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Monte Carlo optimization and experimental validation of a prototype ionization chamber for accurate magnetic resonance image guided radiation therapy (MRgRT) daily output constancy measurements in solid phantoms

Par Conseil national de recherches du Canada

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DOITrouver le DOI : https://doi.org/10.1002/mp.15695
AuteurRechercher : 1; Rechercher : 2; Rechercher : 3; Rechercher : 1
Affiliation
  1. Conseil national de recherches du Canada. Centre de recherche en métrologie
  2. Odette Cancer Centre Sunnybrook Health Sciences Centre Toronto ON Canada
  3. Department of Radiation Oncology University of Toronto Toronto ON Canada
FormatTexte, Article
Sujetionization chamber; magnetic field; Monte Carlo; MRgRT; quality assurance; solid phantom
Résumé
Purpose: To optimize the design, develop and test a prototype ionization chamber for accurate daily output constancy measurements in solid phantoms in clinical magnetic resonance-guided radiation therapy (MRgRT) radiotherapy beams. Up to 4% variations in response using commercial ionization chambers have been previously reported; the prototype ionization chamber developed here aims to minimize these variations. Methods: Monte Carlo simulations with the EGSnrc code system are used to optimize an ionization chamber design by increasing the thickness of a brass (high-density, nonferromagnetic, easy-to-machine) wall until results consistent with no air gap are produced for simulations with a 1.5 T and 0.35 T magnetic field, with a 0.2 mm air gap and varying the placement of the chamber model within the air gap. Based on the results of these simulations, prototype ionization chambers are manufactured and tested in conventional linac beams and in a 7 MV Elekta Unity MR-linac. The chambers are rotated about their axes, both parallel and perpendicular to the 1.5 T magnetic field, through 360º in a plastic phantom with measurements made at each cardinal angle. This reveals any variation in chamber response by varying the thickness of the air gap between the chamber and the phantom. Results: Monte Carlo simulations demonstrate that the optimal thickness of the chamber wall to mitigate the effect of an asymmetric air gap between the chamber and the plastic phantom is 1.1 mm of brass. With this thickness, the differences between simulations with and without an air gap and with asymmetric placement of the chamber within the air gap are less than 0.2%. A prototype chamber constructed with a 1.1 mm brass wall thickness exhibits less than 0.3% variation in response when rotated about its axis in the plastic phantom in a beam from an MR-linac, independent of whether its axis is parallel or perpendicular to the magnetic field. Conclusion: The optimized ionization chamber design and validated prototype for accurate MR-linac daily output constancy measurements allows utilization of conventional phantoms and procedures in MRgRT systems. This can minimize disruption to clinical workflow for MR-linac quality assurance measurements.
Date de publication
Maison d’éditionAmerican Association of Physicists in Medicine
Wiley
Dans
  • Medical Physics 49, nº 8 (août 2022) : 5483–5490.
Langueanglais
Publications évaluées par des pairsOui
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Identificateur de l’enregistrement23a7c0ba-e5eb-42d9-95a0-a56380066057
Enregistrement créé2022-07-18
Enregistrement modifié2022-10-19
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