NRC Metrology Research Centre, National Research Council of Canada, Ottawa, ON, K1A 0R6, Canada.
Med Phys. 2020 Jun;47(5):2267-2276. doi: 10.1002/mp.14048. Epub 2020 Mar 23.
To present and demonstrate the accuracy of a modified formalism for electron beam reference dosimetry using updated Monte Carlo calculated beam quality conversion factors.
The proposed, simplified formalism allows the use of cylindrical ionization chambers in all electron beams (even those with low beam energies) and does not require a measured gradient correction factor. Data from a previous publication are used for beam quality conversion factors. The formalism is tested and compared to the present formalism in the AAPM TG-51 protocol with measurements made in Elekta Precise electron beams with energies between 4 MeV and 22 MeV and with fields shaped with a 10 × 10 cm clinical applicator as well as a 20 × 20 cm clinical applicator for the 18 MeV and 22 MeV beams. A set of six ionization chambers are used for measurements (two cylindical reference-class chambers, two scanning-type chambers and two parallel-plate chambers). Dose per monitor unit is derived using the data and formalism provided in the TG-51 protocol and with the proposed formalism and data and compared to that obtained using ionization chambers calibrated directly against primary standards for absorbed dose in electron beams.
The standard deviation of results using different chambers when TG-51 is followed strictly is on the order of 0.4% when parallel-plate chambers are cross-calibrated against cylindrical chambers. However, if parallel-plate chambers are directly calibrated in a cobalt-60 beam, the difference between results for these chambers is up to 2.2%. Using the proposed formalism and either directly calibrated or cross-calibrated parallel-plate chambers gives a standard deviation using different chambers of 0.4%. The difference between results that use TG-51 and the primary standard measurements are on the order of 0.6% with a maximum difference in the 4 MeV beam of 2.8%. Comparing the results obtained with the proposed formalism and the primary standard measurements are on the order of 0.4% with a maximum difference of 1.0% in the 4 MeV beam.
The proposed formalism and the use of updated data for beam quality conversion factors improves the consistency of results obtained with different chamber types and improves the accuracy of reference dosimetry measurements. Moreover, it is simpler than the present formalism and will be straightforward to implement clinically.
提出并演示一种使用更新的蒙特卡罗计算的束质转换系数的改进的电子束参考剂量学公式的准确性。
所提出的简化公式允许在所有电子束中使用圆柱形电离室（即使是那些具有低束能的电子束），并且不需要测量梯度校正系数。使用以前出版物中的数据来获得束质转换系数。该公式在 AAPM TG-51 协议中进行了测试和比较，并与 Elekta Precise 电子束的测量结果进行了比较，这些电子束的能量在 4 MeV 至 22 MeV 之间，并且使用 10×10 cm 临床适形器以及 20×20 cm 临床适形器对 18 MeV 和 22 MeV 电子束进行了成形。使用六组电离室进行测量（两个圆柱形参考级电离室、两个扫描型电离室和两个平行板电离室）。使用 TG-51 协议中提供的数据和公式以及所提出的公式和数据来推导每个测量单位的剂量，并与直接根据电子束吸收剂量的基准标准校准的电离室获得的剂量进行比较。
当严格遵循 TG-51 时，使用不同电离室的结果的标准偏差在平行板电离室与圆柱形电离室交叉校准的情况下约为 0.4%。然而，如果平行板电离室直接在钴-60 射束中校准，则这些电离室之间的结果差异最大可达 2.2%。使用所提出的公式和直接校准或交叉校准的平行板电离室，使用不同电离室的标准偏差为 0.4%。使用 TG-51 和基准标准测量结果之间的差异约为 0.6%，在 4 MeV 射束中的最大差异为 2.8%。使用所提出的公式和基准标准测量结果之间的差异约为 0.4%，在 4 MeV 射束中的最大差异为 1.0%。
所提出的公式和束质转换系数的更新数据的使用提高了不同类型电离室获得的结果的一致性，并提高了参考剂量测量的准确性。此外，它比现行公式更简单，在临床上实施起来也很简单。
