2009
Systematic uncertainties in the Monte Carlo calculation of ion chamber replacement correction factors
Publication
Publication
Medical Physics , Volume 36  Issue 5 p. 1785 1789
In a previous study [Med. Phys. 35, 17471755 (2008)], the authors proposed two direct methods of calculating the replacement correction factors (Prepl or pcav pdis) for ion chambers by Monte Carlo calculation. By "direct" we meant the stoppingpower ratio evaluation is not necessary. The two methods were named as the highdensity air (HDA) and lowdensity water (LDW) methods. Although the accuracy of these methods was briefly discussed, it turns out that the assumption made regarding the dose in an HDA slab as a function of slab thickness is not correct. This issue is reinvestigated in the current study, and the accuracy of the LDW method applied to ion chambers in a C 60 o photon beam is also studied. It is found that the two direct methods are in fact not completely independent of the stoppingpower ratio of the two materials involved. There is an implicit dependence of the calculated Prepl values upon the stoppingpower ratio evaluation through the choice of an appropriate energy cutoff Δ, which characterizes a cavity size in the SpencerAttix cavity theory. Since the Δ value is not accurately defined in the theory, this dependence on the stoppingpower ratio results in a systematic uncertainty on the calculated Prepl values. For phantom materials of similar effective atomic number to air, such as water and graphite, this systematic uncertainty is at most 0.2% for most commonly used chambers for either electron or photon beams. This uncertainty level is good enough for current ion chamber dosimetry, and the merits of the two direct methods of calculating Prepl values are maintained, i.e., there is no need to do a separate stoppingpower ratio calculation. For high Z materials, the inherent uncertainty would make it practically impossible to calculate reliable Prepl values using the two direct methods.
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doi.org/10.1118/1.3115982  
Medical Physics  
Organisation  Department of Physics 
Wang, L.L.W., La Russa, D.J., & Rogers, D.W.O. (2009). Systematic uncertainties in the Monte Carlo calculation of ion chamber replacement correction factors. Medical Physics, 36(5), 1785–1789. doi:10.1118/1.3115982
