Buckley Lesley A, Rogers D W O
Ottawa-Carleton Institute of Physics, Carleton University, Ottawa, K1S 5B6, Canada.
Med Phys. 2006 Feb;33(2):455-64. doi: 10.1118/1.2161403.
The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, Pwall, for thimble ionization chambers in photon and electron beams. CSnrc calculated values of Pwall give closer agreement with previous experimental results than do the values from the standard formalism used in current dosimetry protocols. A set of Pwall values, computed at the reference depth in water, is presented for several commonly used thimble chambers. These values differ from the commonly used values by up to 0.8% for megavoltage photon beams, particularly for nominal beam energies below 6 MV. The sleeve effect, which is not currently taken into account by the TG-51 dosimetry protocol, is computed to be up to 0.3% and is in some cases larger than the Pwal1 correction itself. In electron beams, where dosimetry protocols assume a wall correction of unity, CSnrc calculations show Pwall values of up to 0.6% at the reference depth, depending on the wall material. Pwall is shown to be sensitive to the depth of measurement, varying by 2.5% for a graphite-walled cylindrical Farmer-like chamber between a depth of 0.5 cm and R50 in a 6 MeV electron beam.
EGSnrc蒙特卡罗用户代码CSnrc用于计算光子束和电子束中针箍电离室的壁修正因子Pwall。与当前剂量测定协议中使用的标准形式主义所得值相比，CSnrc计算得到的Pwall值与先前的实验结果更为吻合。给出了几种常用针箍电离室在水中参考深度处计算得到的一组Pwall值。对于兆伏级光子束，这些值与常用值的差异高达0.8%，特别是对于标称束能量低于6MV的情况。目前TG-51剂量测定协议未考虑的套管效应经计算高达0.3%，在某些情况下大于Pwall修正本身。在电子束中，剂量测定协议假定壁修正为1，CSnrc计算表明，在参考深度处，Pwall值高达0.6%，具体取决于壁材料。结果表明，Pwall对测量深度敏感，对于一个石墨壁圆柱形类似Farmer室，在6MeV电子束中，从0.5cm深度到R50处，其变化为2.5%。
