Identification of the main factors in the 3-D simulation of laser-induced thermal damage in choroidal melanomas

Abstract

Computational numerical simulation is a valuable tool for studying processes that involve living tissues, since it is not invasive, safer, and cheaper than in vivo experiments. The study of heat transfer within human eyes raises special interest given the numerous existing treatments that involve exposing the eye to external heat sources. Furthermore, there is not much blood flow in most tissues of the eye, which means there is a limited cooling effect and a higher chance of severe thermal damage. Choroidal melanomas are malignant ocular tumors that arise in the choroid, that can be treated by a variety of thermal procedures, among other techniques. Transpupillary thermotherapy (TTT) is one of these treatments, which consists of irradiating the tumor with diode laser, thus heating the tumor within a specific temperature range. In our previous studies, the influence of some parameters on the temperature and thermal damage within the human eye following TTT treatment was studied– the parameters were Vitreous Humor (VH) viscosity, laser output power, and the size of the tumor. Since the data available that are needed for mathematical modeling are scarce and uncertain, the quality of the results was also uncertain. In this study, the main goal was to perform sensitivity analyses to identify which thermal-physical properties significantly influence the results of simulations. A 3-D model of the human eye with a small choroidal melanoma was used. The Navier-Stokes equations were included to consider natural convection inside the VH. A numerical strategy was used to represent tumor shrinkage due to thermal damage. A fractional factorial design of experiments was adopted, and the results were analyzed by building ANOVA (analysis of variance) tables for each case. Our results indicate that the radiation absorption coefficient for the tumor and for the choroid are the only parameters that significantly influence the results among those analyzed ones. This suggests that the uncertainties of most thermophysical properties do not substantially harm the predictions of the simulations. Also, this reinforces that the level of melanin on the tumor should be carefully assessed in order to decide the treatment protocol, since this level is directly related to the absorption coefficient.