Applications of the Italian protocol for the calibration of brachytherapy sources

The Associazione Italianà di Fisica Biomedica (AIFB; Italian Association of Biomedical Physics) has adopted the Italian protocol for the calibration of brachytherapy sources. The AIFB protocol allows measurements of the reference air kerma rate, Kr, within 1.7% (1 sigma). To measure Kr the AIFB protocol has identified a direct and an indirect procedure. The direct procedure is based on the use of spherical or cylindrical ionization chambers as local reference dosimeters positioned along the transverse bisector axis of the source. Once the source is specified by a Kr value, this can be used to calibrate a field instrument, such as a well-type ionization chamber, for further source calibrations by means of an indirect procedure. This paper reports the results obtained by the Physics Laboratory of the Università Cattolica del S Cuore (PL-UCSC), in terms of Kr calibration of five types of source, 169Yb, 192Ir and 137Cs. The role of the Kr determination for a brachytherapy source has been underlined when a new source such as the 169Yb seed model X1267 has been proposed for clinical use. The Kr values for 137Cs spherical sources differed by 5% from the vendor's mean value. The five types of source calibrated in terms of Kr were used to obtain the calibration factor. NKrsource, of an HDR-1000 well-type ionization chamber.