西门子直线加速器均整和未均整射束的光子束剂量学特性

Photon beam dosimetric characteristics of SIEMENS linear accelerator with/without flattening filter

利用BEAMnrc和DOSXYZnrc蒙特卡罗程序研究西门子直线加速器6 MV能量均整和未均整光子束的剂量学特点。比较两种射束的深度剂量分布、离轴比剂量分布、中心轴剂量率和总散射因子。结果表明,未均整射束各射野的百分深度剂量均比均整射束的低;中心轴剂量率比均整射束大,且两种射束中心轴剂量率比值随射野和深度的增加逐渐下降;除此之外,未均整射束的半影宽度及总散射因子随深度的变化趋势均比均整射束小。未均整射束的剂量学优势可用于临床应用。

In this study, dosimetric characteristics of percentage depth dose, beam profiles, dose rate on the central axis, and total scattering factor, were estimated by using BEAMnrc and DOSXYZnrc Monte Carlo code, for 6 MV photon beams from a SIEMENS linac with and without flattening filter. The results showed that the percentage depth dose of flattening filter free （FFF） beam were lower than flattening filter （FF） beam; the dose rate on central axis was higher than FF beam, and the ratio of FFF/FF decreased with field size and depth increasing. Besides, the width of penumbra for FFF beam was slightly smaller than that of with FF case. And the variation of total scatter factor showed the flattening filter free mode had smaller depth dependence than flattening filter mode. Our study shows that the beams without flattening can be advantageous in clinical applications.