Quality assurance in brachytherapy: the displacement effect in the vicinity of 60Co and 192Ir brachytherapy sources.

Cylindrical ionization chambers with varying radii have been used to determine the displacement effect at distances between 10 mm and 40 mm from 60Co and 192Ir brachytherapy sources. Agreement with published values of the displacement factor for external 60Co beams is obtained. The shift between the centre of the ionization chamber and the effective point of measurement is found to be only a few per cent of the internal radius of the cylindrical ionization chamber. The shift is found to be dependent on the source to ionization chamber centre distance.