Dosimetry audits in Brazil for 192Ir high dose rate brachytherapy systems

In Brazil, among 200 radiotherapy centres, 30 have high dose rate (HDR) 192Ir brachytherapy systems. In August 2001, the Brazilian National Nuclear Energy Commission (CNEN) started a biennial audit program to those centres having HDR systems. This program consists of visiting each centre in order to investigate the radiation protection aspects of the centres and also to measure the intensity of the brachytherapy source, in terms of air kerma strength, with a well type chamber specially designed for HDR 192 Ir sources. The audit dosimetry results are compared to measurements carried out by the local institution physicist and to the source intensity value provided by the manufacturer. Two methods have been used by the Brazilian physicists for HDR 192Ir brachytherapy source dosimetry, namely the employment of a farmer type chamber calibrated according to the interpolation methodology and the use of a well type chamber to provide direct intercomparison. The larger difference obtained was 18.9% and it can be explained in terms of the lack of knowledge of the institution physicist about the interpolation methodology using the farmer type chamber. Another difference of 5.82% was found as being the lack of an updated calibration factor for the clinic well type chamber. On the basis of these results, CNEN is able to establish a maximum deviation value for the dosimetry of HDR system. Additionally, with this program the radiotherapy services have an opportunity to have their HDR 192Ir sources calibrated and to test the validity of the calibration factors for their own well type chambers, using their calibrated sources. (author)