Higgins P D, Mihailidis D N, Khan F M, Lee E J, Ahuja A S
University of Minnesota Dept. of Therapeutic Radiology, Minneapolis 55455, USA.
Phys Med Biol. 1997 Dec;42(12):2435-47. doi: 10.1088/0031-9155/42/12/010.
In routine dosimetry we assume separability of the collimator (Sc) and phantom (Sp) scatter components that together comprise the total scatter factor (Sc,p). In practice, the addition of blocking also affects the photon fluence attributable to the treatment head and flattening filter in a complicated way. The reduced aperture blocks out some of the head scatter contribution, while the block and tray add back secondary scatter. In the following we present techniques for directly measuring the aperture effect on Sc in air or in a full-scatter phantom. The change in Sc is found to be a scaleable quantity that can be modelled as a simple linear fit to the ratio of projected open-to-blocked equivalent square fields. Measurements have been made for 6, 18 and 24 MV photon beams on one Varian 2500 and two Varian 2100c accelerators. Results indicate a progressive loss of collimator scatter contribution with increased field blocking that is amplified with increasing energy. Block and tray scatter only contribute significantly to Sc for large fields and treatment distances of 80 cm or less. Application of these corrections in monitor unit calculations is presented.
在常规剂量测定中，我们假设准直器散射分量（Sc）和模体散射分量（Sp）是可分离的，它们共同构成了总散射因子（Sc,p）。实际上，添加挡块也会以复杂的方式影响治疗头和平坦滤过器产生的光子注量。孔径减小会阻挡部分机头散射贡献，而挡块和托盘又会产生二次散射。在本文中，我们介绍了直接测量空气中或全散射模体中孔径对准直器散射因子（Sc）影响的技术。发现Sc的变化是一个可缩放的量，可以通过对射野开放与挡块等效方野比值进行简单线性拟合来建模。我们在一台Varian 2500和两台Varian 2100c加速器上对6、18和24 MV光子束进行了测量。结果表明，随着射野挡块增加，准直器散射贡献逐渐减少，且能量越高这种减少越明显。挡块和托盘散射仅在大射野和80 cm及以下的治疗距离时对Sc有显著贡献。本文还介绍了这些校正因子在监测单位计算中的应用。
