Development and implantation of calibration of well type ionization chamber cameras with ¹⁹²Ir source of high dose rate in Brazil

The use of brachytherapy with after-loading equipment with remote source of high dose rate ¹⁹²Ir has increased worldwide and in Brazil, which presents about 60 facilities with this equipment nowadays. The need to ensure the traceability of the source dosimetric quantity to the International Ionizing Radiation Metrology Network has become evident and urgent, since high doses values are delivered in a short time to the tumor and any error in the determination of the dose can provoke much damage to patients. In the absence of an established dosimetric standard for the energy spectrum of ¹⁹²Ir, many studies showed alternative calibration of sources of high dose rate ¹⁹²Ir, with the ionization thimble or well chamber type. The well type chamber is now recommended by international organisms, because of its simplicity to use and to its reproducibility. Nowadays, it is adopted by alI radiotherapy services in Brazil using afterloading equipment with high dose rate ¹⁹²Ir source. As the calibration of this instrument is still limited to some countries in the northern hemisphere, in order to enhance the growth of the use of this instrument in Brazil, this paper proposes a methodology for calibration of well chambers of users of high dose rate ¹⁹²Ir sources, traceable to the international Ionizing Radiation Metrology Network. In the proposed method a thimble chamber is used, calibrated in LNMRI/IRD/CNEN (traceable to the international Ionizing Radiation Metrology Network) in X-ray and ⁶⁰Co beams with an interpolation for the ¹⁹²Ir energy. Using a positioning system to hold the chamber and a high dose rate ¹⁹²Ir source, the quantity reference air kerma rate is determined. This radioactive source is then used to calibrate the user' s chambers. The proposed methodology was tested in various radiotherapy services of the country and was also compared to the existing methodology in University of Wisconsin Accredited Dosimetry Calibration Laboratory (UW ADCL) in the United States of America, which currently calibrate most of the worldwide well chambers. The deviation of 1.0% found between the methodology of this study and the UW ADCL, show that we can introduce the methodology in a Brazilian metrology laboratory. For the implementation of calibration routine of the Laboratorio de Metrologia of the LCRL/UERJ, the technical procedures associated with the methodology were written and the laboratory is now able to provide the calibration of user's well type chambers around the country. (author)