The optical properties of skin tumours measured during superficial photodynamic therapy

Accurate measurement of light distribution in tissue can improve the knowledge of in vivo tissue optical properties, which is essential for precise dosimetry calculations during photodynamic therapy (PDT). In our application of PDT, superficial skin lesions are treated by topical administration of 5-aminolaevulinic acid followed by surface illumination using 630 nm laser light. Several small detector probes inserted under the skin surface prior to illumination record the light intensity at different depths throughout the treatment. Results from 11 patients are presented. These results indicate light distributions described by the diffusion theory for light transport in tissue. The accumulated data do not imply one specific set of optical parameters for the skin, but indicate that although the depth of light penetration is constant for all patients, the variation in skin colour is significant when performing dosimetry calculations. The average value of effective attenuation coefficient was found to be 0.359 mm⁻¹ (±9.5%) and the coefficientk that is used to quantify the build-up of subsurface fluence rate varies between 0.12 and 8.23.