CORRECTION FACTORS FOR FARMER-TYPE CHAMBERS FOR ABSORBED DOSE DETERMINATION IN CO-60 AND IR-192 BRACHYTHERAPY DOSIMETRY

This paper presents experimentally determined correction factors for F armer-type chambers for absorbed dose determination in Co-60 and Ir-19 2 brachytherapy dosimetry. The correction factors were determined from measurements made in a PMMA phantom and calculation of ratios of meas ured charges. The ratios were corrected for the different volumes of t he ionization chambers, determined in external high-energy electron be ams. The correction factors for the central electrode effect and the w all material dependency in Co-60 brachytherapy dosimetry agree with th ose used in external Co-60 beam dosimetry. In Ir-192 dosimetry, the ce ntral aluminium electrode increases the response of an NE2571 chamber compared with that of a chamber with a central graphite electrode. The increase is 1.1 and 2.1% at 1.5 and 5.0 g cm(-2) distance, respective ly. Similar values are obtained with an NE2577 chamber. The wall corre ction factor in Ir-192 dosimetry for a chamber with an A-150 wall has been determined to be 1.018, independent of the measurement distance. For a graphite walled chamber, the correction factor is 0.996 and 1.00 1 at 1.5 and 5.0 g cm(-2) distance, respectively. The values of the wa ll correction factors are evaluated by a theory presented. If the cham ber is used according to the 'large cavity' principle, the correction factor to account for the replacement of the phantom material by the i onization chamber was determined to be 0.982 for an NE2571 chamber whe n used with a Delrin cap, and 0.978 for an NE2581 when used with a pol ystyrene cap. The correction factors for the 'large cavity' principle are valid at both Co-60 and Ir-192 qualities.