Calibration of photon and beta ray sources used in brachytherapy. Guidelines on standardized procedures at Secondary Standards Dosimetry Laboratories

It has generally been recognized that international harmonization in radiotherapy dosimetry is essential. Consequently, the IAEA has given much effort to this, for example by publishing a number of reports in the Technical Reports Series (TRS) for external beam dosimetry, most notably TRS-277 and more recently TRS-398. Both of these reports describe in detail the steps to be taken for absorbed dose determination in water and they are often referred to as 'dosimetry protocols'. Similar to TRS-277, it is expected that TRS-398 will be adopted or used as a model by a large number of countries as their national protocol. In 1996, the IAEA established a calibration service for low dose rate (LDR) 137 Cs brachytherapy sources, which is the most widely used source for treatment of gynecological cancer. To further enhance harmonization in brachytherapy dosimetry, the IAEA published in 1999 IAEA-TECDOC-1079 entitled 'Calibration of Brachytherapy Sources. Guidelines on Standardized Procedures for the Calibration of Brachytherapy Sources at Secondary Standard Dosimetry Laboratories (SSDLs) and Hospitals'. The report was well received and was distributed in a large number of copies to the members of the IAEA/WHO network of SSDLs and to medical physicists working with brachytherapy. The present report is an update of the aforementioned TECDOC. Whereas TECDOC-1079 described methods for calibrating brachytherapy sources with photon energies at or above those of 192Ir, the current report has a wider scope in that it deals with standardization of calibration of all the most commonly used brachytherapy sources, including both photon and beta emitting sources. The latter sources have been in use for a few decades already, but their calibration methods have been unclear. Methods are also described for calibrating sources used in the rapidly growing field of cardiovascular angioplasty. In this application, irradiation of the vessel wall is done in an attempt to prevent restenosis after cardiovascular interventions. The present report includes a description of suitable detector systems that can be used for the calibration. It must be emphasized that for safe use of brachytherapy a comprehensive quality assurance (QA) programme should be developed at the radiotherapy center using this modality. A QA programme cannot rest on a source calibration alone, but in addition it should address all the different steps included in the treatment process. Such a programme is described in IAEA- TECDOC-1040, 'Design and Implementation of a Radiotherapy Programme: Clinical, Medical Physics, Radiation Protection and Safety Aspects'. As summarized in the present report, omission of a QA programme may have serious consequences for a patient undergoing brachytherapy treatment. The parts of this publication describing the calibration of low energy photon sources and beta ray sources have been written in close collaboration with members of the International Commission on Radiation Units and Measurements (ICRU)