1. Introduction Spectrophotometry has been the conventional technique for measuring optical density of irradiated gels [1]. Later, magnetic resonance imaging (MRI) was found to be capable of imaging radiation dose patterns in 3D [2]. Optical methods are still under investigation in the form of optical tomography [3-6]. They promise to give three dimensional dose distributions without the cost attached to MRI. Spectrophotometry has an important role in this context. It provides a benchmark for optical density measurements which are then used to calibrate optical tomography results. Spectrophotometry measures transmittance T: (1) P 0 is the power of a monochromatic beam before entering the absorbing medium. P represents the radiant power of the transmitted (unabsorbed) radiation that emerges from the absorbing medium. Absorbance is calculated directly from transmittance: (2) Methods This quantity must be normalised by the size of cuvettes used, leading to the material’s optical density measured in cm -1. Preliminary optical density results on irradiated Presage™ dosimeter are outlined in this poster. Presage™ is a solid dosimeter, based on a clear polyurethane combined with the leuco-dye leuco-malachite green [7]. The purpose of these measurements was a) to obtain spectra for optimizing wavelength of the new light source for equipment described in [6] and b) to obtain a dose response relation. 10 Presage™ cuvettes were given different doses and later read out using a CAMSPEC M350 double beam spectrophotometer. Spectrophotometry of Presage™* polyurethane dosimeters [1] H. L. Andrews, R. E. Murphy, and E. J. LeBrun, Review of Scientific Instruments 28, 329-32 (1957). [2] J.C. Gore, Y.S. Kang, and R.J. Schulz, Physics in Medicine and Biology 29, 1189-97 (1984). [3] J.C. Gore, M.Ranade, M. Maryanski and R. J. Schulz. Phys. Med. Biol.. 41(12), 2695-2704, (1996) [4] M. Oldham, J.H. Siewerdsen, S. Kumar, J. Wong, and D.A. Jafray. Medical Physics, 30(4),623–634,(2003) [5] R.G. Kelly, K.J. Jordan, and J.J. Battista. Medical Physics, 25(9),1741–1750, (1998) [6] S.J. Doran, K.K. Koerkamp, M. Bero, P. Jenneson, E. Morton, and W. Gilboy. Phys. Med. Biol., 46(12)3191– 3213, (2001) [7] J. Adamovics and M.J. Maryanski. Medical Physics, 30(6), 1349-1349, (2003) S N Krstajić 1, P Wai 1, J Adamovics 2, S Doran 1 1 Department of Physics, University of Surrey, Guildford, GU2 7XH, UK Results The absorbance spectrum is shown in figure 1. Peak sensitivity is at wavelengths between 630nm and 640nm. Figure 2 gives an idea of linearity of Presage™ as well as the change in optical density with time. Highest absorbance points from each curve (at 630nm) are selected for 1 day and 9 weeks after irradiation. The linear least squares method was used to provide the best line of fit of absorbance against dose. The photometric accuracy of the spectrophotometer used is 1%. The temperature dependence measurement was performed as follows. Each cuvette was cooled in a water bath to 5°C and then gradually heated to 35°C. During the heating, the cuvette was placed in the spectrophotometer and its transmittance measured. From Figure 3 it is clear Presage™ shows negligible temperature dependence. The temperature error was +/- 5 ºC due to high temperature inside the spectrophotometer. Figure 1: Absorbance vs wavelength curve Figure 2: Time-dependence of dose- response characteristic Figure 3: Temperature dependence Figure 4: Optical tomography setup Figure 5: Relative irradiance of the parallel beam upon exiting the tank Figure 6: Relative irradiance line profile along X and Y Figure 7: The crazing effect of diallyl and dibutyl phthalate mixture upon the perspex scanning tank Matching refractive index Once the best wavelength for studying Presage™ was found the next step was to scan it with the Optical Tomography setup [6] (see figure 4). This involves surrounding the dosimeter with liquid having the same refractive index[3-6]. Transparent liquid used to match Presage™ was a mixture of diallyl and dibutyl phthalate. First attempts at scanning showed that matching Presage™ can be time consuming. Very small refractive index mismatch matters a lot [6]. A model of the setup was developed using optical design software to prove this. Differences of as little as 0.0003 in refractive index have a significant effect [6]. The effect on irradiance on the surface of focusing lens is given in figure 5 and figure 6. The red line in figure 6 shows that even this small mismatch gives almost 50% difference in irradiance along the parallel projection of the edges of the container. Tips on using Presage™ The matching liquids used are toxic and appropriate precautions are necessary. Mixing diallyl and dibutyl can take up to 45 minutes. After scanning Presage™, the tank was cleaned with water and ethanol. Severe crazing developed on the perpsex scanning tank windows as shown on Figure 7. Conclusion We have described some aspects of using Presage™ dosimeter including spectrophotometry and some practical issues regarding using Presage™ for optical tomography readout. Future work will focus on rescanning irradiated Presage™ dosimeter with improved optical tomography setup. Acknowledgements Christopher Bunton, Royal Surrey Hospital, Guildford, helped with building the scanner. Dr Joe Keddie supplied the spectrophotometer. Simon Hall from NPL, Teddington, provided very useful insights into the optics. 2 Heuris Pharma LLC, Skillman, NJ USA *Presage™ is a registered trademark of Heuris Pharma LLC, Skillman, NJ, USA