Accuracy of Spencer-Attix cavity theory and calculations of fluence correction factors for the air kerma formalism.

EGSnrc calculations of ion chamber response and Spencer-Attix (SA) restricted stopping-power ratios are used to test the assumptions of the SA cavity theory and to assess the accuracy of this theory as it applies to the air kerma formalism for 60Co beams. Consistent with previous reports, the EGSnrc calculations show that the SA cavity theory, as it is normally applied, requires a correction for the perturbation of the charged particle fluence (K(fl)) by the presence of the cavity. The need for K(fl) corrections arises from the fact that the standard prescription for choosing the low-energy threshold delta in the SA restricted stopping-power ratio consistently underestimates the values of delta needed if no perturbation to the fluence is assumed. The use of fluence corrections can be avoided by appropriately choosing delta, but it is not clear how delta can be calculated from first principles. Values of delta required to avoid K(fl) corrections were found to be consistently higher than delta values obtained using the conventional approach and are also observed to be dependent on the composition of the wall in addition to the cavity size. Values of K(fl) have been calculated for many of the graphite-walled ion chambers used by the national metrology institutes around the world and found to be within 0.04% of unity in all cases, with an uncertainty of about 0.02%.

Systematic uncertainties in the Monte Carlo calculation of ion chamber replacement correction factors.

In a previous study [Med. Phys. 35, 1747-1755 (2008)], the authors proposed two direct methods of calculating the replacement correction factors (P(repl) or P(cav)P(dis)) for ion chambers by Monte Carlo calculation. By "direct" we meant the stopping-power ratio evaluation is not necessary. The two methods were named as the high-density air (HDA) and low-density 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 60Co photon beam is also studied. It is found that the two direct methods are in fact not completely independent of the stopping-power ratio of the two materials involved. There is an implicit dependence of the calculated P(repl) values upon the stopping-power ratio evaluation through the choice of an appropriate energy cutoff delta, which characterizes a cavity size in the Spencer-Attix cavity theory. Since the delta value is not accurately defined in the theory, this dependence on the stopping-power ratio results in a systematic uncertainty on the calculated P(repl) 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 P(repl) values are maintained, i.e., there is no need to do a separate stopping-power ratio calculation. For high-Z materials, the inherent uncertainty would make it practically impossible to calculate reliable P(repl) values using the two direct methods.

Sci-Fri PM: Planning-09: An EGSnrc investigation of ion chamber response to Co-60 beams.

The EGSnrc Monte Carlo code was evaluated for its ability to calculate the relative response of a variety of ion chambers to Co-60 beams as a means of justifying the use of this code in future investigations of cavity theory. EGSnrc calculations were compared to measurements with four separate ion chambers, which were each configured with several wall materials (ranging from plastic to lead) and cavity sizes (or cavity air pressures). The experimental results included measurements by Nilsson et al. in 1992, and experiments by Whyte, Attix et al. and Cormack and Johns in the mid-to-late 1950's designed to evaluate Spencer-Attix cavity theory. Experiments by Whyte involved measurements of the response per unit mass as a function of cavity air pressure for a large cylindrical chamber, whereas the other experiments consisted of measurements of the response per unit mass (or ionization current) as a function of the distance between the front and back wall (cavity height) of a plane-parallel chamber. EGSnrc calculations, which could account for the change in response associated with changes in wall material in most cases, were generally within 1-3% of experimental values, even for experimental data that required calculations of unreported wall corrections determined using experimental techniques. The ability of EGSnrc to accurately model these experiments, which showed variations up to 300%, confirms its suitability for detailed Monte Carlo studies of cavity theory.