Fluence non-uniformity effects in air kerma determination around brachytherapy sources.

In ionization chamber dosimetry close to brachytherapy sources, the non-uniform photon fluence in the vicinity of the air cavity of the ionization chamber must be corrected for. In this paper, experimentally determined relative non-uniformity correction factors are determined for a PTW23331 chamber and a number of Farmer-type chambers. It is concluded that the 'anisotropic' theory of Bielajew agrees better with the experimental results than the 'isotropic' theory of Kondo and Randolph. The experiments show that neither the material choice nor the radius of the central electrode have any significant effect on the non-uniformity correction factors. Experiments on the wall-material dependence in the non-uniformity correction factor, which is predicted by the anisotropic theory, were inconclusive. Parameters for the Farmer-type chambers for calculation of theoretical non-uniformity correction factors are given. Data to derive anisotropic fluence non-uniformity correction factors are given in tabular form. An error in the Kondo and Randolph equations is reported and the corrected equations presented.