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 caesium sources, which uses a Farmer-type ionization chamber, has been further developed 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 results of this calibration method have been compared with the calibration figures 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 achieved in all cases, establishing that the method is satisfactory for acceptance 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 this method and in the method used by the manufacturer is discussed.