THE MODIFIED DIFFUSION DIPOLE MODEL

Being able to predict how light interacts with tissue is important in photodynamic therapy (PDT). Mathematical models have been constructed to predict light dose within a tissue, the nature of the model being dependent upon the illumination geometry. One of the more complex models to develop involves a superficial point source, and to date, existing models for this illumination geometry have involved simplifying assumptions or have been mathematically complicated. A modified diffusion dipole model for predicting the light fluence rate (or space irradiance) due to a point source of light on a tissue surface is derived in this paper. Based on the diffusion dipole model of Fretterd and Longini, the modified diffusion dipole model incorporates a more realistic boundary condition, obviates the need to approximate the monopole separation, and is mathematically simple. Space irradiance isodoses within optical phantoms illuminated by a superficial point source were measured using an isotropic detector. The six phantoms studied had a range of absorption and scattering characteristics, and of those, two were tissue equivalent in that the diffusion coefficient for a given phantom was the same as that of muscle and liver. The positions of measured isodoses were compared with those predicted by the modified diffusion dipole model. For all phantoms studied, agreement between measured and theoretical isodoses was good (within about 1 mm), even near the surface. Comparison with an ordinary diffusion dipole model showed much poorer agreement, especially near the surface, indicating an inappropriate boundary condition in the ordinary model.