Keller H, Fix M, Rüegsegger P
Swiss Federal Institute of Technology ETH, Institute for Biomedical Engineering, Zurich, Switzerland.
Med Phys. 1998 Oct;25(10):1891-902. doi: 10.1118/1.598378.
Today electronic portal imaging devices (EPID's) are used primarily to verify patient positioning. They have, however, also the potential as 2D-dosimeters and could be used as such for transit dosimetry or dose reconstruction. It has been proven that such devices, especially liquid filled ionization chambers, have a stable dose response relationship which can be described in terms of the physical properties of the EPID and the pulsed linac radiation. For absolute dosimetry however, an accurate method of calibration to an absolute dose is needed. In this work, we concentrate on calibration against dose in a homogeneous water phantom. Using a Monte Carlo model of the detector we calculated dose spread kernels in units of absolute dose per incident energy fluence and compared them to calculated dose spread kernels in water at different depths. The energy of the incident pencil beams varied between 0.5 and 18 MeV. At the depth of dose maximum in water for a 6 MV beam (1.5 cm) and for a 18 MV beam (3.0 cm) we observed large absolute differences between water and detector dose above an incident energy of 4 MeV but only small relative differences in the most frequent energy range of the beam energy spectra. It is shown that for a 6 MV beam the absolute reference dose measured at 1.5 cm water depth differs from the absolute detector dose by 3.8%. At depth 1.2 cm in water, however, the relative dose differences are almost constant between 2 and 6 MeV. The effects of changes in the energy spectrum of the beam on the dose responses in water and in the detector are also investigated. We show that differences larger than 2% can occur for different beam qualities of the incident photon beam behind water slabs of different thicknesses. It is therefore concluded that for high-precision dosimetry such effects have to be taken into account. Nevertheless, the precise information about the dose response of the detector provided in this Monte Carlo study forms the basis of extracting directly the basic radiometric quantities photon fluence and photon energy fluence from the detector's signal using a deconvolution algorithm. The results are therefore promising for future application in absolute transit dosimetry and absolute dose reconstruction.
如今，电子射野影像装置（EPID）主要用于验证患者体位。然而，它们也有作为二维剂量仪的潜力，可用于传输剂量测定或剂量重建。业已证明，此类装置，尤其是充液电离室，具有稳定的剂量响应关系，这可以根据EPID的物理特性和脉冲直线加速器辐射来描述。然而，对于绝对剂量测定，需要一种精确的绝对剂量校准方法。在这项工作中，我们专注于在均匀水模体中针对剂量进行校准。使用探测器的蒙特卡罗模型，我们以每入射能量注量的绝对剂量为单位计算剂量扩展核，并将它们与不同深度水中计算得到的剂量扩展核进行比较。入射笔形束的能量在0.5至18 MeV之间变化。对于6 MV束（1.5 cm）和18 MV束（3.0 cm）在水中剂量最大值深度处，我们观察到在入射能量高于4 MeV时，水和探测器剂量之间存在较大的绝对差异，但在束能谱最常见的能量范围内仅存在较小的相对差异。结果表明，对于6 MV束，在1.5 cm水深处测量的绝对参考剂量与绝对探测器剂量相差3.8%。然而，在水中1.2 cm深度处，相对剂量差异在2至6 MeV之间几乎恒定。还研究了束能谱变化对水和探测器中剂量响应的影响。我们表明，对于不同厚度水板后的不同入射光子束质量，差异可能大于2%。因此得出结论，对于高精度剂量测定，必须考虑此类影响。尽管如此，这项蒙特卡罗研究中提供的关于探测器剂量响应的精确信息构成了使用反卷积算法从探测器信号中直接提取基本辐射量光子注量和光子能量注量的基础。因此，这些结果对于未来在绝对传输剂量测定和绝对剂量重建中的应用很有前景。
