Spectrometry for Dosimetric Characterization of Electronic Brachytherapy Systems
Abstract
Purpose
Within the Joint Research Project PRISM-eBT to establish primary standards and traceable measurement methods for electronic brachytherapy devices (eBT), three different devices were spectrometrically investigated: the “Intrabeam®”-system from Carl Zeiss Meditec with a maximum tube voltage of 50 kV and a maximum tube current of 40 µA, the “Axxent®” tube from Xoft (now Elekta) with a maximum tube voltage of 50 kV and a maximum tube current of 300 µA and the “iort-50” from BEBIG Medical with a maximum tube voltage of 70 kV and a maximum tube current of 7 mA.
Methods
An HPGe-Detecor with transistor reset preamplifier and pulse-pile-up rejector circuits for high count rate spectrometry was utilized. An algorithm was developed in MATLAB to eliminate remaining pulse-pile up artefacts. Due to the higher tube current and thus photon fluence the third system was investigated with a ‘X-123’ CdTe-spectrometer. For both detectors the response function was determined by Monte Carlo Calculations, with the EGSnrc user code dosrznrc. Spectra were unfolded using a custom-developed program that employed an iterative method based on Bayes' theorem.
Results
In Figure 1 three spectra of the three devices taken at 50 kV tube voltages are presented (Xoft: green, Intrabeam: blue and Womed: black). An additional spectrum for the Womed device taken at 70 kV tube voltage in given in red. The spectra measured with the HPGe detector show a good to very good line resolution despite the high-count rate and in addition no artifacts of the reconstruction algorithm are recognizable. The spectra of the Womed tube show that realistic spectra can be obtained with the 'X-123' even at tube currents of a few mA.
Conclusion
It turns out that the spectra of the different eBT systems deviate considerably from each other and that therefore spectrometry is essential for dosimetry of these devices.