组织模体比对X射线中心轴剂量特性的研究  

Description of tissue phantom ratio in a quadratic exponential function of depth

作　　者：任青[1] 

机构地区：[1]中国医科大学附属第一医院放射治疗科,沈阳110001

出　　处：《中华放射肿瘤学杂志》2001年第4期258-261,共4页Chinese Journal of Radiation Oncology

摘　　要：目的　提出一种用于描述高能X射线中心轴剂量特性的数学模型 ,并且给出一种用于计算临床组织模体比 (TPR)表的方法。方法　分别对 6、18和 2 4MVX射线的实测TPR数据与深度和照射野之间的关系进行二阶指数线性回归分析 ,推导出描述TPR值随深度和照射野变化的数学模型。通过对模型的计算结果与实测数据之间的误差分析来评价模型精度 ,并给出确定模型参数所需的最少实测数据。结果　只需 8个TPR数据即可确定由 5个参数构成的二阶指数模型 ,其参数均与辐射质指数线性相关。对应 6、18和 2 4MVX射线 ,使用该模型计算的TPR数据表与实测表的误差分别在 1.5 %、1.2 %和 1.1%之内。并可同时求得描述原射线中心轴剂量特性的 2个主要指标 :衰减系数 (μ)和硬化系数 (η)。 结论　该模型建立了一种从有限实测数据得出临床TPR数据表的有效方法 。Objective To develop a mathematical model for clinical calculation of and to verify its practical accuracy in tissue phantom ratios (TPRS). Methods An exponential attenuation algorithm was designed to express the TPRS through a function of any depth and field size of the primary beam with a beam hardening correction coefficient. In our model,the variation of attenuation coefficient with the field size was described by an attenuation factor and the hardening coefficient was replaced by a correction factor. Results From fitting in with the published TPR data of 6, 18 and 24?MV X-rays at the normalization depth sufficient for electron equilibrium, it was found that the attenuation factor can be described by a second-order function of field sizes. And the correction factor exhibited a linear relation to the attenuation factor. In order to determine the five parameters needed to calculate the above attenuation factor and correction factor, only eight TPR values at four extreme and mode rate field sizes were measured. A linear fit to the quality index(QI) showed that these parameters were strongly energy-dependent with R2 ranging from 0.844 to 1.000. The attenuation coefficient and hardening coefficient of the primary beam were calculated directly from two parameters. The derived initial attenuation coefficients from QI fitted well to published experimental data and the hardening coefficients were also consistent with measured data. Compared with practical measured results,the error rate of derived functional TPRS,presented within 1.5% at 6?MV, 1.2% at 18?MV and 1.1% at 24?MV X-ray.The principal attenuation and havdening coefficients were also obtained from our developed model.Conclusions The derived model is able to accurately describe the central axis dose distribution of the primary photon beam and be used to calculate the clinical TPR table from limited TPR values.

关 键 词：衰减系数 硬化系数 辐射质指数 TPR表 组织模体比 X射线中心轴剂量 

分 类 号：R730.5[医药卫生—肿瘤]