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Setting up of a spectrometry unit for brachytherapy

04.01.2010

Working Group 6.22 operates primary standard measuring devices for the calibration of radiation sources which are used in brachytherapy, in particular for the calibration of I-125 photon sources which are used for prostate therapy. In the course of the treatment, the radioactive photon sources were implanted into the body. To do this, the source is enclosed in a capsule which has a defined inner structure. The entire assembly is called a "seed".

The manufacturers offer different types of seeds and, according to their structure, a variation of the emitted radiation field can be observed. In the main, it is a variation of the spectral composition in the case when materials are used due to which additional fluorescence lines are emitted. In addition, the intensity and also the shape of the spectra vary with varying polar or azimuthal angle. This leads to a considerable change in the calibration factor and must be taken into account.

To determine the spectra emitted by the seeds, a spectrometry unit has been built for the requirements in brachytherapy. It is composed of two mobile components: one component for the alignment of the source, and one for the detection of the spectra. This modular set-up allows the strength of the photon field to be adapted at the position of the detector according to the ability of the detector electronics via a variation of the distance between the two components.

The source holder is mounted to a computer-interfaced rotary stage, providing angular control while maintaining the proper orientation of the detector entrance window to the source, which is controlled via a laser pointer. This allows both integral spectra over all rotary angles and the respective spectra for a given angle to be determined.

Figure: Spectrometry unit built for the requirements in brachytherapy. It is composed of two mobile components: one component for the alignment of the source, and one for the detection of the spectra. In the upper left corner a magnification of the source holder, mounted to a computer-interfaced rotary stage, is given.