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Radiation therapy: Method for the 3D characterization of active volumes of new therapy dosimeters using PTB’s microbeam has been established


Advances in radiation therapy are leading to ever better ways to irradiate even small tumors precisely and to spare the surrounding tissue. For this highly exact task, radiation devices and radiation plans are metrologically verified by means of therapy dosimeters before use on patients. New, very small dosimeters which are appropriate for this application were at the focus of PTB’s research: Together with the University of Oldenburg and the PTWFreiburg GmbH company, PTB has developed and published a method to characterize the three-dimensional active volume of these solid-state dosimetry detectors.

The response of the therapy dosimeter (in this case: the microDiamond detector; see arrow) was characterized two-dimensionally by mounting it in air directly under the exit in the focus of the microbeam and positioning it with a computer-controlled X-Y table. The eective detector thickness was determined by varying the proton range in the detector chip using layers of aluminum foil as absorbers and by recording the detector signals.

Two-dimensional response map of the microDiamond detector: The color scale represents the detector current which has been normalized using the monitor signal. The white circle denotes the calculated surface of the active volume.

The method was established at PTB’s microbeam facility using 10 MeV protons. Three of PTW’s detectors were studied: the new microSilicon detector, the micro- Diamond detector, and the Diode E detector. e detailed measurements provided eective geometric data which serve as the basis for Monte Carlo simulations. Using this type of simulations, among other things, correction factors can be determined which must be taken into account for the dosimetry in small irradiation volumes.