Mehnati Parinaz, Biglari Farideh, Jomehzadeh Ali
Department of Medical Physics, School of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran.
Department of Medical Physics, School of Paramedical, Kerman University of Medical Sciences, Kerman, Iran.
J Med Signals Sens. 2019 Apr-Jun;9(2):117-122. doi: 10.4103/jmss.JMSS_36_18.
The collimator scatter factor (S) is one of the most important parameters in monitor unit (MU) calculation. There are several factors that impact S values, including head structures, backscatter in dose monitoring chambers, and wedges. The objective of this study was to investigate the variation of S with different buildup cap materials, wall thickness of buildup caps, source-to-chamber distances (SCDs), ionization chambers, and wedge angles in 6 MV photon beam.
In this study, copper and Perspex buildup caps were made with two different thicknesses for each buildup cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK dosimeter with a sensitive volume of 0.120 cm and Farmer-type ion chamber with a sensitive volume of 0.65 cm. In all measurements, buildup caps and ionization chambers were positioned such as to stand vertically to the beam central axis. It was also investigated the effect of internal wedge with different angles (30° and 60°) different SCDs on S.
It was found in large field sizes, S values in Perspex buildup cap were higher than copper. Different SCDs and type of ion chamber and wall thickness of buildup caps had no significant influence on S values. The presence of wedge influenced S values significantly. Variation of S in wedged fields compared to open fields had a maximum deviation of 0.9% and 6.8% in 30° and 60° wedge angles, respectively.
It was found that the presence of wedges had a significant influence on S and increases with wedge angles. As such, it should be taken into account in manual MU calculations.
准直器散射因子（S）是监测单位（MU）计算中最重要的参数之一。有几个因素会影响S值，包括头部结构、剂量监测室中的反向散射以及楔形板。本研究的目的是调查在6兆伏光子束中，S随不同建成帽材料、建成帽壁厚、源到室距离（SCD）、电离室和楔形角的变化情况。
在本研究中，为每种建成帽制作了两种不同厚度的铜和有机玻璃建成帽。使用灵敏体积为0.120立方厘米的RK剂量计和灵敏体积为0.65立方厘米的 Farmer 型电离室，在 Elekta Compact 医用直线加速器（6兆伏）上进行测量。在所有测量中，建成帽和电离室的放置方式是使其垂直于束中心轴。还研究了不同角度（30°和60°）的内置楔形板和不同SCD对S的影响。
发现在大野尺寸下，有机玻璃建成帽中的S值高于铜建成帽。不同的SCD、电离室类型和建成帽壁厚对S值没有显著影响。楔形板的存在对S值有显著影响。与开放野相比，楔形野中S的变化在30°和60°楔形角下的最大偏差分别为0.9%和6.8%。
发现楔形板的存在对S有显著影响，且随楔形角增大而增加。因此，在手动MU计算中应予以考虑。
