Med Phys. 2010 Mar;37(3):1161-3. doi: 10.1118/1.3314072.
Verify experimentally the theoretical prediction of F. Tessier and I. Kawrakow [Med. Phys. 37, 96-107 (2010)] that it is possible to design a thimble ionization chamber with no shift in its effective point of measurement (EPOM), i.e., a chamber that provides a measure of the dose to the medium at the location of its central axis.
Measure dose from a 25 MV photon beam incident on water with an Exradin A1SL ionization chamber inside a thin sleeve (as a means of effectively increasing the thimble wall thickness). The depth-dose curve is compared to that obtained using a well-characterized PTW Roos parallel-plate chamber.
With an appropriate increase in thimble wall thickness, the EPOM shift of the Exradin A1SL vanishes. Further increase of the wall thickness yields a chamber with a positive (downstream) shift in its point of measurement.
It is possible to design a thimble ionization chamber with a zero EPOM shift by adjusting the wall thickness.
验证 F. Tessier 和 I. Kawrakow [Med. Phys. 37, 96-107 (2010)] 的理论预测，即设计一个没有有效测量点（EPOM）位移的薄壁电离室是可能的，也就是说，一个提供介质在其中心轴位置处剂量测量的腔室。
用 Exradin A1SL 电离室在薄套管内测量 25 MV 光子束入射水时的剂量（作为有效增加薄壁厚度的手段）。将深度剂量曲线与经过良好表征的 PTW Roos 平行板室获得的曲线进行比较。
通过适当增加薄壁厚度，可以消除 Exradin A1SL 的 EPOM 位移。进一步增加壁厚度会导致测量点出现正向（下游）位移。
通过调整壁厚度，可以设计出具有零 EPOM 位移的薄壁电离室。
