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Flattening-Filter-Free Beam Quality Correction Factor Determination Using Experimental and Monte Carlo Methods

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M Wood

M Wood*, V Desai , E Simiele , S Taneja , L DeWerd , University of WI-Madison/ADCL, Madison, WI


TU-F-BRE-6 Tuesday 4:30PM - 6:00PM Room: Ballroom E

Purpose: To investigate beam quality correction factors for the flattening-filter-free (FFF) energies of the TrueBeamâ„¢ accelerator based on a dosimetry formalism for small and nonstandard fields.

Methods: Three detectors – an Exradin W1 scintillator, Sun Nuclear EDGE diode, and LiF(Mg,Tl) TLD-100 chips – were investigated to determine their applicability as tools to measure quality correction factors for ionization chambers in the small and nonstandard fields of the TrueBeam™. Volume-averaging effects and energy dependence were observed in fields ranging from 1x1 to 40x40 cm² for 6 MV and 10 MV beam energies using both FFF and flattened beam modes. Correction factors were determined for three ionization chambers: an Exradin A12 Farmer-type chamber, an Exradin A1SL scanning chamber, and an Exradin A26 reference-class microchamber. Beam quality corrections were also obtained using a benchmarked model of the TrueBeam™ created with the BEAMnrc user code of EGSnrc.

Results: All three detectors demonstrated measureable energy dependence in the megavoltage range. The EDGE diode was deemed the most appropriate tool for beam quality correction factor measurements due to its low energy dependence and small size; however, alanine will be used in the future to reduce energy dependent effects even further. Measured kQmsr,Q corrections of up to 4% were found for the 6MV FFF and 10 MV FFF beams, corresponding to a discrepancy of up to 3% compared to TG-51-determined dose. Up to a 10% kQclin,Qmsr correction was measured for small fields referenced to a 10x10 cm² field of the same energy. Much larger corrections were determined using the Monte Carlo model, and these discrepancies require further investigation.

Conclusion: Progress has been made toward determining beam quality correction factors for the small and nonstandard fields of the TrueBeamâ„¢ accelerator. Further work must be done to ensure greater accuracy in patient treatments with this new modality.

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