Stasko John T, Culberson Wesley S
Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin, USA.
Med Phys. 2025 Apr;52(4):1991-1996. doi: 10.1002/mp.17629. Epub 2025 Jan 25.
A cylindrical free-air chamber, the Attix FAC, is used for absolute air-kerma measurements of low-energy photon beams at the University of Wisconsin Medical Radiation Research Center. Correction factors for air-kerma measurements of specific beams were determined in the 1990s. In order to measure air-kerma rates of beams in development, new correction factors must be computed.
We aimed to compute monoenergetic correction factors for air-kerma measurements with the Attix FAC that could be used to determine corrections for arbitrary polyenergetic beams.
A model of the Attix FAC was created in the Monte Carlo code, EGSnrc. The EGSnrc user codes, egs_fac, and egs_chamber, were utilized to calculate aperture transmission, scatter, collecting rod electron loss, and wall electron loss correction factors for incident monoenergetic photon beams with energies between 5 and 50 keV. Beam-specific correction factors were then derived from the monoenergetic correction factors and compared with the currently accepted values.
Correction factors were computed in 0.5 keV intervals. The newly calculated beam-specific correction factors and the old conventional values agreed within 0.1% for all beams investigated.
The process for determining monoenergetic correction factors for air-kerma measurements with a free-air chamber is detailed in this work. Beam-specific correction factors can then be calculated if photon spectra are known. This process can be carried out for any free-air chamber, given specific materials and dimensions for modeling.
威斯康星大学医学辐射研究中心使用一种圆柱形自由空气电离室（Attix FAC）来对低能光子束进行绝对空气比释动能测量。特定束流空气比释动能测量的校正因子在20世纪90年代已确定。为了测量正在研发的束流的空气比释动能率，必须计算新的校正因子。
我们旨在计算使用Attix FAC进行空气比释动能测量的单能校正因子，可用于确定任意多能束流的校正。
在蒙特卡罗代码EGSnrc中创建了Attix FAC的模型。利用EGSnrc用户代码egs_fac和egs_chamber，计算能量在5至50 keV之间的入射单能光子束的孔径传输、散射、收集棒电子损失和壁电子损失校正因子。然后从单能校正因子中推导出特定束流的校正因子，并与当前公认值进行比较。
以0.5 keV的间隔计算校正因子。对于所有研究的束流，新计算的特定束流校正因子与旧的常规值在0.1%以内相符。
这项工作详细介绍了使用自由空气电离室进行空气比释动能测量的单能校正因子的确定过程。如果已知光子光谱，则可以计算特定束流的校正因子。给定用于建模的特定材料和尺寸，此过程可针对任何自由空气电离室进行。
