CALIBRATION OF IR-192 HIGH-DOSE-RATE AFTERLOADING SYSTEMS

A method is described for calibration of Ir-192 high-dose-rate (HDR) brachytherapy afterloading systems. Since NIST does not offer calibration of ionization chambers with the gamma-ray spectrum of iridium-192, an interpolation procedure is employed, using calibrations above (Cs-137, 662 keV) and below (250 kVcp, 146-keV x rays) the exposure-weighted average Ir-192 energy of 397 keV. The same total wall + cap thickness must be used for both calibrations, and for the Ir-192 measurements. A wall + cap thickness of 0.3 g/cm2 is recommended to assure charged particle equilibrium and to exclude secondary electrons emitted from the source encapsulation. Procedures are described for determining the corrections for source-chamber distance and room scatter during the source calibration in inverse-square-law geometry. A new well-type ionization chamber has been designed specifically for convenient routine use with the HDR afterloading system. It can be calibrated by means of a previously calibrated Ir-192 source, and offers a simple means for verifying the decay rate and for calibrating Ir-192 replacement sources.