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Program Information

Modernization of the Large-Volume Extrapolation Chamber GROVEX at PTB

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S Ketelhut

S Ketelhut1*, M Meier1 , W Culberson2 , (1) Physikalisch-Technische Bundesanstalt, Braunschweig, Germany, (2) Univ of Wisc Madison, Madison, WI


TU-FG-205-9 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: 205

Purpose: The GROVEX at the PTB is the German national primary standard for reference air-kerma rate (RAKR) for low-dose rate brachytherapy seeds. After being in use for over 10 years, modernization and updates have become necessary. These included changes to the setup, re-measuring of key dimensions and re-determination of the correction factors. In order to verify the changes, a comparison with the University of Wisconsin VAFAC (Variable-Angle Free-Air Chamber) has been performed.

Methods: In the course of the modernization, a wooden box around the chamber was removed and the orientation of the collimator was corrected. Measurements of the distance from the source to the reference plane, of the effective area of the electrode, and of the aluminum holder for the seeds were performed. Furthermore, the equation for the divergence correction was updated, a source anisotropy correction was introduced, and the scatter and attenuation corrections were recalculated with EGSnrc. A comparison with the VAFAC has been performed for a Bebig I25.S16C 125I seed. 16 measurements were acquired (8 measurements for each seed orientation).

Results: Overall, the changes led to a reduction of the measured RAKR by 1.1%, which is significant in view of the stated uncertainty of 0.8%. The changes are comprised of a reduction of 0.7% for the setup, an increase by 0.5% because of the change of the re-measured dimensions, and a reduction of 0.9% due to the update of the correction factors. The comparison shows an agreement within the stated uncertainty, with a deviation of -0.2% relative to the measured air-kerma strength (divided by 1 m² to match the units) from the VAFAC for the investigated seed.

Conclusion: The measured RAKR in the comparison between the PTB primary standard and the University of Wisconsin VAFAC shows very good agreement and thus evidence that the modernization was improvements.

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