AUTHOR=Begg Jarrad , Jelen Urszula , Keall Paul , Liney Gary , Holloway Lois 

TITLE=Ion chamber magnetic field correction factors measured via microDiamond cross-calibration from a conventional linac to MRI-linac

JOURNAL=Frontiers in Physics

VOLUME=10

YEAR=2022

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2022.925890

DOI=10.3389/fphy.2022.925890

ISSN=2296-424X

ABSTRACT=<p>Magnetic field correction factors are required for performing reference dosimetry on Magnetic Resonance Imaging Linear accelerators (MRI-linacs). Methods for measuring magnetic field correction factors usually require specialized equipment and expertise. Our work investigated the use of a microDiamond detector to cross-calibrate an ion chamber between a conventional linac and MRI-linac as a method to measure ion chamber magnetic field correction factors for the MRI-linac. Ratios of the microDiamond and ion chamber were measured on a conventional linac, parallel MRI-linac at 0 T, parallel MRI-linac at 1 T and perpendicular MRI-linac at 1.5 T. The beam quality dependence of the microDiamond was investigated by comparing the measurements on the conventional linac and parallel MRI-linac at 0 T. The magnetic field dependence of the microDiamond was investigated comparing the measurements on a parallel MRI-linac at 0 and 1 T. The ion chamber magnetic field correction factors were calculated by comparing the conventional linac and parallel MRI-linac at 1 T and the conventional linac and perpendicular MRI-linac at 1.5 T for the parallel and perpendicular factors respectively. The FC65-G and PTW30013 ion chambers were investigated. For a parallel MRI-linac, with a beam quality of <inline-formula id="inf1"><mml:math id="m1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mrow><mml:mi>T</mml:mi><mml:mi>P</mml:mi><mml:mi>R</mml:mi></mml:mrow><mml:mn mathvariant="italic">20,10</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> = 0.632, we measured magnetic field correction factors of 0.988 ± 0.016 (<italic>k</italic> = 2) and 0.987 ± 0.016 (<italic>k</italic> = 2) for a FC65-G and PTW30013 respectively, where <italic>k</italic> is the coverage factor. For a perpendicular MRI-linac, with a beam quality of <inline-formula id="inf2"><mml:math id="m2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mrow><mml:mi>T</mml:mi><mml:mi>P</mml:mi><mml:mi>R</mml:mi></mml:mrow><mml:mn mathvariant="italic">20,10</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> = 0.701, we measured magnetic field correction factors of 0.995 ± 0.020 (<italic>k</italic> = 2) and 0.983 ± 0.020 (<italic>k</italic> = 2) for a FC65-G and PTW30013 respectively. The results showed agreement with previously published work which used different approaches. Our work demonstrates the use of a microDiamond to calculate the ion chamber magnetic field correction factor using measurements on a conventional linac and MRI-linac. The high level of uncertainty in our results means the method at present can only be used for validation of magnetic field correction factors.</p>