Brachytherapy is besides chemotherapy and external radiation therapy - an established procedure for the treatment of various tumours. Thereby, a radioactive radiation source is placed in the body, inside - or in the direct vicinity of - the tumourous tissue to be irradiated. A major part of the treatments is done with highly radioactive radiation sources having a high dose rate - so-called HDR sources - which usually contain the radioactive nuclide 192Ir. Presently, the radiation intensity of the source is characterised by indicating its air kerma rate at a distance of 1 m from the centre of the corresponding source. It is converted - by means of dose conversion factors which are published in specialised journals and are internationally accepted - into the unit of absorbed dose rate to water at a distance of 1 cm from the source, which is required for planning the dose to be applied to the patient. These conversion factors have mostly been computed by means of Monte Carlo simulations and are based on only very few experimental results. Therefore, they sometimes have undesirably high uncertainties of up to 5 % (k=1).
To remedy this, several national metrology institutes have, within the scope of a European research project , assigned themselves the task of developing primary standards for the afore-mentioned HDR brachytherapy sources by means of which such sources can be calibrated directly in the unit of absorbed dose rate to water. In this context, PTB has set up a primary standard based on a water calorimeter with which a clinical radiation source - a 370 GBq 192Ir source of the type "GammaMed 12i" - was characterised and calibrated in the unit of absorbed dose rate to water for the first time in 2009 . A second iridium source with an activity of 550 GBq was characterised in the spring of 2010. With the obtained data, it was possible to calibrate, for the first time, a transfer standard - a well-type ionisation chamber - in the units of absorbed dose rate to water of 192Ir source of the same type. This, too, took place within the scope of the above-mentioned European research project; the secondary standard to be calibrated was provided by the Austrian project partner BEV (Austrian Federal Office for Metrology and Surveying). For this calibration, PTB has indicated a measurement uncertainty of 3.5 % (k=2). This represents a considerable reduction compared with the - currently still commonly used - calibration in the unit of air kerma rate with subsequent conversion into absorbed dose to water. Within the scope of the ERA-NET Plus Project, BEV now uses its secondary standard, which is traceably linked up to PTB, for the dissemination of the unit of absorbed dose rate to water for 192Ir sources to hospitals.
After completion of the research project in summer 2011, PTB envisages to officially include the calibration of 192Ir brachytherapy sources and well-type ionisation chambers in the unit of absorbed dose rate to water in its calibration service offer and to, thus, make it available for all hospitals as clients. PTB, thus, strictly follows the recommendation of the German standardisation committee on radiology within DIN which advises using the absorbed dose rate to water, as a matter of principle, to calibrate dosimetric systems which are used in radiation therapy - and this in all areas of radiation therapy, and thus also in brachytherapy.