Chapter 9: Gel-based Radiation Dosimetry Using Quantitative MRI

By dissolving radiation sensitive chemicals into a gel matrix, a three-dimensional (3D) radiation dosimeter can be manufactured. These dosimeters can be used as anthropomorphic shape radiation dosimeters for dose verification of modern radiotherapy treatments. A first class of 3D radiation dosimeters is Fricke gel dosimeters, where the active radiation sensitive component is a bivalent cation such as the ferrous ion (Fe²⁺), which is oxidized upon irradiation. The oxidation reaction results in a change in the spin–lattice (R₁) and spin–spin relaxation rates (R₂), which can be mapped with MRI. A second class of 3D radiation dosimeters is polymer gel dosimeters, which are based on vinyl or acrylate monomers that are dissolved in a gelatin or agarose gel. When exposed to radiation, a dose dependent radical polymerization occurs. The amount of entangled or radiation induced grafted polymer is dose dependent and can be read out by use of MRI, optical scanning techniques or X-ray CT. It is mainly the spin–spin relaxation rate (R₂) that is affected in polymer gel dosimeters. A third class is based on the radiation induced conversion of a coloured dye such as leuco-malachite green in the presence of a radical initiator. The coloured dye and radical initiator are mostly hydrophobic and are suspended in a gel matrix as micelles. These 3D dosimeters do not result in significant changes in NMR properties, but can be read out optically.