Rogers D W, Ross C K
Division of Physics, National Research Council of Canada, Ottawa.
Med Phys. 1988 Jan-Feb;15(1):40-8. doi: 10.1118/1.596292.
A detailed derivation is presented of the formulas required to determine Ngas and Dmed in the AAPM TG-21 dosimetry protocol. This protocol specifies how to determine the absorbed dose in an electron or photon beam when using exposure or absorbed dose calibrated ion chambers. It is shown that the expression given in TG-21's recent letter of clarification is incorrect. Accounting for humidity correctly increases, by 0.4%, all absorbed dose determinations using an exposure calibrated ion chamber. Taking into account other correction factors in the equation for exposure could also have varying, but significant effects (possibly over 1%). These are the stem scatter correction, the axial nonuniformity correction and the electrode correction for electrodes made of different materials from the wall. Attention is drawn to differences in the definitions of the exposure and absorbed dose calibration factors, Nx and ND, respectively, as supplied by the NBS and the NRCC.
本文详细推导了AAPM TG - 21剂量测定协议中确定Ngas和Dmed所需的公式。该协议规定了在使用经曝光或吸收剂量校准的电离室时，如何确定电子或光子束中的吸收剂量。结果表明，TG - 21最近的澄清函中给出的表达式是不正确的。正确考虑湿度会使所有使用经曝光校准的电离室进行的吸收剂量测定值增加0.4%。在曝光方程中考虑其他校正因子也可能产生不同但显著的影响（可能超过1%）。这些校正因子包括干体散射校正、轴向不均匀性校正以及电极材料与壁不同时的电极校正。需要注意的是，NBS和NRCC分别提供的曝光校准因子Nx和吸收剂量校准因子ND的定义存在差异。
