Light distributions in a port wine stain model containing multiple cylindrical and curved blood vessels

Knowledge of the light distribution in skin tissue is important for the understanding, prediction, and improvement of the clinical results in laser treatment of port wine stains (PWS). The objective of this study is to improve modelling of PWS treated by laser using an improved and more realistic PWS model. Light distributions are calculated by the Monte Carlo method for various PWS blood vessel configurations, such as single and multiple vessels oriented horizontally, curved vessels, and "vertically" oriented vessels. Various vessel sizes and wavelengths are used. Our modelling confirms the concept of selective photothermolysis; 577nm laser light gives maximal deposited energy at the top side of the blood vessels closest to the skin surface and 585nm gives a more uniform energy distribution in the vessel. In the distribution of deposited energy multiple vessels mutually influence each other, because of "shadowing" of diffuse light. Modelling PWS laser treatments with multiple vessels confirms the need for successive treatments of vessels layer by layer. The use of different wavelengths affects the local deposited energy profiles in the blood vessels. It predicts that the significance of 585nm laser light lies in the uniform energy distribution in the vessels rather than in gain in energy deposition with depth. The calculated light distributions provide a more realistic input for modelling thermal damage effects in PWS laser treatment and modelling of the epidermal response in thermal imaging of the PWS blood vessel structure