Adoption of ICRU report 90 recommendations in the Canadian Co-60 air-kerma primary standard

Abstract

The Canadian air-kerma primary standard for Co-60 beams is updated according to ICRU report 90 recommendations. The effect of these recommendations and a more detailed chamber model on the primary standard is investigated. Dosimetric quantities and corrections required for the realization of air kerma are calculated using the EGSnrc Monte Carlo simulation system. The validity of Spencer-Attix cavity theory as function of Δ, the secondary particle production threshold energy, is investigated to assess the accuracy of its current selection criterion. Although individual changes in some correction factors are statistically non-negligible, the overall effect amounts to only a 0.04% increase of the standard. The mean restricted mass-electronic stopping-power ratio graphite to air decreases from the previous value of 1.0010 to a value of 0.9949. If this value is used with the recommended Wair value of 33.97 eV, the effect on the standard is a reduction of 0.613%. When the recommended WairSca value of 33.72 eV is used instead, the change results in a 0.835% reduction in Kair. Using SA cavity theory to obtain Kair with a Δ value of 20 keV, corresponding to the cavity's mean chord length, reproduces a direct Monte Carlo calculation at the 0.03% level. Type B uncertainties due to uncertainty in the photon and electron cross sections for the largest corrections Kwall and Kcomp are estimated to be 0.01% and 0.10%, respectively. No statistically significant effect on the standard is observed due to changes in the correction factors. Fano tests and comparison to single scattering calculations demonstrate EGSnrc's electron transport algorithm accuracy to be within 0.03% relative to its own cross sections. The latter also validates the criterion for selecting Δ based on the average chord length of the chamber's cavity. A 0.2% difference in the restricted stopping power ratio obtained in this work and the value resulting from ICRU-90 recommendations has the potential for affecting standards relying on the absolute value of Wair, thus further investigation is granted. The current update results in a 0.8% decrease of the Canadian Co-60 air-kerma primary standard with a reduced uncertainty of 0.2%. This change is in excellent agreement with the reported change to the BIPM Co-60 air-kerma standard

The role of standards laboratories in reducing uncertainty in clinical dosimetry: A Canadian perspective

“…For most standards, accuracy is limited by the measurement uncertainty and therefore reducing uncertainty is often the primary goal for established standards. An example of this at the NRC is the recent review carried out by Mainegra-Hing of the Co-60 air kerma standard [2]. In that work, initiated by the requirement to implement the recommendations of ICRU Report 90 [3], an analysis was carried out to determine each component that is required to determine air kerma using a cavity ionization chamber, resulting in an improved realization (even though no change was made to the physical standard).…”