Jursinic P A, Thomadsen B R
Battle Creek Health System, Radiation Oncology Department, Michigan 49016, USA.
Med Phys. 1999 Apr;26(4):512-7. doi: 10.1118/1.598550.
Head-scatter factors, Sh, also referred to as output factors, are measured in-air with an ion chamber and a semiconductor diode fitted with cylindrical build-up caps and columnar miniphantoms fabricated from materials of different atomic number. Sh increases with field size less rapidly when cylindrical build-up caps are constructed from high atomic number materials. This is a consequence of a net scatter of contamination electrons away from the detector. Ion chambers and diodes give identical results when the same type of build-up caps are used. Contamination electrons can be avoided by the use of columnar miniphantoms that have sufficient wall thickness in the radial direction. This radial wall thickness is characterized in this work for 6, 10, and 18 MV x-ray beams. Sh increases with field size less rapidly when columnar miniphantoms are constructed from high atomic number materials. This is due to the decrease in the average energy of photons at large field sizes. It is concluded that to obtain Sh for dosimetry in water, cylindrical build-up caps and columnar miniphantoms should be constructed from material with an atomic number close to that of water.
头部散射因子Sh，也称为输出因子，是在空气中用配备有圆柱形累积帽和由不同原子序数材料制成的柱状小型体模的电离室和半导体二极管进行测量的。当圆柱形累积帽由高原子序数材料制成时，Sh随射野大小的增加速度较慢。这是污染电子从探测器净散射的结果。当使用相同类型的累积帽时，电离室和二极管给出相同的结果。通过使用在径向方向上具有足够壁厚的柱状小型体模，可以避免污染电子。在这项工作中，对6、10和18MV X射线束的这种径向壁厚进行了表征。当柱状小型体模由高原子序数材料制成时，Sh随射野大小的增加速度较慢。这是由于在大射野尺寸下光子平均能量的降低。得出的结论是，为了在水中进行剂量测定获得Sh，圆柱形累积帽和柱状小型体模应由原子序数接近水的材料制成。
