Modelling the distribution of laser light in port-wine stains with the Monte Carlo method.

Using the Monte Carlo method, we have calculated the distribution of absorbed light in skin during the laser treatment of port-wine stains. Our model includes the effect that the blood capillaries and epidermis have on the propagation of light through skin. It is more complete than those used by previous workers. In this paper, we change the number of scattering dimensions, the spot diameter, and the capillary separation and diameter, and we use the wavelengths 577 nm and 585 nm. One-dimensional scattering calculations are misleading but two-dimensional results suffice for large spot diameters. The model shows that changing the position or diameter of the capillaries has a larger effect on the distribution of absorbed light in the skin than changing the wavelength of the laser beam from 577 nm to 585 nm. Changing the wavelength does increase absorption in the deeper capillaries, but this is not significant. From our results, we discuss the optimal treatment of port-wine stains.