Microdosimetric study on influence of low energy photons on relative biological effectiveness under therapeutic conditions using 6 MV linac
Microdosimetric study on influence of low energy photons on relative biological effectiveness under therapeutic conditions using 6 MV linac
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DOI：
10.1118/1.3613152
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发表时间：
2011-08-01
期刊：
影响因子：
3.8
通讯作者：
Itami, Jun
中科院分区：
文献类型：
--
作者：
Okamoto, Hiroyuki;Kohno, Toshiyuki;Itami, Jun
目的：微剂量学已被发展用于辐射质量的评估，单粒子剂量-平均线能量(Y)/巴(D)可以很好地表示辐射质量。采用微剂量学方法研究了6 MV直线加速器治疗条件下相对生物效应(RBE)值的变化。方法：采用组织等效正比计数器(TEPC)，通过降低加速器的枪栅电压，在极低剂量率下，测量6 MV直线加速器在不同照射条件下的相对生物效应(RBE)值。根据微剂量学动力学模型(MK模型)，如果从TEPC测量得到(Y)Over bar(D)值，就可以得到人唾液腺(HSG)肿瘤细胞杀伤细胞的RBEMK值。利用蒙特卡罗程序GEANT4计算了体模中的光子能量分布，并研究了不同条件下(Y)Over bar(D)值的变化。结果：当体模的视场大小和深度变化时，y Over bar(D)值的变化小于约10%。然而，在垂直于中心束轴的测量中，在场内和场外的(Y)Over bar(D)值之间观察到了很大的变化。与场内相比，场外的(Y)比bar(D)值最大增加了约50%。GEANT4计算表明，与场内相比，场外存在大量相对较多的低能光子。场外低于200keV的光子通量的百分比约为40%，而场内的光子通量百分比约为8%。利用MK模型，场内RBEMK值的场大小和深度依赖关系小于约2%。而场外的RBEMK值比场内的高6.6%。这种增加是由于光子能量分布的变化，特别是场外低能光子相对数目的增加所引起的。(C)2011年美国医学物理学家协会。[DOI：10.1118/1.3613152]
Purpose: Microdosimetry has been developed for the evaluation of radiation quality, and single-event dose-mean lineal energy (y) over bar (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.Methods: The (y) over bar (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 mu Gy/min by decreasing the gun grid voltage of the linac. According to the microdosimetric kinetic model (MK model), the RBEMK values for cell killing of the human salivary gland (HSG) tumor cells can be derived if the (y) over bar (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) over bar (D) values under the various conditions.Results: The changes of the (y) over bar (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) over bar (D) values inside the field and those outside the field. The maximum increase of approximately 50% in the (y) over bar (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.Conclusions: The increase of the RBEMK 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. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3613152]