CRUK/MRC Gray Institute for Radiation Oncology and Biology, Department of Oncology, University of Oxford, ORCRB Roosevelt Drive, Oxford, OX3 7DQ, United Kingdom.
Med Phys. 2013 Aug;40(8):082102. doi: 10.1118/1.4812687.
The Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)] formalism for small field dosimetry proposes a set of correction factors (kQclin,Qmsrfclin,fmsr) which account for differences between the detector response in nonstandard (clinical) and machine-specific-reference fields. In this study, the Monte Carlo method was used to investigate the viability of such small field correction factors for four different detectors irradiated under a variety of conditions. Because kQclin,Qmsrfclin,fmsr values for single detector position measurements are influenced by several factors, a new theoretical formalism for integrated-detector-position [dose area product (DAP)] measurements is also presented and was tested using Monte Carlo simulations.
A BEAMnrc linac model was built and validated for a Varian Clinac iX accelerator. Using the egs++ geometry package, detailed virtual models were built for four different detectors: a PTW 60012 unshielded diode, a PTW 60003 Diamond detector, a PTW 31006 PinPoint (ionization chamber), and a PTW 31018 MicroLion (liquid-filled ionization chamber). The egs_chamber code was used to investigate the variation of kQclin,Qmsrfclin,fmsr with detector type, detector construction, field size, off-axis position, and the azimuthal angle between the detector and beam axis. Simulations were also used to consider the DAP obtained by each detector: virtual detectors and water voxels were scanned through high resolution grids of positions extending far beyond the boundaries of the fields under consideration.
For each detector, the correction factor (kQclin,Qmsrfclin,fmsr) was shown to depend strongly on detector off-axis position and detector azimuthal angle in addition to field size. In line with previous studies, substantial interdetector variation was also observed. However, it was demonstrated that by considering DAPs rather than single-detector-position dose measurements the high level of interdetector variation could be eliminated. Under small field conditions, mass density was found to be the principal determinant of water equivalence. Additionally, the mass densities of components outside the sensitive volumes were found to influence the detector response.
kQclin,Qmsrfclin,fmsr values for existing detector designs depend on a host of variables and their calculation typically relies on the use of time-intensive Monte Carlo methods. Future moves toward density-compensated detector designs or DAP based protocols may simplify the methodology of small field dosimetry.
Alfonso 等人[Med. Phys. 35, 5179-5186 (2008)]提出的小场剂量学公式提出了一组校正因子（kQclin，Qmsrfclin，fmsr），用于说明探测器在非标准（临床）和机器特定参考场中的响应差异。在这项研究中，使用蒙特卡罗方法研究了四种不同探测器在各种条件下受辐照时使用这些小场校正因子的可行性。由于单个探测器位置测量的 kQclin，Qmsrfclin，fmsr 值受多种因素影响，因此还提出了一种用于集成探测器位置[剂量面积乘积（DAP）]测量的新理论公式，并使用蒙特卡罗模拟进行了测试。
建立并验证了基于 BEAMnrc 直线加速器模型的瓦里安 Clinac iX 加速器。使用 egs++几何包，为四个不同的探测器建立了详细的虚拟模型：PTW 60012 无屏蔽二极管、PTW 60003 钻石探测器、PTW 31006 PinPoint（电离室）和 PTW 31018 MicroLion（液体填充电离室）。使用 egs_chamber 代码研究了校正因子（kQclin，Qmsrfclin，fmsr）随探测器类型、探测器结构、射野大小、离轴位置以及探测器与射束轴之间的偏转角的变化。模拟还用于考虑每个探测器获得的 DAP：虚拟探测器和水体素通过远超出所考虑的射野边界的位置的高分辨率网格进行扫描。
对于每个探测器，校正因子（kQclin，Qmsrfclin，fmsr）都强烈依赖于探测器的离轴位置和探测器的偏转角，除了射野大小之外。与先前的研究一致，也观察到了大量的探测器间差异。然而，研究表明，通过考虑 DAP 而不是单个探测器位置的剂量测量，可以消除高水平的探测器间差异。在小射野条件下，发现质量密度是水等效性的主要决定因素。此外，还发现敏感体积外的组件的质量密度会影响探测器的响应。
现有探测器设计的 kQclin，Qmsrfclin，fmsr 值取决于许多变量，其计算通常依赖于使用时间密集型蒙特卡罗方法。未来朝着密度补偿探测器设计或基于 DAP 的协议的发展可能会简化小场剂量学的方法。
