H. Tolli et Ka. Johansson, ABSORBED DOSE DETERMINATION AT SHORT-DISTANCE FROM CO-60 AND IR-192 BRACHYTHERAPY SOURCES, Physics in medicine and biology (Print), 43(11), 1998, pp. 3183-3194
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%.