A review study on application of gel dosimeters in low energy radiation dosimetry

Polymer gel dosimeters are fabricated from radiation sensitive chemicals which, upon irradiation, polymerize as a function of the absorbed radiation dose. These gel dosimeters, with the capacity to uniquely record the radiation dose distribution in three-dimensions (3D), have specific advantages when compared to one-dimensional dosimeters, such as ion chambers, and two-dimensional dosimeters, such as film. These advantages are particularly significant in dosimetry situations where steep dose gradients exist such as in intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery. Polymer gel dosimeters also have specific advantages for brachytherapy dosimetry. Potential dosimetry applications include those for low-energy x-rays, high-linear energy transfer (LET) and proton therapy, radionuclide and boron capture neutron therapy dosimetries. These 3D dosimeters are radiologically soft-tissue equivalent with properties that may be modified depending on the application. The 3D radiation dose distribution in polymer gel dosimeters may be imaged using magnetic resonance imaging (MRI), optical-computerized tomography (optical-CT), x-ray CT or ultrasound. The fundamental science underpinning polymer gel dosimetry is reviewed along with the various evaluation techniques. Clinical dosimetry applications of polymer gel dosimetry are also presented.

A method is described for determining the spatial distribution of radiation dose in a tissue-equivalent phantom using nuclear magnetic resonance imaging. The conversion of ferrous ions to ferric by ionising radiation alters the magnetic moment and electron spin relaxation times of the metal ion. The spin relaxation times (T1 and T2) of the hydrogen nuclei in an aqueous solution of a ferrous salt are consequently reduced substantially. These changes in T1 and T2 can be measured using standard NMR techniques. The same conversion is used in conventional Fricke dosimetry, which can be used to calibrate the technique.

Polymer gels whose NMR and optical properties change when irradiated offer unique advantages for measuring radiation dose distributions. To date, all acrylic polymer gel dosimeters must be manufactured, stored and irradiated in hypoxic conditions which severely limits their use and stability. A new formulation of acrylic dosimeter gel has been developed that responds well in normal atmosphere and which we have named MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper). To produce dosimeter gels, an aqueous solution of gelatin, open to the atmosphere, is mixed with methacrylic acid, copper(II) ions, ascorbic acid and hydroquinone. It is believed that the copper(II) and ascorbic acid form a complex with oxygen which (with radiolysis of water) serves as a free radical source for the initiation of the polymerization of methacrylic acid. At room air the water proton spin relaxation rate R2 in MAGIC gels is proportional to absorbed dose though the precise relationship depends on the composition of the gel and the initiating complex. For example, in the range 0-30 Gy the slope of the response of R2 versus dose at 20 MHz was 0.300, 0.519 and 0.681 s(-1) Gy(-1), respectively, when the concentration of MAA was 3, 6 and 9%. The slopes increased to 0.310, 0.567 and 0.868 s(-1) Gy(-1) at 85 MHz. An important determinant of the sensitivity to detect small dose changes is shown to be the slope-to-intercept ratio of the dose-response curve. These varied from 0.08 to 0.17, comparable to hypoxic gels described earlier. MAGIC gels can be manufactured and used much more easily than the previous formulations and can be imaged by magnetic resonance imaging or optical scanning, and thus they will likely be of considerable interest to radiation physicists.