Kubo H
Department of Radiation Oncology, University of Rochester Cancer Center, New York 14642.
Med Phys. 1989 Mar-Apr;16(2):295-8. doi: 10.1118/1.596381.
A contribution to field-size dependent output by backscattered radiation (BSR) from secondary collimator jaws to a beam monitor chamber of a linear accelerator was measured with a Farmer ionization chamber, positioned 200 cm from the source behind a low-melting-point alloy slab with a 10-cm wide slit. Another slit was positioned against the collimator jaws. Both slits were in the form of a 6.3-mm-diam hole in the middle and were aligned to the source. The use of a pair of slits was intended to eliminate any influence on the ion chamber readings due to field-size dependent charge contribution from the flattening filter and collimator jaw forward scattering. In addition, the setup permits to observe the degree of field-size dependence on BSR. Charge measurements from the Therac-20 18-MV x rays showed a 7.5% field-size dependence on BSR whereas 6- and 18-MV x rays from Varian Clinac-1800 showed less than 2% dependence on BSR. The telescopic method was found to be easy to use and permitted direct determination of BSR contributions.
利用一个 Farmer 电离室测量了二次准直器钳口的反向散射辐射（BSR）对直线加速器束流监测电离室的输出中与射野大小相关的贡献。该电离室位于距源 200 cm 处，置于一块 10 cm 宽狭缝的低熔点合金板后面。另一个狭缝紧贴准直器钳口放置。两个狭缝中间均为直径 6.3 mm 的孔的形式，并与源对齐。使用一对狭缝旨在消除由于均整滤过器和准直器钳口向前散射导致的与射野大小相关的电荷贡献对电离室读数的任何影响。此外，该设置允许观察射野大小对 BSR 的依赖程度。来自 Therac - 20 18 MV X 射线的电荷测量显示，BSR 对射野大小的依赖为 7.5%，而来自 Varian Clinac - 1800 的 6 MV 和 18 MV X 射线对 BSR 的依赖小于 2%。发现该伸缩法易于使用，并能直接确定 BSR 的贡献。
