Nilsson B, Brahme A
Phys Med Biol. 1979 Sep;24(5):901-12. doi: 10.1088/0031-9155/24/5/003.
The absorbed dose in high energy photon beams due to scattered electrons from the irradiated air volume and from beam-shaping platforms has been calculated using the Fermi-Eyges theory of multiple scattering. The results are presented as lateral surface absorbed dose distributions across the field for three different radiation qualities, namely 60Co, 6 MV and 21 MV X-rays. For 60Co the relative absorbed dose due to electrons expelled in air reaches a value as high as 30% of the absorbed dose at dose maximum at a field size 40 X 40 cm2 and an SSD of 100 cm. The absorbed dose from electrons emanating from beam-shaping platforms contribute significantly to the absorbed dose at the surface when the platform is placed closer than 20--40 cm from the surface for field sizes greater than 10 X 10 cm2 to 40 X 40 cm2 respectively.
利用多次散射的费米 - 艾格斯理论，计算了高能光子束中由于来自受照空气体积和射束整形平台的散射电子而产生的吸收剂量。结果以三种不同辐射质（即钴 - 60、6兆伏和21兆伏X射线）下穿过射野的侧面表面吸收剂量分布的形式呈现。对于钴 - 60，在射野尺寸为40×40平方厘米且源皮距为100厘米时，在剂量最大值处，空气中射出的电子导致的相对吸收剂量高达吸收剂量的30%。当射野尺寸分别大于10×10平方厘米至40×40平方厘米时，若射束整形平台放置在距离表面小于20 - 40厘米处，平台产生的电子的吸收剂量对表面吸收剂量有显著贡献。
