Dosimetric verification of source strength for HDR afterloading units with Ir-192 and Co-60 photon sources: Comparison of three different international protocols

Before clinical use of a brachytherapy source, regulations or recommendations by medical physics societies require an independent measurement of its air kerma strength by a qualified medical physicist. Currently, in addition to Ir-192, also HDR-Co-60 sources are increasingly coming into operation. However, the existing dosimetry protocols do not provide any guidelines for Co-60 sources. The purpose of this work was therefore to compare air kerma rate measurements as recommended by different dosimetry protocols for Ir-192 HDR sources and to test their applicability to Co-60 sources. Dosimetric verification of HDR afterloading source specification was performed according to three protocols, DIN 6809-2 (1993) in combination with DGMP-Report 13 (2006), IAEA-TECDOC-1274 (2002) and AAPM Report 41 (1993) for the nuclides Ir-192 and Co-60. Measurements of the sources reference air kerma rate were performed with 3 different methods (with a cylindrical chamber both in a solid phantom and in free air, and with a well chamber) and evaluated using all three protocols for each type of source and method of measurement. The measurements with all protocols and methods show deviations from the certified specification smaller than about 1.2% for Ir-192 and 2.5% for Co-60-Sources. The measurements with the well chamber showed the lowest deviations from the certificate value. Air kerma rate measurements for Co-60 HDR sources using the existing protocols are possible with accuracy sufficient to verify source calibration as provided by the source certificate. However, extension of the protocols by correction factors for measurement with Co-60 sources would be helpful.