FLUENCE NONUNIFORMITY EFFECTS IN AIR KERMA DETERMINATION AROUND BRACHYTHERAPY SOURCES

In ionization chamber dosimetry close to brachytherapy sources, the no n-uniform photon fluence in the vicinity of the air cavity of the ioni zation chamber must be corrected for. In this paper, experimentally de termined relative non-uniformity correction factors are determined for a PTW23331 chamber and a number of Farmer-type chambers. It is conclu ded that the 'anisotropic' theory of Bielajew agrees better with the e xperimental 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-unifor mity correction factors. Experiments on the wall-material dependence i n the non-uniformity correction factor, which is predicted by the anis otropic theory, were inconclusive. Parameters for the Farmer-type cham bers for calculation of theoretical non-uniformity correction factors are given. Data to derive anisotropic fluence non-uniformity correctio n factors are given in tabular form. An error in the Kondo and Randolp h equations is reported and the corrected equations presented.