Ramsey C R, Spencer K M, Oliver A L
Thompson Cancer Survival Center, Department of Radiation Oncology, Knoxville, Tennessee 37916, USA.
Med Phys. 1999 Feb;26(2):214-9. doi: 10.1118/1.598507.
Plane-parallel ionization chambers are the instrument of choice for use in electron calibration and dosimetry, but these chambers may exhibit large polarity effects. This study concentrates on measuring the dependence of the polarity error at various mean energies using different linear accelerator, field size, ion chamber, and electrometer combinations. The polarity error was shown to increase for all four ionization chambers as the mean energy at depth decreased, but was always less than one percent at d(max). Polarity error dependence was also observed for similar plane-parallel chambers, varying field sizes, and different linear accelerators, but no polarity error dependence was observed for similar cylindrical chambers and different electrometers. Measurements of the polarity error can be used to develop correction factors for future measurements that will help minimize the time spent performing electron dosimetry and calibrations. These correction factors can be used to calculate the correct reading without the need to reverse the chamber bias, thus reducing the number of measurements required.
平行板电离室是用于电子校准和剂量测定的首选仪器，但这些电离室可能会表现出较大的极性效应。本研究专注于使用不同的直线加速器、射野大小、电离室和静电计组合，测量在各种平均能量下极性误差的依赖性。结果表明，随着深度处平均能量的降低，所有四个电离室的极性误差均增大，但在d(max)处始终小于1%。对于类似的平行板电离室、不同的射野大小和不同的直线加速器，也观察到了极性误差依赖性，但对于类似的圆柱形电离室和不同的静电计，未观察到极性误差依赖性。极性误差的测量结果可用于为未来的测量制定校正因子，这将有助于减少进行电子剂量测定和校准所需的时间。这些校正因子可用于计算正确读数，而无需反转电离室偏压，从而减少所需的测量次数。
