Dose distribution measurements by MRI of a phantom containing lung tissue equivalent compartments made of ferrous sulphate gel.

Gel dosimetry based on magnetic resonance imaging (MRI) has previously been shown to provide verification of calculated dose distributions in soft tissue equivalent homogeneous phantoms. This study demonstrates how measurements of dose distribution can also be achieved in a phantom containing porous, lung-equivalent, Fricke gel. A phantom was made of Fe2+ infused low-density gel and conventional ferrous sulphate gel, filled in separate compartments in a Perspex container. Absorbed dose measurements were accomplished by MR imaging and by calibrating the proton spin-lattice relaxation rate (R1) versus absorbed dose by means of TLD measurements. This study shows that the production of lung-equivalent low-density (LD) dosimeter gel (mean CT number of -610 HU) is feasible. The MR signal detected in the LD gel dosimeter was substantially more noisy (i.e. displayed larger random fluctuations) than the signal from conventional gel, as expected. A deviation between calculated (TPS) and measured dose of about 3% (6 MV) and 4-7% (15 MV) was found in the LD region of the phantom. These results correspond well with data from other studies of dose distribution in lung-equivalent phantoms. The Fe2+ infused LD gel therefore seems suitable for measurement of absorbed dose distribution in phantoms that contain lung tissue compartments.