Can diffusion be assumed in correction for oblique incidence in laser photodynamic therapy?

Abstract

By using a diffusing bulb implanted in muscle tissue, it has been shown that the isodose pattern of space irradiance arising from a pencil beam of photons is characteristic of a diffusion-dominant process. This means that photons incident from any direction will behave similarly within the tissue, so that, for plane-beam irradiation of given muscle tissue, the only parameter which determines the space irradiance at depth is the surface irradiance. In order to keep the treatment dose constant at any given depth within the tissue, the surface radiant exposure should be kept constant by adjusting the exposure duration according to the reciprocal of the cosine of the angle of incidence. This has relevance for many clinical situations where the tissues involved have effective attenuation coefficients equal to or less than that found in muscle (e.g. brain). In tissues such as liver, where the absorption is high (Svaasand et al., 1981) no such conclusion may necessarily be drawn.