Ottawa Medical Physics Institute (OMPI), Ottawa Carleton Institute for Physics, Carleton University Campus, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada.
Med Phys. 2011 Aug;38(8):4600-9. doi: 10.1118/1.3600697.
The journal Medical Physics recently published two papers that determine beam quality conversion factors, k(Q), for large sets of ion chambers. In the first paper [McEwen Med. Phys. 37, 2179-2193 (2010)], k(Q) was determined experimentally, while the second paper [Muir and Rogers Med. Phys. 37, 5939-5950 (2010)] provides k(Q) factors calculated using Monte Carlo simulations. This work investigates a variety of additional consistency checks to verify the accuracy of the k(Q) factors determined in each publication and a comparison of the two data sets. Uncertainty introduced in calculated k(Q) factors by possible variation of W/e with beam energy is investigated further.
The validity of the experimental set of k(Q) factors relies on the accuracy of the NE2571 reference chamber measurements to which k(Q) factors for all other ion chambers are correlated. The stability of NE2571 absorbed dose to water calibration coefficients is determined and comparison to other experimental k(Q) factors is analyzed. Reliability of Monte Carlo calculated k(Q) factors is assessed through comparison to other publications that provide Monte Carlo calculations of k(Q) as well as an analysis of the sleeve effect, the effect of cavity length and self-consistencies between graphite-walled Farmer-chambers. Comparison between the two data sets is given in terms of the percent difference between the k(Q) factors presented in both publications.
Monitoring of the absorbed dose calibration coefficients for the NE2571 chambers over a period of more than 15 yrs exhibit consistency at a level better than 0.1%. Agreement of the NE2571 k(Q) factors with a quadratic fit to all other experimental data from standards labs for the same chamber is observed within 0.3%. Monte Carlo calculated k(Q) factors are in good agreement with most other Monte Carlo calculated k(Q) factors. Expected results are observed for the sleeve effect and the effect of cavity length on k(Q). The mean percent differences between experimental and Monte Carlo calculated k(Q) factors are -0.08, -0.07, and -0.23% for the Elekta 6, 10, and 25 MV nominal beam energies, respectively. An upper limit on the variation of W/e in photon beams from cobalt-60 to 25 MV is determined as 0.4% with 95% confidence. The combined uncertainty on Monte Carlo calculated k(Q) factors is reassessed and amounts to between 0.40 and 0.49% depending on the wall material of the chamber.
Excellent agreement (mean percent difference of only 0.13% for the entire data set) between experimental and calculated k(Q) factors is observed. For some chambers, k(Q) is measured for only one chamber of each type--the level of agreement observed in this study would suggest that for those chambers the measured k(Q) values are generally representative of the chamber type.
《医学物理学杂志》最近发表了两篇确定大量离子室束质转换系数 k(Q)的论文。在第一篇论文[McEwen Med. Phys. 37, 2179-2193 (2010)]中，k(Q)是通过实验确定的，而第二篇论文[Muir 和 Rogers Med. Phys. 37, 5939-5950 (2010)]则提供了使用蒙特卡罗模拟计算的 k(Q)因子。本工作研究了各种其他一致性检查，以验证两篇论文中确定的 k(Q)因子的准确性，并对这两个数据集进行比较。还进一步研究了束能变化对计算 k(Q)因子的不确定性的影响。
实验 k(Q)因子的有效性依赖于 NE2571 参考室测量的准确性，所有其他离子室的 k(Q)因子都与该测量值相关联。确定了 NE2571 水吸收剂量校准系数的稳定性，并对其与其他实验 k(Q)因子进行了比较。通过与提供 k(Q)蒙特卡罗计算的其他出版物以及对套管效应、腔长效应和石墨壁 Farmer 室之间的自一致性的分析进行比较，评估了蒙特卡罗计算的 k(Q)因子的可靠性。用两篇论文中给出的 k(Q)因子之间的百分比差异来表示两个数据集之间的比较。
对 NE2571 室吸收剂量校准系数进行了 15 年以上的监测，结果显示，其一致性水平优于 0.1%。观察到 NE2571 的 k(Q)因子与来自标准实验室的所有其他实验数据的二次拟合相吻合，误差在 0.3%以内。蒙特卡罗计算的 k(Q)因子与大多数其他蒙特卡罗计算的 k(Q)因子非常吻合。对套管效应和腔长对 k(Q)的影响都观察到了预期的结果。实验和蒙特卡罗计算的 k(Q)因子之间的平均百分比差异分别为 Elekta 6、10 和 25 MV 标称束能的-0.08%、-0.07%和-0.23%。用钴-60 到 25 MV 的光子束确定 W/e 的变化上限为 0.4%，置信度为 95%。重新评估了蒙特卡罗计算的 k(Q)因子的组合不确定度，其值在 0.40%到 0.49%之间，具体取决于室壁的材料。
观察到实验和计算的 k(Q)因子之间极好的一致性（整个数据集的平均百分比差异仅为 0.13%）。对于一些室，每种类型的室仅测量一个室的 k(Q)，本研究中的一致性水平表明，对于这些室，测量的 k(Q)值通常代表该室的类型。
