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Detectors for a measuring system for 3D dose distributions around HDR brachytherapy sources have been completed

30.09.2019

The software that is used in HDR brachytherapy for treatment planning of radiation therapies in hospitals is currently being validated with Monte Carlo simulations. In the energy regions that are typical of HDR brachytherapy (and especially of electronic brachytherapy, i.e. 400 keV and 50 kV, respectively), the exact material composition of all the materials in the radiation path – for example in the path of the applicators – is very important. To avoid this possible source of error, it is necessary to have a system that measures the dose distributions with the required spatial resolution: in the near field of the radiator, a positioning error of only 50 µm already leads to a 1 % change in the dose rate. The measuring system shown here, which has now been completed at PTB, positions the detectors with an uncertainty of approximately 50 µm at any position within a 600 l water phantom and thus achieves the desired measurement accuracies.

Plastic scintillation detectors (PSDs) which were specially constructed at PTB together with a partner university in Aarhus, Denmark, with volumes of 0.05 mm3, 0.10 mm3, 0.39 mm3 and 0.79 mm3, are used. Such small‑volume PSDs are not commercially available. Furthermore, new techniques are applied which reduce the measurement uncertainties but have not yet been reported in international scientific publications. Here, a UV filter is placed between the scintillator and the optical waveguide to avoid undesired scintillation excitation due to Cherenkov radiation (partly in the UV range) which develops in the optical waveguide. Also, by using a constant light source, the relative signal loss that is caused by different bendings of the waveguides is determined for each detector position.

Using this method, the target of limiting the maximum positioning error to 50 µm can be achieved since the total measurement uncertainty (simple standard deviation) is 47 µm at maximum.

Image: A detector with a 0.05 mm3 volume (shown here without the lightproof protection cap) glows under ultraviolet light.

 

Contact:

Opens local program for sending emailR. Behrens, Department 6.3, Working Group 6.34