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Polymer-gel dosimeters may be used for quality assurance procedures for radiotherapy treatments whenever three dimensional dose distributions are required. They can be used for routine measurements of radiation dose distributions produced by irradiation devices delivering doses in the range 0-30 Gy.

Polymer-Gel Dosimeters consist of various monomers dispersed in a gelatin matrix. These monomers are polymerized by the free radicals produced by ionizing radiations, and the extent of polymerization is proportional to the absorbed dose. Both the NMR and optical properties of the irradiated gel are thus changed, and dose distributions may be imaged and quantitated using magneticresonance imaging (MRI) or computerized optical tomographic scanning (currently under development by MGS). The polymerization converts the original solution of monomers into a suspension of microparticles that are each much smaller than one micron, and which are fixed in space by the gel, and so the resolution of polymer-gel dosimeters is limited mainly by the imaging device employed. Using a standard head coil and a clinical MRI, pixel sizes on the order of 1 mm are obtained when imaging 2-4 liter gels. For the higher resolution that may be required for brachytherapy sources, gels of about 0.5 liters can be imaged in small-bore, higher-frequency MRI's and pixel sizes of fractions of a millimeter obtained. Polymer-Gel Dosimeters contain only organic molecules and water, and so their average and effective atomic numbers, and mass densities are, depending upon the specific formulation, very nearly the same as that for muscle tissue. Also, their dose response curves exhibit little or no radiation-quality dependence over the range of xand gamma-ray energies employed in radiation therapy, nor do they show any doserate dependence for dose rates in the range 0.06-16 Gy/min. Of equally great importance is that polymer gels take the shape of their containers which could simulate various parts of the anatomy, and even contain bone and air cavities. This latter feature will provide data not obtainable by any other practical means, and which can provide a benchmarks for treatment-planning-computer algorithms. As polymerization of the monomers in polymer-gel dosimeters is inhibited by oxygen, it is essential that the vessel that contains the gel be oxygen-free when it is filled and impermeable to oxygen during the period of irradiation and for one hour post-irradiation during which time polymerization goes to completion. This requirement places severe constraints upon the techniques employed for gel preparation, and upon the materials and methods of fabrication of the gel vessels. At the present time, MGS provides polymer gels in 2-liter, spherical glass vessels for confirmation of stereotactic radiosurgery, and 1.0-liter glass bottles for brachytherapy-source dosimetry. These vessels are shipped in nitrogen-filled pouches made from a unique aluminum/Saran/polyethylene foil. The fabrication of vessels made from Barex plastic, a product of the BP Chemicals Corp., and which is impermeable to oxygen, is also possible. The images recorded by polymer-gel dosimeters are permanent thus permitting comparisons between, for example, the dose distributions for a particular x-ray beam which were made even years apart.