WE-AB-BRB-07: Alanine and Monte Carlo Determined Beam Quality Corrections for Nonstandard Fields of the Varian TrueBeam Accelerator

Abstract

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 cm2 60Co 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 (kQ, kQmsr,Q, and kQclin,Qmsr) 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 kQ values agreed within uncertainties, showing the chamber models to be reliable. A comparison of measured and calculated kQmsr,Q results with TG-51-based values showed that TG-51 adequately accounts for variations in beam quality between flattened and unflattened 10×10 cm2 fields. Small field corrections, kQclin,Qmsr, were determined to be up to 5%, referenced to a 10×10 cm2 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 kQ 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.