Session Item

The quantity absorbed dose to water for the Intrabeam®-system from Carl Zeiss Meditec AG in the bare needle configuration is realized for the first time worldwide by a European metrology institute.
The Zeiss dosimetry system is based on determining and passing on the dose or depth dose distribution for the bare needle configuration. In addition, the company determines an applicator transfer function (ATF) for each applicator, which is the ratio of the depth dose curve of the applicator to the depth dose curve of the bare needle configuration. This already existing system offers the possibility to provide traceability for many applicator configurations, just by determining and transferring the dose for the bare needle configuration.
Therefore, PTB has decided to realise the unit of absorbed dose to water for the INTRABEAM®-system in the bare needle configuration, even though this is more demanding because the radiation field is not hardened by an applicator and the spatial distribution of the radiation field is not homogenised by multiple scattering in the applicator material.

For the development of the primary standard device the source was characterised: X-ray emission spectra were measured at various angles, relative 3D dose distributions were determined with radiochromic gels. Monte Carlo simulations were performed to mimic the characteristics of the sources and to calculate the conversion and correction factors for the primary standard.
In forward direction the intensity of the emitting field is maximal and the variation sufficient small for reference dosimetry. Therefore, the reference point was defined in 1cm in forward direction which is in accordance with the vendors dosimetry system.
The principle of the primary standard (iPFAC, in phantom free-air chamber) is based on a free-air chamber located in a phantom of water-equivalent material. It has parallel-plate geometry, and the gap between the two plates embodying the measuring volume can be varied continuously up to a distance of 20 cm. The proximal front plate is fixed and its thickness defines the depth of measurement.

In Fig. 1 measurements with the iPFAC for the Intrabeam source at 50 kV tube voltages and with 40 µA beam current are shown. Given are the values of the determined water-kerma rate at plate separation zero for specific plate separations xi+1 (xi=40mm) within the phantom of the primary standard.

From the mean value: KwPh= 71,30 ± 0,01 µGy/s the absorbed dose rate at a distance of 1 cm within a water phantom (10x10x10 cm³) was determined to be: Dw=65,4 mGy/s.
The determined dose is about 60% higher than the dose defined according to the TARGIT protocol and about 20% higher than the dose according to the Zeiss calibration V4.0.

The absorbed dose to water for electronic brachytherapy sources was realized in the needle configuration. This will be the basis for a calibration chain directly traceable to a metrology institute for many applicators of the Zeiss system.