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Variability of Gamma Knife Output Calibration Based On Chamber Type, Phantom Material, and Dosimetry Protocol

A Boczkowski1*, S Graves2 , L Poplawski3 , D Hyer4 , (1) University of Iowa, Iowa City, Iowa, (2) University Of Iowa, Iowa City, IA, (3) West Virginia University, Morgantown, WV, (4) University of Iowa, Iowa City, IA


(Sunday, 7/14/2019) 2:00 PM - 3:00 PM

Room: 302

Purpose: There is currently no consensus on the most accurate method for Gamma Knife output calibration, and significant variability can result from various ‘acceptable’ methodologies. The purpose of this work was to compare dose calibration formalisms for the Leksell Gamma Knife (GK) Icon unit using commercially available ionization chambers and solid phantoms.

Methods: Absorbed dose to water calibrations based on the TG-21 formalism and a modified TG-51 formalism were performed using two ionization chambers (Exradin A26, PTW 31010) and two spherical phantoms (solid water, lucite). Modifications to the TG-51 formalism were made in accordance with literature recommendations (Alfonso et. al, 2008), whereby a correction factor was applied to account for differences in field size, irradiation and phantom geometry, and phantom material. The modified TG-51 formalism was used only for the solid water phantom because appropriate correction factors for lucite are not available. As a verification, thermoluminescent dosimeters (TLDs) were irradiated and independently processed by a third-party.

Results: The absorbed dose rate to water between all measurements showed a maximum difference of 3.8%. Measurements using the same chamber, but different formalisms and phantom materials, yielded results with a spread of less than 1%. The results for the PTW 31010 chamber were 3% – 4% lower than that of the Exradin A26 chamber which is most likely due to the volume averaging effect. The TLD-measured machine output was approximately 3% less than the average of measurements made with the A26 chamber using the TG-21 and modified TG-51 formalisms, however the TLD measurements carry an associated 3% uncertainty.

Conclusion: GK output measurements were performed by a variety of methods, and significant variation in the measured output, up to 3.8%, was observed. As there is currently no community consensus, it is critical for these differences be communicated to end-users of the Gamma Knife.


Gamma Knife, Calibration, Dosimetry Protocols


TH- External beam- photons: gammaknife

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