Magnetic resonance imaging of radiation dose distributions using a polymer-gel dosimeter

Abstract

A new formulation of a tissue-equivalent polymer-gel dosimeter for the measurement of three-dimensional dose distributions of ionizing radiation has been developed. It is composed of aqueous gelatin infused with acrylamide and N, N'-methylene-bisacrylamide monomers, and made hypoxic by nitrogen saturation. Irradiation of the gel, referred to as BANG, causes localized polymerization of the monomers, which, in turn, reduces the transverse NMR relaxation times of water protons. The dose dependence of the NMR transverse relaxation rate, R2, is reproducible (less than 2% variation) and is linear up to about 8 Gy, with a slope of 0.25 s(-1)Gy(-1) at 1.5 T. Magnetic resonance imaging may be used to obtain accurate three-dimensional dose distributions with high spatial resolution. Since the radiation-induced polymers do not diffuse through the gelatin matrix, the dose distributions recorded by BANG gels are stable for long periods of time, and may be used to measure low-activity radioactive sources. Since the light-scattering properties of the polymerized regions are different from those of the clear, non-irradiated regions, the dose distributions are visible, and their optical densities are dependent on dose.

MAGAT gel and EBT2 film‐based dosimetry for evaluating source plugging‐based treatment plan in Gamma Knife stereotactic radiosurgery

“…2b). Then the R2 images of the glass sphere that was treated in the Gamma Knife unit were converted to dose images by applying the calibration equation 37 , 38 , 39 . Absolute dose distribution measurement was not evaluated to avoid introducing errors to the measurement due to the difference in the response of gel vials of different volumes, since the calibration vials were much smaller than the spherical phantom.…”

3D dosimetry fundamentals: gels and plastics

“…However, the porosity of a gelatin matrix is such that small molecules [36], and perhaps also monomers, can almost freely diffuse. A direct consequence is that the radiation dose profile from a half-blocked field differs slightly from the expected "step" irradiation profile: a polymerization "overshoot" is observed at the edge of the field [37,38]. This was interpreted as diffusion of monomers from the unirradiated region to the irradiated region and later confirmed by computer simulations [39].…”

3D Dosimetric Tools in Radiotherapy for Photon Beams

“…In 1992, formulations like BANANA, BANG and PAG polymer gels were proposed [25,26]. The suppression of polymerization by oxygen, present in these gels, was improved with a proposition of MAGIC gel (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) [27], which allows the MAGIC polymeric gels to be prepared in normal weather conditions [28][29][30], which for higher temperatures beyond 25 °C loss stability.…”