Simulation of the dose control for vascular targeted photodynamic therapy

Dose control is one of the key factors of clinical treatment for port wine stains (PWS) under vasculature-targeted photodynamic therapy. A skin simulation model was proposed to show the PDT Type II reaction around the microcapillaries, and to establish the optimal PDT protocol such as light modulation for different PWS vascular types. A simplified two-dimensional cross section of PWS vascular composed of a single superficial epidermal layer, a deeper dermal layer, and a microcapillaries was used. A series of capillary diameters of 40, 70, 100 and 130 micrometers were used to model typical PWS lesions with different blood flow rates. Oxygen and photosensitizers (PS) are pumped from the microartery at heartbeat frequency and exit the vasculature from the microvein, and a PDT type II reaction occurs near the vessel wall: PS excited by light absorption combines with free oxygen, which leads to a reactive singlet state of oxygen (SSO) that in turn causes direct endothelial cell damage. The mathematical simulation model equations are composed of light transmission, oxygen diffusion, photosensitizer diffusion, singlet oxygen generation and photobleaching, which were solved by finite element method. With the drug diffusion and optical absorption properties of human skin, the photon consumption, drugs and oxygen diffusion and photochemical processes within the vessel wall can be simulated. This simulation can provide a quantitative method to optimize the light and drug dose for clinical treatment of PWS.