QUALITY ASSURANCE IN BRACHYTHERAPY - THE DISPLACEMENT EFFECT IN THE VICINITY OF CO-60 AND IR-192 BRACHYTHERAPY SOURCES

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