Institut für Medizinische Physik und Strahlenschutz-IMPS, Technische Hochschule Mittelhessen, Wiesenstr 14, D-35390 Giessen, Germany.
Phys Med Biol. 2012 Apr 7;57(7):1831-54. doi: 10.1088/0031-9155/57/7/1831. Epub 2012 Mar 13.
Current dosimetry protocols (AAPM, IAEA, IPEM, DIN) recommend parallel-plate ionization chambers for dose measurements in clinical electron beams. This study presents detailed Monte Carlo simulations of beam quality correction factors for four different types of parallel-plate chambers: NACP-02, Markus, Advanced Markus and Roos. These chambers differ in constructive details which should have notable impact on the resulting perturbation corrections, hence on the beam quality corrections. The results reveal deviations to the recommended beam quality corrections given in the IAEA TRS-398 protocol in the range of 0%-2% depending on energy and chamber type. For well-guarded chambers, these deviations could be traced back to a non-unity and energy-dependent wall perturbation correction. In the case of the guardless Markus chamber, a nearly energy-independent beam quality correction is resulting as the effects of wall and cavity perturbation compensate each other. For this chamber, the deviations to the recommended values are the largest and may exceed 2%. From calculations of type-B uncertainties including effects due to uncertainties of the underlying cross-sectional data as well as uncertainties due to the chamber material composition and chamber geometry, the overall uncertainty of calculated beam quality correction factors was estimated to be <0.7%. Due to different chamber positioning recommendations given in the national and international dosimetry protocols, an additional uncertainty in the range of 0.2%-0.6% is present. According to the IAEA TRS-398 protocol, the uncertainty in clinical electron dosimetry using parallel-plate ion chambers is 1.7%. This study may help to reduce this uncertainty significantly.
现行的剂量学协议（AAPM、IAEA、IPEM、DIN）建议在临床电子束中使用平行板电离室进行剂量测量。本研究对四种不同类型的平行板电离室（NACP-02、Markus、Advanced Markus 和 Roos）的束流品质校正因子进行了详细的蒙特卡罗模拟。这些电离室在结构细节上存在差异，这些差异应该对最终的扰动量校正产生显著影响，从而对束流品质校正产生影响。结果表明，根据 IAEA TRS-398 协议给出的推荐束流品质校正值，在能量和电离室类型的不同范围内，存在 0%-2%的偏差。对于保护良好的电离室，这些偏差可以追溯到非单位和能量相关的壁扰动量校正。对于无保护的 Markus 电离室，由于壁和腔扰动量的相互补偿，导致几乎与能量无关的束流品质校正。对于这种电离室，与推荐值的偏差最大，可能超过 2%。从包括由于基础横截面数据的不确定性以及由于腔材料组成和腔几何形状的不确定性而导致的 B 型不确定度的计算结果来看，计算得到的束流品质校正因子的总不确定度估计小于 0.7%。由于国家和国际剂量学协议中给出的不同电离室定位建议，存在 0.2%-0.6%范围内的附加不确定性。根据 IAEA TRS-398 协议，使用平行板电离室进行临床电子剂量学的不确定度为 1.7%。本研究可能有助于显著降低这种不确定性。
