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TH‐CD‐304‐08: Small Air‐Gaps Affect the Response of Ionization Chambers in the Presence of a 1.5 T Magnetic Field
Author(s) -
O'Brien D J,
Hackett S L,
van Asselen B,
Ibbott G,
Raaymakers B W,
Sawakuchi G O,
Wolthaus J W H
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 - 10.1118/1.4926214
Subject(s) - ionization chamber , linear particle accelerator , imaging phantom , monte carlo method , beam (structure) , optics , materials science , physics , field size , magnetic field , range (aeronautics) , detector , field (mathematics) , ionization , atomic physics , computational physics , ion , statistics , mathematics , quantum mechanics , pure mathematics , composite material
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 cm 2 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.