Methods for routine calibration of brachytherapy sources

Two types of routine geometry calibrators, the nuclear medicine dose calibrator and a Lucite jig holding a 30-ml external beam ion chamber with the source in a rigid geometry, were compared with open-air measurements for ¹³⁷Cs, ¹⁹²Ir, and ²²⁶Ra brachytherapy sources. The proximity of scattering surfaces in the second apparatus resulted in significant distortion of the buildup effect and deviation from the inverse square law (5% to 15%). For the dose calibrator, the response/Roentgen was found to be dependent not only on source energy, but on source capsule thickness as well. Approximately one half of the observed variation (27%) in calibration factors was accounted for by differences in filtration among sources. A mathematical logarithm that corrects for these geometric and filtration effects is presented. In addition, the activity assay procedure provided by the manufacturer of the dose calibrator is shown to be unsuitable for brachytherapy sources. Steps to overcome these problems are discussed.