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WE‐AB‐BRB‐07: Alanine and Monte Carlo Determined Beam Quality Corrections for Nonstandard Fields of the Varian TrueBeam Accelerator
Author(s) -
Hyun M,
DeWerd L
Publication year - 2015
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4925848
Subject(s) - truebeam , monte carlo method , physics , ionization chamber , beam (structure) , laser beam quality , optics , linear particle accelerator , materials science , atomic physics , computational physics , ionization , ion , laser , statistics , mathematics , quantum mechanics , laser beams
Purpose: To determine beam quality correction factors for flattened and unflattened beams of the Varian TrueBeam™ accelerator using alanine measurements and Monte Carlo calculations. Methods: Measurements were performed using L‐α‐alanine pellets encased in Virtual Water™ paddles. These were irradiated in liquid water using various field sizes of the 6MV and 10MV beam energies of the TrueBeam™, with both flattening‐filter‐free (FFF) and flattened beam modes. Measurements were also performed in a 10×10 cm 2 60 Co field. Pellets were read out by the National Physical Laboratory (NPL, Teddington, UK) using electron paramagnetic resonance (EPR) spectrometry, and results were corrected for energy dependence using graphite calorimetry. Beam quality correction factors ( k Q , k Q msr,Q, and k Q clin,Q msr ) were determined for three ionization chambers: an Exradin A12 Farmer‐type chamber, an Exradin A1SL scanning chamber, and an Exradin A26 microchamber. All chambers were modeled in the egs_chamber user code of EGSnrc. Sources were created using Varian‐supplied IAEA‐compliant phase spaces and the BEAMnrc user code, and were validated by comparing measured and simulated dose profiles. Results: Measured and calculated k Q values agreed within uncertainties, showing the chamber models to be reliable. A comparison of measured and calculated k Q msr,Q results with TG‐51‐based values showed that TG‐51 adequately accounts for variations in beam quality between flattened and unflattened 10×10 cm 2 fields. Small field corrections, k Q clin,Q msr , were determined to be up to 5%, referenced to a 10×10 cm 2 field of the same energy and mode. Conclusion: Beam quality corrections were determined for several beam energies of the TrueBeam™ accelerator using Monte Carlo calculations and were validated using measurements with alanine. Values of k Q determined using TG‐51 were found to be adequate for FFF reference fields. However, small field corrections are necessary for both flattened and unflattened beams to ensure greater treatment planning accuracy.