Zhu T C, Bjärngard B E
Department of Radiation Oncology, Roger Williams Cancer Center, Brown University, Providence, Rhode Island 02908.
Med Phys. 1994 Jan;21(1):65-8. doi: 10.1118/1.597256.
The head-scatter factors H were examined for four different linear accelerators and were found to be similar at field sizes larger than 3 x 3 cm2. Sharply reduced values for small collimator openings were observed for all the accelerators. It is concluded that the head-scatter (or collimator-scatter) factor has two major components. Scatter of photons in various structures in the beam path, especially the flattening filter, causes a slow (about 10%) increase with increased collimator opening. Insertion of a built-in wedge may double this number. When the collimators are closed, they ultimately block photons from the periphery of the source. This may cause a considerable reduction of the primary photon fluence and typically affects fields smaller than 3 x 3 cm2. The effect can be used to estimate the source size, with results that correlate with the design of the bending magnet.
对四台不同的直线加速器的头部散射因子H进行了研究，发现在射野尺寸大于3×3 cm2时它们是相似的。对于所有加速器，在小准直器开口处观察到明显降低的值。得出的结论是，头部散射（或准直器散射）因子有两个主要成分。光子在射线路径中的各种结构（特别是均整器）中的散射，会随着准直器开口的增加而缓慢（约10%）增加。内置楔形板的插入可能会使这个数字翻倍。当准直器关闭时，它们最终会阻挡来自源周边的光子。这可能会导致原发光子注量显著降低，并且通常会影响小于3×3 cm2的射野。这种效应可用于估计源的尺寸，其结果与弯曲磁铁的设计相关。
