The wall correction factor for a spherical ionization chamber used in brachytherapy source calibration.

The effect of wall chamber attenuation and scattering is one of the most important corrections that must be determined when the linear interpolation method between two calibration factors of an ionization chamber is used. For spherical ionization chambers the corresponding correction factors A(w) have to be determined by a non-linear trend of the response as a function of the wall thickness. The Monte Carlo and experimental data here reported show that the A(w) factors obtained for an Exradin A4 chamber, used in the brachytherapy source calibration, in terms of reference air kerma rate, are up to 1.2% greater than the values obtained by the linear extrapolation method for the studied beam qualities. Using the Aw factors derived from Monte Carlo calculations, the accuracy of the calibration factor N(K,Ir) for the Exradin A4, obtained by the interpolation between two calibration factors, improves about 0.6%. The discrepancy between the new calculated factor and that obtained using the complete calibration curve of the ion-chamber and the 192Ir spectrum is only 0.1%.