Advances in calibration of high dose rate brachytherapy sources

High Dose Rate (HDR) brachytherapy 192Ir sources have been in existence for more than 15 years. The calibration of this isotope was first developed at the University of Wisconsin, using what has come to be termed the seven-distance technique. A schematic of the apparatus is shown, showing that repositioning of the distance is within ±0.1mm. Since its inception multiple manufacturers of this source have entered the marketplace. The design of these sources consists of a single pellet or two pellets encapsulated in different materials. There are different afterloaders used to deliver the source through a catheter to a patient. Each of the manufacturer sources for 192Ir has been measured with this apparatus. In addition, another high dose rate source, 169Yb has been measured with the apparatus. The calibration method for the energies involved for the ion chamber will be described, including the changes in the NIST air kerma standard for the cesium calibration point. Another high dose rate source with low energy (50keV) is a miniature X ray source. This source can be calibrated with a free air chamber. Comparison of sources from different 192Ir manufacturers, including the new design versus the older design, and the subsequent use of well chambers to determine the air kerma strength will be explained. The calibrations for all the manufacturers of this source agree within ±1% and well chambers can be calibrated to air kerma strength within the total uncertainty of 2% (k=2). New advances in sources used for HDR Brachytherapy utilize lower energies than the 192Ir nuclide. The intent of the lower energy is a better dose distribution and less transmission through the patient into the room. The 169Yb source was calibrated with the seven-distance technique using the apparatus shown. A miniature X ray source was calibrated using a free air chamber designed for energies at 50 keV or less. The air kerma strength of both of these sources has been made allowing subsequent well chamber calibrations to be performed. These sources can now be calibrated to air kerma strength and used for clinical trials after other dosimetric parameters, such as dose rate constant, are determined