value layers (HVLs) values. To validate the simulation results, x-ray spectra of the ExacTrac system were measured for various tube potentials using a CdTe spectrometer with multiple added narrow collimators. The raw spectra were calibrated using a Co source and corrected for the escape peaks and detector efficiency. HVLs in mm of Al for various energies were measured using a calibrated RaySafe detector. Spektr and MC HVLs were calculated and compared to the measured values. The patient surface dose was calculated for different clinical imaging protocols from the measured air kerma and HVL values following the TG-61 methodology. The x-ray focal spot was measured by slanted edge technique using gafchromic films. ExacTrac imaging system beam profiles were simulated for various energies by MC simulation and the results were benchmarked by experimentally acquired beam profiles using gafchromic films. The effect of 6D IGRT treatment couch on beam hardening, dynamic range of the flat panel detector and scatter effect were determined using both Spektr simulation and experimental measurements. The measured and simulated spectra (of both MC and Spektr) for various kVps were compared and agreed within acceptable error. As another validation, the measured HVLs agreed with the Spektr and MC simulated HVLs on average within 1.0% for all kVps. The maximum and minimum patient surface doses were found to be 1.06 mGy for shoulder (high) and 0.051 mGy for cranial (low) imaging protocols, respectively. The MC simulated beam profiles were well matched with experimental results and replicated the penumbral slopes, the heel effect, and out-of-field regions. Dynamic range of detector (in terms of air kerma at detector surface) was found to be in the range of [6.1 × 10, 5.3 × 10] mGy. Accurate MC and Spektr models of the ExacTrac image guidance system were successfully developed and benchmarked via experimental validation. While patient surface dose for available imaging protocols were reported in this study, the established MC model may be used to obtain 3D imaging dose distribution for real patient geometries.
这项工作的目的是开发准确的计算方法，以全面描述和建模临床 ExacTrac 成像系统，该系统用作立体定向治疗应用的图像引导系统。利用 Spektr 工具包模拟系统的光谱和成像特性。由于 Spektr 仅模拟初级光束（忽略散射），因此还在蒙特卡罗（MC）中开发了 ExacTrac 的完整模型，以对成像系统进行特征描述。为了确保两个模拟模型的适当性能，将 Spektr 和 MC 数据与测量的光谱和半值层（HVL）值进行了比较。为了验证模拟结果，使用带有多个附加窄准直器的 CdTe 光谱仪测量了 ExacTrac 系统的 X 射线光谱，针对各种管电压。使用 Co 源对原始光谱进行校准，并对逃逸峰和探测器效率进行校正。使用经过校准的 RaySafe 探测器测量了各种能量下 HVL 的 Al 厚度为 0.5mm。使用 MC 计算并比较了 Spektr 和 HVL 与测量值。根据 TG-61 方法，根据测量的空气比释动能和 HVL 值，为不同的临床成像方案计算了患者表面剂量。使用倾斜边缘技术，通过 Gafchromic 胶片测量 X 射线焦点。使用 MC 模拟模拟了各种能量的 ExacTrac 成像系统束流轮廓，并使用 Gafchromic 胶片实验获取的束流轮廓对结果进行了基准测试。使用 Spektr 模拟和实验测量确定了 6D IGRT 治疗床对束硬化、平板探测器的动态范围和散射效应的影响。比较了针对各种 kVp 的测量和模拟光谱（包括 MC 和 Spektr），并在可接受的误差范围内达成一致。作为另一种验证，测量的 HVL 与 Spektr 和 MC 模拟的 HVL 平均相差 1.0%，所有 kVp 均如此。分别为肩部（高）和颅部（低）成像方案找到了最大和最小的患者表面剂量，分别为 1.06 mGy 和 0.051 mGy。MC 模拟的束流轮廓与实验结果非常吻合，复制了半影斜率、脚跟效应和场外区域。探测器的动态范围（以探测器表面的空气比释动能表示）在[6.1×10，5.3×10]mGy 范围内。成功开发了 ExacTrac 图像引导系统的准确 MC 和 Spektr 模型，并通过实验验证进行了基准测试。虽然本研究报告了可用成像方案的患者表面剂量，但建立的 MC 模型可用于获得真实患者几何形状的 3D 成像剂量分布。
