Li X A, Soubra M, Szanto J, Gerig L H
Ottawa Regional Cancer Centre, Physics Department, Ontario, Canada.
Med Phys. 1995 Jul;22(7):1167-70. doi: 10.1118/1.597508.
The head-scatter factor (Sh) can be measured with a narrow miniphantom or a metal cap provided it is completely covered by the photon beam and its lateral size is thick enough to prevent electron contamination contributions. The effects of lateral electron equilibrium (LEE) and electron contamination on the Sh values were studied. The EGS4 Monte Carlo technique was used to calculate the minimum beam radii (rLEE) required to achieve complete LEE for photon beams ranging from 60Co to 24 MV. The measurement shows that the error introduced to the Sh value due to lateral electron disequilibrium is negligible. The radii of the miniphantoms or the sidewall thicknesses of the caps can be reduced below rLEE provided they are thick enough to prevent the effect of electron contamination.
头部散射因子（Sh）可用窄小体模或金属帽进行测量，前提是其完全被光子束覆盖且其横向尺寸足够厚，以防止电子污染的影响。研究了横向电子平衡（LEE）和电子污染对Sh值的影响。使用EGS4蒙特卡罗技术计算60Co至24MV光子束实现完全LEE所需的最小束半径（rLEE）。测量结果表明，横向电子不平衡对Sh值引入的误差可忽略不计。只要小体模的半径或帽的侧壁厚度足够厚以防止电子污染的影响，就可将其减小至低于rLEE。
