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Comparison of Reference Dosimetry Protocols for Small Radiosurgery Fields

Z Iqbal*, T Chiu, D Parsons, A Godley, X Gu, S Stojadinovic, UT Southwestern Medical Center, Dallas, TX

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
The reference absorbed dose to water rate for Gamma Knife (GK) ICON was determined following recommendations of the American Association of Physicist in Medicine (AAPM) Task Group (TG) report 51 and the International Atomic Energy Agency (IAEA) formalism for small and nonstandard fields.

Methods:
Two 16 cm diameter Leksell spherical phantoms, an acrylonitrile-butadiene-styrene (ABS) and a certified therapy grade solid water Dosimetry Phantom (Elekta, Stockholm, Sweden) were used for dose rate measurements. The measurements were performed utilizing four ionization chambers: 0.125 cm3 Semiflex 31010 and 0.016 cm3 PinPoint 31016 (PTW, Freiburg, Germany) and Exradin 0.053 cm3 A1SL and 0.007 cm3 A16 (Standard Imaging, Middleton, WI). For each detector the absorbed dose to water calibration coefficients traceable to a primary standard were obtained through an Accredited Dosimetry Calibration Laboratory (ADCL) service. Detector specific factors accounting for the change in the response of the ionization chambers for the reference and GK-specific fields were applied for the IAEA formalism. In addition, the GK output factor (OF) measurements in Leksell solid water phantom were validated using EBT3 radiochromic films (Ashland ISP, Wayne, NJ), and a PTW microDiamond detector.

Results:
For all four ionization chambers, on average, the IAEA and TG51 measured variations of GK ICON dose rates were (-0.27±0.62)% relative to the treatment planning system value. The output factors measured with EBT3 film and diamond were both within 1% and 1.5% agreement relative to the nominal GK values of 0.900 and 0.814 for the 8 mm and 4 mm collimators, respectively.

Conclusion:
The IAEA formalism, with detector specific correction factors calculated for GK phantoms, yielded favorable agreement with the calibration results based on the TG51 protocol.

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Keywords

Small Fields, Dosimetry Protocols, Gamma Knife

Taxonomy

TH- External Beam- Photons: gammaknife

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