Development of a national protocol for a high activity (192)Ir brachytherapy source calibration

Determination of the air kinetic energy released in material (KERMA) rate (commonly named source strength) or a brachytherapy source activity represents one of the first steps in the acceptance, as well as in the periodic Quality Assurance (QA) testing procedures. In this paper, calibration of an (192)Ir high-dose rate (HDR) brachytherapy source is presented.

An (192)Ir HDR brachytherapy source calibration was carried out, with two independent procedures - calibration in water and in free air. The dosimeter used consisted of a thimble ionization chamber (NE 2571) and an electrometer (NE 2670). The reference air KERMA rate was calculated according to the International Atomic Energy Agency (IAEA) and the International Commission for Radiation Units and Measurements (ICRU) protocols. All physical quantities and correction factors used were determined, calculated, and re-checked for particular chamber and source type.

In water, the reference air KERMA rate, determined by the IAEA protocol differed by +1.0% from the manufacturer's declared value and was measured with a relative error of 1%. The reference air KERMA rate, calculated by the ICRU protocol differed by -0.4% from the manufacturer's declared value and was measured with a relative error of 1.1%. In air, the value of the reference air KERMA rate, calculated by the IAEA protocol, differed by +1.6% from the manufacturer's declared value and was measured with a relative error of 1%. The value of the reference air KERMA rate for the same source, calculated by the ICRU protocol, differed by +1.1% from the manufacturer's declared value and was measured with a relative error of 0.9%. The results showed that application of the IAEA protocol gave higher values than the application of the ICRU protocol. It stands for both calibration methods. The difference between calibration protocols was not higher than 2% (in the water 1.4% and in the air 0.5%). Finally, calibration results showed that the difference between the calibration methods (in water and in air), for both calibration protocols, was not higher than 2%. For the IAEA protocol the difference was -0.6% and for the ICRU protocol it was -1.5%. We believe that benefits can be derived from the simultaneous application of both methods.

A proposal of a national protocol represents an attempt to formulate a unique protocol for brachytherapy sources calibration in Serbia. The comparison of "in water" and "in free air" calibration has shown systematic differences of in-source strength of about 2%, respectively. It would be worth suggesting to radiotherapy centres to adopt the most optimal calibration method, based on well-known international protocols.