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Alcides Fernandes, Olga Pinheiro Garcia, Paulo Roberto Maciel Lyra, Rita C.F. Lima; A numerical model to calculate the influence of the viscosity of the vitreous humor during laser-induced thermal damage in choroidal melanomas. Invest. Ophthalmol. Vis. Sci. 2016;57(12):4102. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To create a 3D computational model to simulate the thermal damage caused by Transpupillary Thermotherapy (TTT) in choroidal melanomas with changes in vitreous humor (VH) viscosity and the resulting natural convection.
A 3D model of the eye was created (SolidWorks© software) and included seven different domains: cornea, aqueous humor, the crystalline lens, VH, the choroidal melanoma, the choroid/retina complex, and the sclera. All of the domains were considered as solids, except the VH (normal VH viscosity =0.7 Pa.s and liquefied VH viscosity= 7.2 x 10-4 Pa.s). For the simulations, we used a 3mm laser beam and power outputs of 400 and 600 mW. Powers of 800 and 1000 mW resulted in corneal damage. A software (Ansys CFX®) was used to calculate the temperatures, the thermal damage, and the velocity profiles (Finite Volume Method). The tumor exposure time to diode laser was 60s. The effects of the choroidal blood perfusion were considered. The heat transfer coefficient value between the cornea and the environment included the effects of natural convection, radiation, and tear film evaporation. For the posterior surface of the sclera, the only effect considered was the natural convection between its surface and a fluid medium at 37°C. The thermal damage in the ocular tissues was calculated using the Birngruber’s model. A numerical strategy was used to simulate the tumor shrinkage. The tumor thermo-physical properties were replaced by those of the vitreous when a critical thermal damage was=1.
By reducing the VH viscosity, the temperature profiles became asymmetric in relation to the pupillary axis. Using 400 mW, the change in the VH viscosity from 0.7 Pa.s to 7.2 x 10-4 Pa.s resulted in a 10% reduction in the damage depth (from 1.24 mm to 1.12 mm). Using 600 mW, the damage depths between normal and water viscosity values were the same. When the VH was reduced to 7.2 x 10-4 Pa.s, the maximum temperature reached within the tumor was 0.6K smaller.
The viscosity of the VH decreases with age however with little influence on the thermal damage on melanomas treated with TTT. For smaller tumors, this effect could be more relevant. With liquefaction of the VH , increased laser exposure time and/or stronger laser power outputs may be warranted in order to achieve optimal tumor destruction.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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