用加铅挡块的方法直接测定体模散射因子Sp的探讨

Direct measurement of phantom scattering factor through using blocks

目的：按 Sp的定义，直接测量体模散射因子 Sp。方法：在准直器开口不变的情况下，用加挡块的方法改变各射野在模体中的散射面积。分别测定各射野在模体中参考点处的吸收剂量率和加挡块后同一点的吸收剂量率，再根据 Sp的定义，计算出 Sp因子。结果：通过测量，直接得到各射野的体模散射因子 Sp和总散射校正因子 Scp，然后求出各射野的准直器散射修正因子 Sc。结论：用本文加挡块的方法测定 Sp因子，可以降低制作高能 X线的平衡帽的要求，而且也可避免平衡帽有限体积带来 Sc的测量误差。

Objective: The phantom scatter factor Sp was measured directly according to its definition. Method: Fixing collimator size, changing the port size with block, the dose rates at the reference point were measured with and without blocks, separately. Result: The Sc, Sp and Scp were derived from dose rates and the equation of Sp=Scp/Sc. Conclusion: Without the Built up cap of ionization chamber, the Sc, Sp can be obtained. It does not need to make the cap and may also avoid the influence of the scatter ray from the cap.