1. Introduction The effect of air-cavities within the human body, for example in the head and neck regions, present possible sources of error when calculating a treatment plan for use in radiotherapy. Monte Carlo studies can simulate these effects, but, as yet there has been little work done to investigate these effects experimentally in a three dimensional model. Polymer gel dosimetry [3] has been used in a number of radiotherapy dosimetry scenarios. Previous work by the authors has involved the insertion of inhomogeneous "bone- like" regions into a polymer gel dosimeter [4]. Up until now, little work has been reported in which air cavities are inserted into an active polymer gel dosimeter. In this work a polymer gel dosimeter has been used with an MRI "readout" to map the dose distributions at the distal surface of an air cavity and the subsequent dose pattern beyond. We believe it is the first time that such dose distributions  previously measured on a point-by- point basis by other dosimetric techniques and predicted by Monte Carlo methods  have been observed directly in 3D. Figure 1:A schematic diagram of the polymer gel phantom with air cavity irradiated from the linear accelerator with 6MV X-rays at a gantry angle of 90 degrees. Results Figure 2 shows the R 2 calculated parameter maps of (a) the (10  10) and (b) the (5  5) cm 2 irradiated samples. Using the computer package IDL, three adjacent profiles extracted along the central axis depth dose have been averaged to increase signal to noise. R 2 curves, corresponding to the central axis depth dose for the (10  10) cm 2 and (5  5) cm 2 X-ray field are shown in Figure 3. Dose distributions beyond air-cavities mapped using polymer gel dosimetry Observations These initial results (Figure 3) show features which are accepted radiation effects: Clearly seen are the build-up effects and the subsequent radiation attenuation as energy is deposited within the gel. As expected the data from the (5  5) cm 2 field size falls off more sharply compared to that of the larger (10  10) cm 2 field size since less dose is deposited due to less lateral scattered electrons. In this preliminary work, the PAGs came from two separate batches and have not been calibrated against each other. The (5  5) cm 2 depth dose curve has been scaled so that the R 2 values at d max are the same for both curves. This correction involving at most a 3% alteration to the R 2 value is justified since the geometry up to d max is the same and effects from lateral scatter at this point are negligible so the d max should be the same. Acknowledgements The authors are indebted to Prof. M. O. Leach (Institute of Cancer Research, Sutton) for use of imaging facilities and to EPSRC for a studentship. Experiment Two samples (a and b) of polymer gel based upon the BANG recipe [3] were made in a nitrogen filled glove box and decanted into pyrex round bottom flasks. Each flask was filled to the same level and sealed. The flasks were then placed on their side and the gel allowed to set. This method produces a phantom which, when stood upright, has one hemisphere filled with gel and the other hemisphere filled with nitrogen (simulating an air cavity) and a large surface area of nitrogen/gel interface References 1) T. P. Wong et al., Australasian Physical & Engineering Sciences in Medicine. 12, 138-146, 1992 2) T. P. Wong et al., Australasian Physical & Engineering Sciences in Medicine. 19, 237-247, 1996 3) M. Maryanski et al., Phys. Med. Biol. 39, 1437-1455, 1994 4) S. J. Hepworth et al., Nucl. Instr. and Meth. in Phys. Res. A. 422, 756-760, 1999 Irradiating and Imaging Each sample was submerged in a water cylindrical water bath (creating a water/air/gel phantom) and positioned on the couch of a Philips 75/6 linear accelerator. The gantry angle was set to 90 degrees and the X-ray field centered on the geometrical centre of the phantom such that the radiation beam entered the air cavity prior to the active polymer gel (Fig. 1). Field sizes of (10  10) cm 2 and (5  5) cm 2 were used to irradiate phantoms a and b respectively. A focus to skin distance (FSD) of 100 cm was used and each sample received 400 monitor units which equates to a dose of 4 Gy at d max in the water. A Siemens Vision 1.5 tesla MRI scanner was used to image the gel samples. R 2 parameter maps were obtained by calculating the straight line fit to the logarithms of pixel intensities using a standard multi-echo sequence with echo times varying from 50 to 800 ms in 50 ms increments. (In these preliminary experiments, no parallel “calibration batch” of gel was produced and so we are unable to do a direct conversion from R 2 to dose. Figure 3: MRI R 2 parameter map of (a) the (10  10) cm 2 field and (b) the (5  5) cm 2 irradiated PAG sample. The vertical white lines show the regions extracted to create the profiles of Fig. 3. The light area corresponds to highly polymerised PAG regions which received the greatest dose. Figure 2: Central axis depth dose curves calculated from R 2 parameter maps for a (5  5) cm field and a (10  10) cm field. The X-ray field is incident from left to right, FSD was 100 cm and each phantom received 400 MU (~4 Gy at d max ). Conclusions This experiment has shown that polymer gel dosimetry can be used measure dose distributions beyond air cavities. Coupled with the technique of embedding dense regions within the gel [4], the PAG provides a method of moving towards realistic anatomically shaped phantoms. This may well prove useful for the assessment of new complex radiotherapy treatment regimes. S Stephen J. Hepworth and Simon J. Doran Department of Physics, University of Surrey, Guildford, GU2 5XH, UK