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 C 60 o 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 (Kfl) by the presence of the cavity. The need for Kfl corrections arises from the fact that the standard prescription for choosing the low-energy threshold Δ in the SA restricted stopping-power ratio consistently underestimates the values of Δ needed if no perturbation to the fluence is assumed. The use of fluence corrections can be avoided by appropriately choosing Δ, but it is not clear how Δ can be calculated from first principles. Values of Δ required to avoid Kfl corrections were found to be consistently higher than Δ 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 Kfl 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%.

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Medical Physics
Department of Physics

La Russa, D.J., & Rogers, D.W.O. (2009). Accuracy of spencer-attix cavity theory and calculations of fluence correction factors for the air kerma formalism. Medical Physics, 36(9), 4173–4183. doi:10.1118/1.3174862