DETERMINING PHOTOSENSITIZER CONCENTRATION IN TISSUE BY REFLECTANCE SPECTROPHOTOMETRY

Abstract

During photodynamic therapy of cancer it is desirable to know the concentration of the photosensitizerin the tumour. Reflectance spectrophotometry is a technique which could provide thisinformation by the spectral andthe spatial distribution oflightdiffuselyreflected from the tissue. The absorption of light by a photosensitizer will result in one or more dips in the reflectance spectrum, compared to a reference. The amplitude of a dip, known as the absorbance, depends upon the concentration of the photosensitizer in the tissue. The sensitivity is defined as the initial slope of an absorbance-photosensitizer concentration graph. The sensitivity has been shown to depend on the tissue optical properties. The central goal ofthis research was to non-invasively determine the optical parameters required to predict the sensitivity. A helium-neon laser, 632.8 nm, was used as the photon source. A reproducible tissue phantom consisting of a scattering component, Intralipid or Polystyrene spheres, and an absorbing component, India ink, was used. Measurements of the local and total diffuse reflectances were made. Three tissue samples were also examined. A simple model based on diffusion of neutral particles was used to predict the sensitivity of this technique, as well as the effective attenuation, \itff, and the total diffuse reflectance,Rtot. Differences between diffusion theory and experimentfor n,/z were 5 to 10%. Fortotal diffuse reflectance, theory and experiment agreed within 5%, while forsensitivity the difference was 7 to 15 %. Differences between the diffusion model and experiment were examined by Monte Carlo simulation.