(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Due to the near 1/r²-characteristic of the photon fluence and the volume averaging effect, accurate ion chamber measurements are only feasible beyond a minimum source-to-detector-distance of a few centimetres. The cross-calibration of smaller detectors can allow accurate measurements closer to the source. The referenced cross calibration setup positions the detector tip towards the source, thus aligning the source centre and the symmetry axis of the detector (angle of incidence = 0°). In measurements of spatial dose distributions, the angle of incidence can deviate from zero.
Methods: The directional dependence of the response of two ionization chambers, the PinPoint 3D 31016 and the PinPoint 3D 31022, the silicon diode 60017 and the MicroDiamond 60019 (all PTW, Freiburg, Germany) were measured and simulated for an angular range of ± 110°. The measurements were performed with the source centred in an MP3 water phantom. After locating the source the directional dependence was be measured by moving the detector on a circular path around the source without changing its orientation. Monte Carlo simulations were performed in a corresponding geometric setup. Three source-to-detector-distances were regarded.
Results: The directional dependence generally decreases with increasing distance to the source. At a source-to-detector-distance of 25 mm the diode shows up to 32% signal loss, the PinPoint3D 31022 up to 35 % in relation to the reference point 0°. The PinPoint3D 31016 and the MicroDiamond 60019 show less than 6% respectively 9% signal change.
Conclusion: The MicroDiamond 60019 and the PinPoint3D 31016 show the smallest angular dependence of the detector response. With appropriate correction factors these detectors could be suitable for measuring spatial dose distributions near 192Ir-brachytherapy sources.
Keywords
Taxonomy
TH- Brachytherapy: Calibration & Quality Assurance
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