Calorimetric determination of the absorbed dose to water in the near field of 192Ir brachytherapy sources

The measurand in the dosimetry of brachytherapy sources is the absorbed dose rate to water  at a distance of 1 cm from the centre of activity of the radiation source, vertical to the source axis. So far, this measurand has been determined indirectly via the measurement of the reference air kerma rate at a distance of 1 m from the centre of activity of the radiator and by using a - mostly calculated - dose conversion coefficient. This method leads to standard measurement uncertainties for   of several percent. With the aid of a water calorimeter, the desired measurand can be determined directly and with a smaller standard measurement uncertainty. As a first step, a water calorimeter already existing at PTB (type: GammaMed 12i; initial activity: approx. 370 GBq) was modified for use with 192Ir brachytherapy sources so that the absorbed dose rate to water in the near field of the source could be determined at a minimum distance of 24.35 mm.

The water calorimeter used is operated at a water temperature of 4°C and is largely identical in construction to the calorimeter used at PTB as a primary standard measuring device for the realization of the unit of the absorbed dose to water at ⁶⁰Co radiation. With the aid of an afterloader, the brachytherapy source is transported through a Teflon catheter in the calorimeter's outer enclosure and further into the water phantom of the calorimeter. In the area of the calorimetric detector, the Teflon catheter leads to a stainless steel needle (inner diameter: 1.35) which is closed at its end and which can be fixed with the aid of additional Plexiglas rods in front of or behind the water-filled glass cylinder of the detector. Thus, distances of 24.35 mm to approx. 48 mm can be realized between the stainless steel needle and the thermistors of the detector, which are melted into the tips of glass pipettes (see figure).The geometric distances between source and thermistors, measured as the distance between the axis of symmetry of the steel needle and the centre of the single thermistors, were determined with a measurement uncertainty of less than 50 µm with the aid of an alignment telescope.

The water phantom of the calorimeter is centered inside the external housing (edge length: approx. 1 m) which has been temperature-regulated to 4°C. Between successive measurements with the calorimeter, the ¹⁹²Ir source remains inside this external housing in a lead block (edge length: approx. 10 cm) which has also been cooled to 4°C and which provides the necessary "pre-cooling" and sufficient radiation shielding.

For the calorimetric determination of the absorbed dose rate to water, the heat conductance effects occurring during the measurements must be corrected. The cause of the heat conductance effects is both the absorption of the radiation within the source and its enclosure which, e.g. for a 350 GBq ¹⁹²Ir source, leads to self-heating with approx. 21 mW, and the steep depth dose distribution in the area of the measuring point. To investigate these effects and to verify or correct them with the aid of finite element calculations, radiation measurements of nominally 60 s, 90 s and 120 s duration were carried out in three different measuring positions. The effective dwell time of the source in front of the detector and its reproducibility were measured prior to the experiments with a measurement uncertainty of less than 0.1 s with the aid of a non-radioactive dummy source and of light sensors positioned at the end of a prepared plastics needle. To minimize the influence on the final result of random changes in the source positions (outer diameter of the source: 1.1 mm) inside the stainless steel needle, approx. 100 measurements had to be carried out with the calorimeter for each irradiation duration and for each measuring position.

With the measuring method presented here it could be shown for the first time that the determination of the absorbed dose rate to water of ¹⁹²Ir brachytherapy sources at the clinically relevant distance is possible with the aid of a water calorimeter. The relative standard measurement uncertainty of the calorimetric method is less than 1 %.

Figure : Detector fixture with supplementary Plexiglas rods for fixing the stainless steel needles in front of or behind the glass cylinder of the detector (left: distance of 48 mm from the thermistors; right: distance of 24.35 mm). The ¹⁹²Ir source is transported with the aid of the afterloader to the tip of the needle.