高能电子线射野中心轴百分深度剂量测定

Dosimetry of electron beam central axis percent depth dose

目的：寻找限光筒、挡块开孔、电子线能量对电子线百分深度剂量的影响，明确它们的变化趋势和幅度，针对性地用于临床。方法：采用三维水箱系统测量Ｐｒｉｍｕｓ直线加速器各种条件下的中心轴百分深度剂量。结果：（１）电子线的百分深度剂量与射野大小（即挡块开孔）有很大的关系，特别是照射野较小及能量较高时，小野的时候，随着照射野的减小，最大剂量点深度移向表面，治疗深度变浅，剂量跌落变缓，能量升高，这种效应更明显；当射野尺寸大于射程时，已看不出这种效应。（２）百分深度剂量曲线与限光筒的关系不大，能量较高时，限光筒的影响变大，但差异不会超过４％。（３）电子线矩形射野的百分深度量可根据平方根的算法计算。结论：应分别实测限光筒和射野的百分深度量。选择能量时要同时考虑照射野的大小，防止由于深度剂量不足而引起复发，小射野时更要小心。

Objective: To examine the effects of various energies, applications and cutouts on electron beam central-axis percent depth dose, and to explain their change tendency and scopes, as well as to comment pertinently in clinical use. Methods: Using a 3D water scanning dosimeter for delermining the depth dose on various conditions, the experiments were done with electron beams in the energy range from 6 to 21 MeV delivered by linear accelerator Primus. Results: 1. The PDDs of electron beam depend on field size, especially for high energy and small size beam (i. e. cutout). With decreasing filed size, the generally changing tendency in small field was the decrease of therapeutic depth, the consequent shift of R100 toward the surface, and reduction of the dose gradient. These changes become more profound at higher energy. But the depth-dose curve does not significantly change with the field width greater than the electron range. 2. The applications have minor influence on depth dose curves except the higher energy (the maximum difference is within 4% ). 3. The PDD for rectangutar fields could be calculated by the square-root method. Conclusions: Percent depth-dose measurement for electron beams should he performed for each application and various cutout separately. The field size must be taken into account when the electron beam energy was determined in order to prevent the relapse of the tumor from insufficiency depth dose, especially using small fields.