Division of Medical Physics, CancerCare Manitoba,Winnipeg, Manitoba R3E 0V9, Canada.
Med Phys. 2012 Feb;39(2):697-705. doi: 10.1118/1.3675972.
The aim of this work is to characterize a new online dose verification device, COMPASS transmission detector array (IBA Dosimetry, Schwarzenbruck, Germany). The array is composed of 1600 cylindrical ionization chambers of 3.8 mm diameter, separated by 6.5 mm center-to-center spacing, in a 40 × 40 arrangement.
The line spread function (LSF) of a single ion chamber in the detector was measured with a narrow slit collimator for a 6 MV photon beam. The 0.25 × 10 mm(2) slit was formed by two machined lead blocks. The LSF was obtained by laterally translating the detector in 0.25 mm steps underneath the slit over a range of 24 mm and taking a measurement at each step. This measurement was validated with Monte Carlo simulation using BEAMnrc and DOSXYZnrc. The presampling modulation transfer function (MTF), the Fourier transform of the line spread function, was determined and compared to calculated (Monte Carlo and analytical) MTFs. Two head-and-neck intensity modulated radiation therapy (IMRT) fields were measured using the device and were used to validate the LSF measurement. These fields were simulated with the BEAMnrc Monte Carlo model, and the Monte Carlo generated incident fluence was convolved with the 2D detector response function (derived from the measured LSF) to obtain calculated dose. The measured and calculated dose distributions were then quantitatively compared using χ-comparison criteria of 3% dose difference and 3 mm distance-to-agreement for in-field points (defined as those above the 10% maximum dose threshold).
The full width at half-maximum (FWHM) of the measured detector response for a single chamber is 4.3 mm, which is comparable to the chamber diameter of 3.8 mm. The pre-sampling MTF was calculated, and the resolution of one chamber was estimated as 0.25 lp∕mm from the first zero crossing. For both examined IMRT fields, the χ-comparison between measured and calculated data show good agreement with 95.1% and 96.3% of in-field points below χ of 1.0 for fields 1 and 2, respectively (with an average χ of 0.29 for IMRT field 1 and 0.24 for IMRT field 2).
The LSF for a new novel online detector has been measured at 6 MV using a narrow slit technique, and this measurement has been validated by Monte Carlo simulation. The detector response function derived from line spread function has been applied to recover measured IMRT fields. The results have shown that the device measures IMRT fields accurately within acceptable tolerance.
本工作旨在描述一种新型在线剂量验证设备，即 COMPASS 传输探测器阵列（IBA 剂量学，施瓦岑布鲁克，德国）。该阵列由 1600 个直径为 3.8 毫米的圆柱形电离室组成，中心间距为 6.5 毫米，呈 40×40 排列。
使用窄缝准直器测量探测器中单电离室的线扩展函数（LSF），用于 6 MV 光子束。0.25×10mm(2)狭缝由两个加工的铅块形成。LSF 是通过在狭缝下方以 0.25 毫米的步长横向平移探测器，在 24 毫米的范围内进行测量，并在每个步骤进行测量来获得的。该测量使用 BEAMnrc 和 DOSXYZnrc 的蒙特卡罗模拟进行了验证。预采样调制传递函数（MTF），即线扩展函数的傅里叶变换，已确定并与计算（蒙特卡罗和解析）MTF 进行比较。使用该设备测量了两个头颈部强度调制放射治疗（IMRT）场，并用于验证 LSF 测量。这些场使用 BEAMnrc 蒙特卡罗模型进行模拟，蒙特卡罗生成的入射通量与二维探测器响应函数（由测量的 LSF 得出）卷积，以获得计算剂量。然后使用 3%剂量差异和 3mm 距离一致性的 χ 比较标准，对测量和计算剂量分布进行定量比较，以用于场内点（定义为高于 10%最大剂量阈值的点）。
单个腔室的探测器响应的半峰全宽（FWHM）为 4.3mm，与 3.8mm 的腔室直径相当。已计算出预采样 MTF，并根据第一个过零点估计单个腔室的分辨率为 0.25 lp∕mm。对于两个检查的 IMRT 场，测量和计算数据之间的 χ 比较表明，场 1 和场 2 的场内点分别有 95.1%和 96.3%的 χ 值低于 1.0（平均 χ 值分别为场 1 的 0.29 和场 2 的 0.24）。
使用窄缝技术在 6 MV 下测量了新型在线探测器的 LSF，并通过蒙特卡罗模拟对该测量进行了验证。从线扩展函数得出的探测器响应函数已应用于恢复测量的 IMRT 场。结果表明，该设备在可接受的容差范围内准确测量 IMRT 场。
