THE MEASUREMENT OF DIHEMATOPORPHYRIN ETHER CONCENTRATION IN TISSUE BY REFLECTANCE SPECTROPHOTOMETRY

Abstract The in vivo quantitation of local photosensitizer concentration is an important problem in photodynamic therapy because tumor response depends on this parameter. This paper describes a new method for measuring dihematoporphyrin ether (DHE) concentration by reflectance spectrophotometry—a technique which could be applied to other photosensitizers. The absorbance due to the 630 nm absorption peak of DHE was determined by obtaining diffuse reflectance spectra before and after the addition of DHE in the form of Photofrin II. Spectra were obtained by placing an optical fiber bundle source and single fiber detectors in contact with the tissue surface. The sensitivity of the technique was measured for three tissues in vitro with a range of optical properties as well as for Nutralipid—a liquid with a very high scattering to absorption ratio. The results were in qualitative agreement with the predictions of a diffusion model of light propagation although a systematic discrepancy was observed. The technique was also successfully demonstrated in vivo for VX‐2 carcinomas implanted in rabbit ears by correlating simultaneous absorbance measurements with gamma counting of radiolabeled 6‐4 Cu Photofrin II. Our results suggest that reflectance spectrophotometry may be a useful clinical and research tool for the in vivo quantitation of DHE and other photosensitizers.