The Optical Properties of Tissues at 633 Nanometers as Related to Light Dosimetry in Photodynamic Therpy

In Photodynamic therapy with hematoporphyrin derivative the usual irradiation wavelength is around 630 nm. Here we will discuss the optical properties of tissues in this region and examine the accuracy required in order to adequately calculate the spatial distributions of light in tissue. The fundamental, “microscopic” optical properties of tissues are the absorption and scattering coefficients, Σa and Σs and the scattering phase function S(Θ), which yield the total attenuation coefficient Σt = Σa + Σs and the mean cosine of scatter $$\bar \mu = \smallint s(\theta ).\cos \theta d\cos \theta $$. As reviewed in Ref. 1, for lightly-pigmented soft tissues, the reported values for the effective attenuation coefficient Σeff, which describes the rate of decrease of space irradiance with depth in tissue, are typically ~0.1–1 mm-1. Recently we have measured the microscopic optical properties of a variety of soft tissues in vitro,2–4 the results are summarized in Fig. 1. The essential findings are that the scattering coefficient is very large (Σs ~10–100 mm-1),the ratio of Σs to Σa is very large (~100–1000), so that the albedo is close to unity (c = Σs/Σt>0.99)the scattering is very forward-directed (y>0.9 for normal tissues).