Department of Energy Sciences, Tokyo Institute of Technology, Kanagawa 226-8502, Japan.
Med Phys. 2011 Aug;38(8):4714-22. doi: 10.1118/1.3613152.
Microdosimetry has been developed for the evaluation of radiation quality, and single-event dose-mean lineal energy y(D) is well-used to represent the radiation quality. In this study, the changes of the relative biological effectiveness (RBE) values under the therapeutic conditions using a 6 MV linac were investigated with a microdosimetric method.
The y(D) values under the various irradiation conditions for x-rays from a 6 MV linac were measured with a tissue-equivalent proportional counter (TEPC) at an extremely low dose rate of a few tens of microGy/min by decreasing the gun grid voltage of the linac. According to the microdosimetric kinetic model (MK model), the RBE(MK) values for cell killing of the human salivary gland (HSG) tumor cells can be derived if the y(D) values are obtained from TEPC measurements. The Monte Carlo code GEANT4 was also used to calculate the photon energy distributions and to investigate the changes of the y(D) values under the various conditions.
The changes of the y(D) values were less than approximately 10% when the field size and the depth in a phantom varied. However, in the measurements perpendicular to a central beam axis, large changes were observed between the y(D) values inside the field and those outside the field. The maximum increase of approximately 50% in the y(D) value outside the field was obtained compared with those inside the field. The GEANT4 calculations showed that there existed a large relative number of low energy photons outside of the field as compared with inside of the field. The percentages of the photon fluences below 200 keV outside the field were approximately 40% against approximately 8% inside the field. By using the MK model, the field size and the depth dependence of the RBEMK values were less than approximately 2% inside the field. However, the RBEMK values outside the field were 6.6% higher than those inside the field.
The increase of the RBE(MK) values by 6.6% outside the field was observed. This increase is caused by the change of the photon energy distributions, especially the increase of the relative number of low energy photons outside the field.
微剂量学已被开发用于评估辐射质量，而单事件剂量平均线性能量 y(D) 常用于表示辐射质量。在这项研究中，使用微剂量学方法研究了在治疗条件下使用 6 MV 直线加速器时相对生物效应 (RBE) 值的变化。
通过降低直线加速器的枪栅电压，使用组织等效比例计数器 (TEPC) 在极低的几十微戈瑞/分钟剂量率下测量来自 6 MV 直线加速器的 X 射线的各种照射条件下的 y(D) 值。根据微剂量动力学模型 (MK 模型)，如果从 TEPC 测量中获得 y(D) 值，则可以得出人唾液腺 (HSG) 肿瘤细胞杀伤的 RBE(MK) 值。还使用蒙特卡罗代码 GEANT4 计算光子能谱分布，并研究各种条件下 y(D) 值的变化。
当照射野大小和体模深度变化时，y(D) 值的变化小于约 10%。然而，在垂直于中央射束轴的测量中，在照射野内和照射野外观察到 y(D) 值的较大变化。与照射野内相比，照射野外的 y(D) 值最大增加约 50%。GEANT4 计算表明，与照射野内相比，照射野外存在大量相对低能光子。照射野外的光子通量低于 200 keV 的百分比约为 40%，而照射野内约为 8%。使用 MK 模型，照射野内的 RBEMK 值的大小和深度依赖性小于约 2%。然而，照射野外的 RBEMK 值比照射野内高 6.6%。
观察到照射野外 RBE(MK) 值增加 6.6%。这种增加是由光子能谱分布的变化引起的，特别是照射野外低能光子的相对数量增加。
