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

Abstract

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.

Medical physics challenges in clinical MR-guided radiotherapy

“…Due to growing interest in 3D dosimetric detectors, gel and plastic-based solutions have been tested in magnetic fields [127][128][129]. Polymeric gels have been shown to be comparable to Gafchromic films for checking the isocenter accuracy [128].…”

MRI-guided lung SBRT: Present and future developments

“…Awareness of the dependence of the detector readout on its orientation relative to the magnetic field, as well as the use of dedicated detectors and phantoms are necessary for dosimetry inside a magnetic field [113][114][115][116]. Furthermore, gel-based dosimeters and a shift towards software-based QA procedures are being investigated on MRI-guided treatment units [117,118]. An electronic portal imaging device integrated with the hybrid MRI treatment unit could also be used for treatment beam QA [119].…”

APESM statistics and summary of 2017–2018

“…In 2018 the award went to the authors of Predicting prostate tumour location from multiparametric MRI using Gaussian kernel support vector machines: a preliminary study [8]. In 2017 the award went to the authors of Initial experiments with gel-water: towards MRI-linac dosimetry and imaging [9]. The Springer citation award for 2018 (most cited article of those published in the years from 2013 to 2017) went to Mobile healthcare applications: system design review, critical issues and challenges [10].…”