半导体和电离室探头在直线加速器数据测量中的比较与分析

Analysis and Comparison of Data of Linear Accelerators Measured with Diode and Ionization Detector

目的:比较分析半导体探头和电离室探头在三维水箱测量中的差异,为能够提高数据测量精度从而实现治疗计划系统建立准确的计算模型提供依据;方法:在加速器8 MV光子线下,使用0.13 cm3的指形电离室和半导体探头在三维水箱中分别测量照射野1 cm×1 cm,2 cm×2 cm,3 cm×3 cm,4 cm×4 cm,5 cm×5 cm,6 cm×6 cm,8 cm×8 cm,10 cm×10 cm的总散射因子、百分深度剂量曲线、离轴比曲线,对测量结果进行比较和分析;结果:对于总散射因子,在较大照射野测量时结果一致,在小野测量时存在差异,1 cm×1 cm照射野的两者测量结果偏差15.32%;对于百分深度曲线,在建成区差异最大,各照射野的在水面处的测量结果均偏差10%以上;对于离轴比曲线,在半影区存在显著差异,半导体探头在最大剂量点深度测量的射野大小均小明显小于电离室测量的结果。结论:总散射因子,小照射野测量时建议使用半导体探头或者较小体积的电离室;百分深度剂量曲线,建议使用电离室探头;离轴比曲线,使用半导体探头可测量到较好的射野半影区。

Objective： This study was to analyze and compare the data measured with ionization chambers and diode detector in three dimensional water tanks and provided theory basis for foundation of precise calculating model in treatment planning sys- tem. Methods： The treatment planning system needs plenty of accurate physical data to model treatment machine. These data include the total scatter factors, percent depth dose curves, off-axis ratio curves and so on. These data were acquired in linear accelerator of 8 MV photon beam with different beam fields. Results： For total scatted factors ,the great discrepancy exist in small fields, for field of lcm×lcm, the discrepancy from two detectors is 15.32% ; For percent depth dose, the great discrepan- cy exists in build-up region. The discrepancy for two types detectors are more than 10% for all field sizes. For off-axis ratio curves, the data showed differ in penumbra areas. The field sizes calculated by data measured from diode detector were smaller than that from ion chamber. Conclusions： For total scatter factors of small fields, diode and ion chamber with small volume can get good result. For percent depth dose curves measurement, ion chamber is recommended. For off-axis ratio curves mea- surement, diode detectors can performance well than ion chamber.