Phys Med Biol. 2010 Mar 7;55(5):L9-16; author reply L17-9. doi: 10.1088/0031-9155/55/5/L01. Epub 2010 Feb 16.
The authors of a recent paper (Wang and Rogers 2009 Phys. Med. Biol. 54 1609) have used the Monte Carlo method to simulate the 'classical' experiment made more than 30 years ago by Johansson et al (1978 National and International Standardization of Radiation Dosimetry (Atlanta 1977) vol 2 (Vienna: IAEA) pp 243-70) on the displacement (or replacement) perturbation correction factor p(dis) for cylindrical chambers in 60Co and high-energy photon beams. They conclude that an 'unreasonable normalization at dmax' of the ionization chambers response led to incorrect results, and for the IAEA TRS-398 Code of Practice, which uses ratios of those results, 'the difference in the correction factors can lead to a beam calibration deviation of more than 0.5% for Farmer-like chambers'. The present work critically examines and questions some of the claims and generalized conclusions of the paper. It is demonstrated that for real, commercial Farmer-like chambers, the possible deviations in absorbed dose would be much smaller (typically 0.13%) than those stated by Wang and Rogers, making the impact of their proposed values negligible on practical high-energy photon dosimetry. Differences of the order of 0.4% would only appear at the upper extreme of the energies potentially available for clinical use (around 25 MV) and, because lower energies are more frequently used, the number of radiotherapy photon beams for which the deviations would be larger than say 0.2% is extremely small. This work also raises concerns on the proposed value of pdis for Farmer chambers at the reference quality of 60Co in relation to their impact on electron beam dosimetry, both for direct dose determination using these chambers and for the cross-calibration of plane-parallel chambers. The proposed increase of about 1% in p(dis) (compared with TRS-398) would lower the kQ factors and therefore Dw in electron beams by the same amount. This would yield a severe discrepancy with the current good agreement between electron dosimetry based on an electron cross-calibrated plane-parallel chamber (against a Farmer) or on a directly 60Co calibrated plane-parallel chamber, which is not likely to be in error by 1%. It is suggested that the influence of the 60Co source spectrum used in the simulations may not be negligible for calculations aimed at an uncertainty level of 0.1%.
最近一篇论文的作者（Wang 和 Rogers，2009 年《物理医学与生物学》54 卷 1609 页）使用蒙特卡罗方法模拟了 Johansson 等人（1978 年《国家和国际辐射剂量标准化（亚特兰大 1977 年）》第 2 卷（维也纳：IAEA）第 243-70 页）在 30 多年前进行的关于圆柱形电离室在 60Co 和高能光子射束中位移（或替换）扰动修正因子 p(dis)的“经典”实验。他们的结论是，电离室响应的“在 dmax 处不合理的归一化”导致了错误的结果，对于使用这些结果比值的 IAEA TRS-398 实践规范，“修正因子的差异可能导致类似于 Farmer 的电离室的光束校准偏差超过 0.5%”。本工作批判性地审查并质疑了该论文的一些主张和普遍结论。结果表明，对于实际的、商业用的 Farmer 型电离室，吸收剂量的可能偏差要小得多（通常为 0.13%），而不是 Wang 和 Rogers 所声称的那样，因此他们提出的值对实际高能光子剂量测定的影响可以忽略不计。约 0.4%的差异仅出现在可能用于临床应用的能量上限（约 25MV），而且由于较低的能量更频繁地使用，偏差会大于 0.2%的放射治疗光子束数量非常少。这项工作还对 Farmer 型电离室在 60Co 参考质量下的 pdis 提出了一些关注，因为它对电子束剂量测定有影响，包括使用这些电离室直接测定剂量以及对平面平行电离室进行交叉校准。与 TRS-398 相比，p(dis)提议增加约 1%（相比之下）将使电子束中的 kQ 因子和 Dw 降低相同的量。这将与基于经电子交叉校准的平面平行电离室（与 Farmer 型电离室相比）或直接经 60Co 校准的平面平行电离室的电子剂量测定目前的良好一致性产生严重差异，而后者不太可能有 1%的误差。有人建议，在计算目的是达到 0.1%的不确定度水平时，模拟中使用的 60Co 源谱的影响可能不可忽略。
