Gel dosimetry enables volumetric evaluation of dose distributions from an MR-guided linac

Magnetic resonance-guided radiation therapy (MRgRT) benefits from performing treatment response assessments at the beginning and end of treatment and also during the treatment itself allowing for more normal tissue sparing and better tumor conformality. Gel dosimeters have been shown to demonstrate 3D dose distributions from conventional treatment units. This qualitative study assessed the value of gel dosimetry to measure volumetric dose distributions delivered by an MR-linac (MRL) while using the MR component for readout. Polymer gels in custom-designed glass cylinders of 5 cm diameter and 4 cm height were obtained from MGS Research Inc. (Madison, CT). In addition, Fricke-type dosimeters were prepared in-house with 0.05 mM xylenol orange. For this preliminary study, the dosimeters were irradiated in air, with a part of each dosimeter outside the treatment field to act as a reference. MR imaging was performed with the MRL to observe the change in paramagnetic properties pre and post irradiation using a T₂ sequence for polymer gels and a T₁-weighted sequence for Fricke gels. MRI during irradiation was done in the MRL using a fast sequence of TR = 5 ms and TE = 1.7 ms. Spin-spin relaxation rate (R2) maps were generated from the polymer gel images and line profiles across the penumbra were analyzed. R2 has previously been shown to be proportional to absorbed dose. When Fricke dosimeters are exposed to ionizing radiation, ferrous ions are oxidized to ferric ions. The paramagnetic spin changes, which are dependent on the concentrations of ferrous and ferric ion species, were observable on T₁-weighted images due to changes in the spin-lattice relaxation rate (R₁ = 1/T₁). We observed a mean increase in pixel value of 53% from un-irradiated to irradiated regions of about 20 Gy. The increase in pixel value and corresponding dose was visible during irradiation using a fast MR sequence. The polymer gels also responded quickly. At 5 minutes after irradiation, the line profiles on the R2 map through the penumbra region showed the fall-off of the radiation field in both dosimeters. 20 hours after irradiation the polymerization of the gel was presumably completed. The overall signal was both higher on the R2 gray scale maps and more pronounced on the R2 color maps. The line profiles across the penumbra regions exhibited a similar trend for both field edges compared to the profiles at 5 minutes post-irradiation. Over the time frame tested, the dosimeter appears stable. These qualitative results indicate that gel dosimetry offers a promising means of measuring relative volumetric dose distributions delivered by an MR-Linac.