Detector density and small field dosimetry: integral versus point dose measurement schemes.
Detector density and small field dosimetry: integral versus point dose measurement schemes.
复制标题
探测器密度和小视场剂量测定：积分剂量测量方案与点剂量测量方案。
DOI：
10.1118/1.4812687
复制
发表时间：
2013
期刊：
影响因子：
3.8
通讯作者：
John D. Fenwick
中科院分区：
文献类型：
--
作者：
T. Underwood;H. Winter;Mark A. Hill;John D. Fenwick
目的
阿方索等人[医学物理学35，5179-5186（2008）]提出了一组校正因子（kQclin，Qmsrfclin，fmsr），用于解释非标准（临床）和机器特定参考场中探测器响应之间的差异。在这项研究中，使用蒙特卡罗方法来研究在各种条件下照射的四种不同探测器的小场校正因子的可行性。由于kQclin，Qmsrfclin，fmsr值为单探测器位置测量的影响，由几个因素，一个新的理论形式主义集成探测器位置[剂量面积积（DAP）]测量也提出，并使用Monte Carlo模拟进行了测试。
方法
建立了BEAMnrc直线加速器模型，并对Varian Clinac iX加速器进行了验证。使用egs++几何软件包，为四种不同的探测器构建了详细的虚拟模型：PTW 60012无屏蔽二极管，PTW 60003 Diamond探测器，PTW 31006 PinPoint（电离室）和PTW 31018 MicroLion（充液电离室）。egs_chamber程序用来研究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束轴之间的方位角的变化。模拟也被用来考虑由每个探测器获得的DAP：虚拟探测器和水体素扫描通过高分辨率网格的位置延伸远远超出所考虑的领域的边界。
结果
对于每个检测器，校正因子（kQclin，Qmsrfclin，fmsr）被证明强烈依赖于检测器离轴位置和检测器方位角，除了字段大小。与先前的研究一致，还观察到了大量的检测器间差异。然而，已经证明，通过考虑DAP而不是单探测器位置剂量测量，可以消除探测器间的高水平变化。在小场条件下，质量密度被认为是水当量的主要决定因素。此外，发现敏感体积外的组分的质量密度影响检测器响应。
结论
现有检测器设计的kQclin、Qmsrfclin、fmsr值取决于大量变量，并且它们的计算通常依赖于时间密集型蒙特卡罗方法的使用。未来朝向密度补偿探测器设计或基于DAP的协议的移动可以简化小野剂量测定的方法。
PURPOSE
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.
METHODS
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.
RESULTS
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.
CONCLUSIONS
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.