Dose mapping of inhomogeneities positioned in radiosensitive polymer gels

A gel matrix (gelatin), infused with acrylamide monomer and a crosslinking agent has been used as a three-dimensional dosimeter of X- and γ-rays are used in radiotherapy. Magnetic resonance imaging is used to create a parameter map of transverse relaxation time (T2) from which a map of absorbed dose is calculated. We are using the technique to investigate the interfacial radiation patterns resulting from the presence of implanted inhomogeneities in the gel phantom being irradiated, a concept which is as yet underdeveloped. The inhomogeneities, which may be either low or high density with respect to the surrounding medium, are designed to simulate the effects of air cavities or bone in the body and their effects on the absorbed dose during radiotherapy. This paper explores ways of introducing inhomogeneities into the gel and presents results of dose maps obtained post irradiation. An inherent problem of the dosimeter is the inhibition of polymerisation due to dissolved oxygen. Free radicals produced by the incident radiation are key to the polymerisation and crosslinking effects but are consumed by the dissolved oxygen. This paper describes the observed effects of oxygen contamination within a gel and suggests steps to be taken to minimise the problem.