Abstract
Purpose: :
To develop a mathematical and computational model in order to determine the temperature profiles and the thermal damage in ocular tissues exposed to laser radiation used in Thermal Transpupillary Therapy (TTT) to treat choroidal melanomas.
Methods: :
The temperatures profiles were calculated with the Pennes’s Bioheat Transfer Equation which included a thermal heat source term produced by the blood perfusion. The volumetric rate of heat generation produced by the laser source was determined by Beer's law. The thermal damage was calculated using Birngruber’s equation. A computational mesh was generated using a commercial software (GAMBIT). The temperatures profiles were calculated using a Computational Fluid Dynamics software (CFX). Smaller triangular elements were used for the tumor and surrounding areas, where the greatest and most rapid changes in temperature were expected to occur. The environment temperature used was 25°C. The thermal analyses were performed for several ocular tissues and the choroidal melanoma (12mm in diameter; 8mm in thickness). A modified diode laser with a wavelength of 810nm was used. The laser beams used diameters of 2 or 3mm and intensities of 5.7 or 7.1W/cm2. The exposure time for all simulations was kept constant (60s).
Results: :
During the simulations the highest temperature values were achieved within the tumor itself and they produced the greatest thermal damages. Irreversible thermal damage (ITD) occurred when the function reached Ω = 1. When a 2 mm laser beam diameter with 5.7W/cm2 intensity was used, the ITD occurred at a tumor depth of 2.5mm. By increasing the laser beam diameter to 3mm, the damage extended to a depth of 3.0mm. By increasing the laser intensity to 7.1W/cm2 the ITD extended to a depth of 3.7mm.
Conclusions: :
Our computational model may be a useful tool to determine the TTT induced temperatures and thermal damage to several ocular tissues and tumors. In the case of ocular melanomas it may be possible to determine the optimal time of tissue exposure to laser radiation and other important parameters such as the intensity and diameter of the laser beam in order to promote irreversible damage at specific tumor depths. Our model may potentially produce estimates of the tumor moving front velocity during laser ablations. Further refinement of our model may provide additional guidelines for the laser treatment of choroidal melanomas.
Keywords: melanoma • choroid • laser