Influence of drug-light-interval on photodynamic therapy of port wine stains—Simulation and validation of mathematic models

Summary 

Objectives

We established mathematical models of photodynamic therapy (PDT) on port wine stains (PWS) to observe the effect of drug-light-interval (DLI) and optimize light dose.

Materials and methods

The mathematical simulations included determining (1) the distribution of laser light by Monte Carlo model, (2) the change of photosensitizer concentration in PWS vessels by a pharmacokinetics equation, (3) the change of photosensitizer distribution in tissue outside the vessels by a diffuse equation and photobleaching equation, and (4) the change of tissue oxygen concentration by the Fick's law with a consideration of the oxygen consumption during PDT. The concentration of singlet oxygen in the tissue model was calculated by the finite difference method. To validate those models, a PWS lesion of the same patient was divided into two areas and subjected to different DLIs and treated with different energy density. The color of lesion was assessed 8–12 weeks later.

Results

The simulation indicated the singlet oxygen concentration of the second treatment area (DLI=40min) was lower than that of the first treatment area (DLI=0min). However, it would be increased to a level similar to that of the first treatment area if the light irradiation time of the second treatment area was prolonged from 40min to 55min. Clinical results were consistent with the results predicted by the mathematical models.

Conclusions

The mathematical models established in this study are helpful to optimize clinical protocol.

Keywords: Photodynamic therapy (PDT), Port wine stains (PWS), Monte Carlo method, Simulation, Singlet oxygen