The photon spectra in vacuum around four types of 192Ir HDR brachytherapy sources are calculated using the Monte Carlo code EGS4 and the most recent spectral information on 192Ir decay. The air-kerma strengths per unit activity are calculated based on the photon fluence around a bare 192Ir source and around each of four types of encapsulated sources using recent mass energy-absorption coefficients. For the full spectrum the bare vs encapsulated difference is up to 23% due to the large air-kerma contribution from the unfiltered low-energy photons. For the penetrating part of the photon spectrum (> 11.3 keV), the air-kerma strength per unit source activity on the transverse axis for a bare source is 2-15% higher than for the encapsulated sources due to the attenuation and absorption in the core and the encapsulating material. The contribution to the air-kerma strength from photons scattered in the capsule and from bremsstrahlung are calculated to increase the air-kerma strength by 2-4% and 0.2-0.3%, respectively. Air-kerma strengths for a variety of sources agree well with previously reported results for sources from Nucletron International, Best Industries, Inc., and Alpha-Omega Services, Inc. In addition we present air-kerma strengths for the present model of the HDR source from Nucletron International and the source from Varian Associates, Inc.

To develop a primary standard for 192Ir sources, the basic science on which this standard is based, i.e., Spencer-Attix cavity theory, must be established. In the present study Monte Carlo techniques are used to investigate the accuracy of this cavity theory for photons in the energy range from 20 to 1300 keV, since it is usually not applied at energies below that of 137Cs. Ma and Nahum [Phys. Med. Biol. 36, 413-428 (1991)] found that in low-energy photon beams the contribution from electrons caused by photons interacting in the cavity is substantial. For the average energy of the 192Ir spectrum they found a departure from Bragg-Gray conditions of up to 3% caused by photon interactions in the cavity. When Monte Carlo is used to calculate the response of a graphite ion chamber to an encapsulated 192Ir source it is found that it differs by less than 0.3% from the value predicted by Spencer-Attix cavity theory. Based on these Monte Carlo calculations, for cavities in graphite it is concluded that the Spencer-Attix cavity theory with delta = 10 keV is applicable within 0.5% for photon energies at 300 keV or above despite the breakdown of the assumption that there is no interaction of photons within the cavity. This means that it is possible to use a graphite ion chamber and Spencer-Attix cavity theory to calibrate an 192Ir source. It is also found that the use of delta related to the mean chord length instead of delta = 10 keV improves the agreement with Spencer-Attix cavity theory at 60Co from 0.2% to within 0.1% of unity. This is at the level of accuracy of which the Monte Carlo code EGSnrc calculates ion chamber responses. In addition, it is shown that the effects of other materials, e.g., insulators and holders, have a substantial effect on the ion chamber response and should be included in the correction factors for a primary standard of air kerma.

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