Palmans Hugo, Verhaegen Frank, Denis Jean-Marc, Vynckier Stefaan
Subatomic and Radiation Physics Department, Ghent University, Belgium.
Phys Med Biol. 2002 Aug 21;47(16):2895-905. doi: 10.1088/0031-9155/47/16/305.
Reference ionization chamber dosimetry in clinical proton beams is generally performed with cylindrical ionization chambers. However, when the measurement is performed in the presence of a large depth dose gradient or in a narrow spread out Bragg peak (SOBP), it could be advisable to use a plane-parallel chamber. Few recommendations and studies have been devoted to this subject. In this paper, experimental information on perturbation correction factors for four plane-parallel ionization chamber types in proton beams is presented. The experiments were performed in 75 MeV modulated and non-modulated proton beams. Monte Carlo calculations have been performed to support the conclusions of the experimental work. Overall, we were not able to find experimental evidence for significant differences between the secondary electron perturbation correction factors for plane-parallel chambers and those for a cylindrical NE2571. We found experimental ratios of perturbation correction factors that did not differ by more than 0.6% from unity for a Roos and two NACP02 chambers, and by not more than 1.2% for a Calcam-2 and two Markus chambers. Monte Carlo simulations result in corrections that are limited to 0.6% in absolute value, but given the overall uncertainties of the measurements, the deviations of the correction factors from unity could not be resolved from the experimental results. The results of the simulations thus support the experimental conclusion that perturbation correction factors for the set of plane-parallel chambers in both proton beams (relative to NE2571) do not deviate from unity by more than 1.2%. This confirms, within the experimental uncertainties, the assumption that the overall perturbation correction factor for a plane-parallel chamber in a low-energy proton beam is unity, made in IAEA TRS-398 and other dosimetry protocols. Given the large uncertainties of the gradient correction factors to be applied when using a cylindrical ionization chamber in a narrow SOBP or in the presence of a strong depth dose gradient, the level of agreement between plane-parallel and cylindrical ionization chambers observed in this study shows that plane-parallel chambers are a reliable alternative for reference dosimetry in low-energy proton beams.
临床质子束中的参考电离室剂量测定通常使用圆柱形电离室进行。然而，当在存在大深度剂量梯度或窄扩展布拉格峰（SOBP）的情况下进行测量时，使用平行板电离室可能是明智的。很少有建议和研究致力于这个主题。本文给出了四种平行板电离室类型在质子束中微扰校正因子的实验信息。实验在75 MeV调制和未调制质子束中进行。已进行蒙特卡罗计算以支持实验工作的结论。总体而言，我们未能找到实验证据表明平行板电离室和圆柱形NE2571电离室的二次电子微扰校正因子之间存在显著差异。我们发现，对于一个Roos电离室和两个NACP02电离室，微扰校正因子的实验比值与1的差异不超过0.6%，对于一个Calcam - 2电离室和两个Markus电离室，差异不超过1.2%。蒙特卡罗模拟得出的校正绝对值限制在0.6%以内，但考虑到测量的总体不确定性，校正因子与1的偏差无法从实验结果中分辨出来。因此，模拟结果支持了实验结论，即两种质子束中平行板电离室组的微扰校正因子（相对于NE2571）与1的偏差不超过1.2%。这在实验不确定性范围内证实了IAEA TRS - 398和其他剂量测定协议中关于低能质子束中平行板电离室的总体微扰校正因子为1的假设。鉴于在窄SOBP或存在强深度剂量梯度时使用圆柱形电离室时要应用的梯度校正因子存在很大不确定性，本研究中观察到的平行板电离室和圆柱形电离室之间的一致程度表明，平行板电离室是低能质子束参考剂量测定的可靠替代方案。
