Clinical dosimetry is typically performed using ion chambers calibrated in terms of absorbed dose to water. As primary measurement standards for this quantity for low and medium energy x-rays are available only since a few years, most dosimetry protocols for this photon energy range are still based on air kerma calibration. For that reason, data for beam quality correction factors , necessary for the application of dose to water based protocols, are scarce in literature. Currently the international IAEA TRS-398 Code of Practice is under revision and new factors for a large number of ion chambers will be introduced in the update of this protocol. Several international groups provided the IAEA with experimental and Monte Carlo based data for this revision. Within the European Community the EURAMET 16NRM03 RTNORM project was initiated for that purpose. In the present study, Monte Carlo based results for the beam quality correction factors in medium energy x-ray beams for six ion chambers applying different Monte Carlo codes are presented. Additionally, the perturbation factor p _Q , necessary for the calculation of dose to water from an air kerma calibration coefficient, was determined.

The beam quality correction factor for the chambers varied in the investigated energy range by about 4%–5%, and for five out of six chambers the data could be fitted by a simple logarithmic function, if the half-value-layer was used as the beam quality specifier. Corresponding data using different Monte Carlo codes for the same ion chamber agreed within 0.5%. For the perturbation factor p _Q , the data did not obey a comparable simple relationship with the beam quality specifier. The variation of p _Q for all ion chambers was in the range of 3%–4%. Compared to recently published data, our p _Q data is around 1% larger, although the same Monte Carlo code has been used. Compared to the latest experimental data, there are even deviations in the range of 2%.

临床剂量测定通常使用根据水的吸收剂量校准的离子室进行。由于该量的低和中能量 X 射线的主要测量标准仅在几年后才可用，因此该光子能量范围的大多数剂量测定协议仍然基于空气比释动能校准。出于这个原因，对于将剂量应用于水基协议所必需的光束质量校正因子的数据在文献中很少见。目前国际 IAEA TRS-398 实践守则正在修订和新大量离子室的因素将在本协议的更新中引入。几个国际组织向 IAEA 提供了本次修订的实验数据和基于蒙特卡洛的数据。在欧洲共同体内，为此目的启动了 EURAMET 16NRM03 RTNORM 项目。在本研究中，给出了针对应用不同蒙特卡罗代码的六个离子室的中等能量 X 射线束中的光束质量校正因子的蒙特卡罗结果。此外，确定了根据空气比释动能校准系数计算水剂量所必需的扰动因子p _{Q 。}

腔室的光束质量校正因子在所研究的能量范围内变化约 4%–5%，如果将半值层用作光束质量说明符。对同一离子室使用不同蒙特卡罗代码的相应数据在 0.5% 范围内一致。对于扰动因子p _Q，数据不服从与光束质量说明符的可比较的简单关系。所有离子室的p _Q变化在 3%–4% 的范围内。与最近公布的数据相比，我们的p _Q 尽管使用了相同的蒙特卡洛代码，但数据大约大 1%。与最新的实验数据相比，甚至有2%范围内的偏差。