TH-CD-304-08: Small Air-Gaps Affect the Response of Ionization Chambers in the Presence of a 1.5 T Magnetic Field

Abstract

Purpose: Small air-gaps surrounding ionization chambers (ICs) are unavoidable if ICs are used in plastic phantoms. Here we show that air-gaps affect the response of ICs in a 1.5 T magnetic (B-)field from a MR-linac. Methods: We used NE2571 and PTW30013 Farmer chambers with and without a 1.5 T B-field for measurements and Monte Carlo simulations. For measurements, the chambers were exposed in water-equivalent plastic and water phantoms at the University Medical Center Utrecht in a 1.5 T MR-linac and an Elekta Precise linac. The NE2571 Farmer chamber was modeled with Geant4 at a 10 cm depth in a water phantom with a 1.5 T B-field with a beam modeled from a 10×10 cm2 Elekta SL25 6 MV energy spectrum. The detector model was positioned on the central-axis with a cylindrical air block surrounding it (symmetric) or beside it (asymmetric). The chamber cavity dose was calculated for a range of air-gap thicknesses. Chamber, beam and B-field were all orthogonal. Results: To experimentally change the distribution of air-gaps surrounding the chamber in the plastic phantom we rotated the chamber about its cylindrical axis. Variations of 1.8% and 1.3% were observed in the response of the NE2571 and PTW30013 ICs, respectively, in the presence of a B-field. No effect was observed when it was exposed in water instead of plastic or in plastic but without the B-field. The symmetric simulations show that the chamber cavity dose decreases as the air-gap thickness increases. The effect is >1% for thicknesses >1 mm. For a 0.25 mm asymmetric air-gap the dose-response varied by up to 2.2% depending on air-gap orientation. These effects disappear when no B-field is modeled. Conclusion: Small air-gaps surrounding a Farmer chamber may affect its response by over 1% when the chamber is exposed in the presence of a 1.5 T B-field. Funding provided by Elekta Limited.