Zink K, Wulff J
Institut für Medizinische Physik und Strahlenschutz-IMPS, University of Applied Sciences Giessen-Friedberg, Wiesenstr. 14, D-35390 Giessen, Germany.
Phys Med Biol. 2009 Apr 21;54(8):2421-35. doi: 10.1088/0031-9155/54/8/011. Epub 2009 Apr 1.
Current dosimetry protocols recommend the use of plane-parallel ionization chambers for the dosimetry of clinical electron beams. The necessary perturbation corrections p(wall) and p(cav) are assumed to be unity, independent of the depth of measurement and the energy of the primary electrons. To verify these assumptions detailed Monte Carlo studies of a Roos chamber in clinical electron beams with energies in the range of 6-21 MeV are performed at different depths in water and analyzed in terms of Spencer-Attix cavity theory. Separate simulations for the perturbation corrections p(wall) and p(cav) indicate quite different properties of both correction factors with depth. Dose as well as fluence calculations show a nearly depth-independent wall correction factor for a shift of the Roos chamber Deltaz = -0.017 cm toward the focus. This value is in good agreement with the positioning recommendation given in all dosimetry protocols. Regarding the fluence perturbation p(cav) the simulation of the electron fluence inside the air cavity in comparison to water unambiguously reveals an in-scattering of low energy electrons, despite the fact, that the cavity is 'well guarded'. For depths beyond the reference depth z(ref) this effect is superimposed by an increased loss of primary electrons from the beam resulting in p(cav) > 1. This effect is largest for low electron energies but present for all electron energies involved in this study. Based on the different depth dependences of p(wall) and p(cav) it is possible to choose a chamber shift Deltaz in a way to minimize the depth dependence of the overall perturbation factor p. For the Roos chamber this shift is Deltaz = -0.04 cm independent of electron energy.
当前的剂量测定协议建议使用平行板电离室进行临床电子束的剂量测定。假定必要的扰动校正p(wall)和p(cav)为1，与测量深度和初级电子能量无关。为验证这些假设，对能量范围在6 - 21 MeV的临床电子束中的Roos电离室在水中不同深度进行了详细的蒙特卡罗研究，并根据Spencer - Attix空腔理论进行分析。对扰动校正p(wall)和p(cav)的单独模拟表明，两个校正因子随深度的特性有很大差异。剂量以及注量计算表明，对于Roos电离室向焦点偏移Deltaz = -0.017 cm的情况，壁校正因子几乎与深度无关。该值与所有剂量测定协议中给出的定位建议高度一致。关于注量扰动p(cav)，与水相比，对空气腔内电子注量的模拟明确显示出低能电子的向内散射，尽管该空腔“防护良好”。对于超过参考深度z(ref)的深度，这种效应会叠加束中初级电子损失增加的情况，导致p(cav) > 1。这种效应在低电子能量时最大，但在本研究涉及的所有电子能量下都存在。基于p(wall)和p(cav)不同的深度依赖性，可以选择电离室偏移Deltaz，以使总扰动因子p的深度依赖性最小化。对于Roos电离室，该偏移为Deltaz = -0.04 cm，与电子能量无关。
