Calibration of pre-cut iridium-192 wires for low dose rate interstitial brachytherapy using a Farmer-type ionization chamber

A technique, originally developed for calibrating small low activity caesiu m sources, which uses a Farmer-type ionization chamber, has been further de veloped for use with iridium wires. Correction factors have been generated to account for the finite source and detector sizes, and attenuation in the source carriers. The air kerma calibration factor for heavily filtered 280 kV X-rays was used for reference back to the National Standard. The result s of this calibration method have been compared with the calibration figure s given by the manufacturers over a 5 year period for the emissions from 50 batches of wires of varying strengths. Agreement to within +/-3.2% was ach ieved in all cases, establishing that the method is satisfactory for accept ance testing purposes. The mean agreement was good to within 0.2%, but the possibility of a systematic error of between 1% and 3% existing both in thi s method and in the method used by the manufacturer is discussed.