ABSORBED DOSE DETERMINATION AT SHORT-DISTANCE FROM CO-60 AND IR-192 BRACHYTHERAPY SOURCES

This paper presents two methods for absorbed dose determination with i onization chambers at short distance from Co-60 and Ir-192 brachythera py sources. The methods are modifications of the Bragg-Gray and large cavity principles given in the IAEA code of practice for high- and med ium-energy photon beams. A non-uniformity correction factor to account for the non-uniform electron fluence in the air cavity is introduced into the methods. The absorbed dose rates were determined from ionizat ion chamber measurements at distances between 1.5 and 5.0 cm from the brachytherapy sources. The agreement between the two methods is excell ent in Co-60 brachytherapy dosimetry. For Ir-192 dosimetry, the differ ence is less than 2.5% at all distances. In absorbed dose rate calcula tions with the Co-60 brachytherapy source, the ratios between calculat ed and experimentally determined absorbed dose rates are 0.987 and 0.9 94 depending on the method used for absorption and scatter correction. In Ir-192 dosimetry, the large cavity principle gives almost identica l values to those which can be obtained with the AAPM recommendations. Using the chambers according to the Bragg-Gray principle in Ir-192 do simetry, the agreement with AAPM calculated absorbed dose rates is wit hin 2.5% at all distances. The uncertainty, expressed as one standard deviation, in the experimentally determined absorbed dose is estimated to be between 3 and 4%.