Nuclear Instruments and Methods in Physics Research A 422 (1999) 756 — 760 Dose mapping of inhomogeneities positioned in radiosensitive polymer gels S.J. Hepworth*, M. McJury, M. Oldham, E.J. Morton, S.J. Doran  Department of Physics, University of Surrey, Guildford, Surrey GU2 5XH, UK  Royal Marsden Hospital, Institute of Cancer Research, Sutton, Surrey, UK Abstract A gel matrix (gelatin), infused with acrylamide monomer and a crosslinking agent has been used as a three-dimensional dosimeter of X- and -rays as used in radiotherapy. Magnetic resonance imaging is used to create a parameter map of transverse relaxation time (¹  ) 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.  1999 Elsevier Science B.V. All rights reserved. Keywords: PAG; Dosimetry; Magnetic resonance imaging; Polyacrylamide gel; Gel dosimetry 1. Introduction Measurement of absorbed dose in a tissue equiv- alent three-dimensional phantom, from X- and - rays, using MRI was first suggested in 1984 [1]. Initial studies were performed using agarose gels * Correspondence address. University of Surrey, Department of Physics, Guildford, Surrey, UK. infused with iron (II) ions to form a Fricke dosimeter [2]. Since the spatial information given by these dosimeters blurs with time due to ionic diffusion, an alternative dosimeter was developed [3] based upon the radiation-induced polymeris- ation of a polyacrylamide gel (PAG) in a gelatin matrix. In previous work quantitative information has normally been obtained from the PAGs using Magnetic Resonance Imaging (MRI) techniques based on changes in spin—spin relaxation time (¹  ) but recently tomographical methods based upon 0168-9002/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 1 0 3 1 - 6