Nagoya University Graduate School of Medicine, Nagoya, Aichi, Japan.
Nagoya University Hospital, Nagoya, Aichi, Japan.
Radiol Phys Technol. 2022 Dec;15(4):291-297. doi: 10.1007/s12194-022-00669-z. Epub 2022 Aug 6.
Current dosimetry protocols specify an effective point of measurement (EPOM) shift of 0.6r for a cylindrical ionization chamber in photon beams. However, prior studies have reported that this shift was excessively large. The objective of this study was to experimentally evaluate the EPOM shifts in photon beams for cylindrical ionization chambers, which are widely used in clinical practice, and thus determine the appropriate EPOM shift. A microdiamond detector, which is a semiconductor detector with a small sensitive volume, was used as a reference detector, and the EPOM shifts of 11 types of cylindrical ionization chambers were evaluated at 6 MV and 10 MV. The depth shift from the percent depth dose (PDD) of the reference detector to that of the evaluated chamber was calculated using the least-squares method and was defined as the EPOM shift. The EPOM shift of the 10 MV condition was slightly larger than that of the 6 MV condition. However, because this trend was not observed for all chambers, the results of the two energies were averaged, and the EPOM shifts were determined to be 0.33r-0.43r (± 0.05) for 10 types of ionization chambers, and 0.03r (± 0.03) for the A1SL chamber. The shifts for all ionization chambers were smaller than 0.6r, indicating that the recommended EPOM shifts were overestimated and the absorbed dose was underestimated at the calibration depth. Hence, the appropriate EPOM shift of the 10 types of ionization chambers was 0.4r (the geometric center of the A1SL chamber), with a dose uncertainty of 0.05%.
现行的剂量学规程规定，在光子射束中，圆柱形电离室的有效测量点（EPOM）移位应为 0.6r。然而，先前的研究报告表明，这种移位过大。本研究的目的是实验评估光子射束中广泛应用于临床实践的圆柱形电离室的 EPOM 移位，从而确定适当的 EPOM 移位。微金刚石探测器是一种具有小灵敏体积的半导体探测器，被用作参考探测器，评估了 11 种圆柱形电离室在 6MV 和 10MV 下的 EPOM 移位。使用最小二乘法计算从参考探测器的百分深度剂量（PDD）到评估室的深度移位，并将其定义为 EPOM 移位。10MV 条件下的 EPOM 移位略大于 6MV 条件下的 EPOM 移位。然而，由于并非所有腔室都观察到这种趋势，因此对两种能量的结果进行了平均，确定了 10 种电离室的 EPOM 移位为 0.33r-0.43r（±0.05），A1SL 腔室为 0.03r（±0.03）。所有电离室的移位均小于 0.6r，表明推荐的 EPOM 移位过大，在校准深度处低估了吸收剂量。因此，10 种电离室的适当 EPOM 移位为 0.4r（A1SL 腔室的几何中心），剂量不确定度为 0.05%。
