Initial experiments with gel-water: towards MRI-linac dosimetry and imaging.

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Australasian physical & engineering sciences in medicine, ISSN: 1879-5447, Vol: 39, Issue: 4, Page: 921-932

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Alnaghy, Sarah; Gargett, Maegan; Liney, Gary P; Petasecca, Marco; Begg, Jarrad; Espinoza, Anthony A; Newall, Matthew; Duncan, Mitchell; Holloway, Lois C; Lerch, Michael L. F; Lazea, Mircea; Rosenfeld, Anatoly B; Metcalfe, Peter E Show More Hide
Springer Nature
Biochemistry, Genetics and Molecular Biology; Engineering; Medicine; Medicine and Health Sciences
article description
Tracking the position of a moving radiation detector in time and space during data acquisition can replicate 4D image-guided radiotherapy (4DIGRT). Magnetic resonance imaging (MRI)-linacs need MRI-visible detectors to achieve this, however, imaging solid phantoms is an issue. Hence, gel-water, a material that provides signal for MRI-visibility, and which will in future work, replace solid water for an MRI-linac 4DIGRT quality assurance tool, is discussed. MR and CT images of gel-water were acquired for visualisation and electron density verification. Characterisation of gel-water at 0 T was compared to Gammex-RMI solid water, using MagicPlate-512 (M512) and RMI Attix chamber; this included percentage depth dose, tissue-phantom ratio (TPR), tissue-maximum ratio (TMR), profiles, output factors, and a gamma analysis to investigate field penumbral differences. MR images of a non-powered detector in gel-water demonstrated detector visualisation. The CT-determined gel-water electron density agreed with the calculated value of 1.01. Gel-water depth dose data demonstrated a maximum deviation of 0.7% from solid water for M512 and 2.4% for the Attix chamber, and by 2.1% for TPR and 1.0% for TMR. FWHM and output factor differences between materials were ≤0.3 and ≤1.4%. M512 data passed gamma analysis with 100% within 2%, 2 mm tolerance for multileaf collimator defined fields. Gel-water was shown to be tissue-equivalent for dosimetry and a feasible option to replace solid water.