Influence of field size on electron beam central axis dose on a radiotherapy linear accelerator / 中华放射肿瘤学杂志

ABSTRACT

Objective Clinical application of electrons often involves some beam in which the field size varies with the applicators. The work was done to understand the electron beam characteristics in different field sizes. Methods Percent depth dose and the dose output factor were measured for square and rectangular fields at 100?cm source to surface distance ( SSD ) . Central axis percent depth dose (PDD) measurements were made using the RFA 300 three dimensional radiation field analyzer with a shielded p type diode detector . The dose output factors were measured with the RFA 300 three dimensional radiation field analyzer with a PTW 0.1?cm 3 chamber and a Farmer 2570/1 dosimeter with a 2571 ion chamber in a water phantom. Results The measurements showed that the depth dose curves and the output factors were sometimes dependent on how the field sizes were formed. The change in depth dose with field size was more pronounced in the high energy beams than the low energy ones. However, the output factor did not show any systematic energy dependence because each applicator had it's own X ray jaw setting at each energy. Conclusions When using small inserted apertures to treat small lesions, we should verify the conformation of depth dose and output factors. In this case, we should use applicator dependent output factors at each energy to calculate the monitor units for irradiation.