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Alcides Fernandes, Olga Pinheiro Garcia, Virginia Laura Lucas Torres, Paulo Roberto Maciel Lyra, Rita de Cássia Fernandes de Lima; A numerical model to calculate the role of the vitreous humor viscosity in laser-induced thermal damage in choroidal melanomas. Invest. Ophthalmol. Vis. Sci. 2019;60(9):714.
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To create a 3D model in order to simulate the thermal damage (TD) caused by Transpupillary Thermotherapy (TTT) in choroidal melanomas and to perform sensibility analyses of the changes in the thermophysical properties of ocular tissues and of the role of patient position during treatment.
A 3D model of the eye was created (SolidWorks© software) and included eight different domains: cornea, aqueous humor, iris, crystalline lens, vitreous humor (VH), choroidal melanoma, choroid/retina complex, and sclera. All domains were considered as solids except the VH (liquefied VH viscosity=7.2 10(-4) Pa.s). For the numerical simulations, a 3mm diode laser beam was used; outputs were 400 and 500 mW; exposure was 60s. A software (Ansys CFX®) was used to calculate the temperatures, the TD, and the velocity profiles. The effects of the choroidal blood perfusion were also considered. The heat transfer coefficient between the cornea and the environment included the effects of natural convection, radiation, and tear film evaporation. For the posterior surface of the sclera, we considered the natural convection with a fluid medium at 37°C. The TD in the ocular tissues was calculated with Birngruber’s model. A numerical strategy was used to simulate the tumor shrinkage. The tumor thermo-physical properties were replaced by those of the VH when TD was=1. With a laser output of 500 mW, two patient positions were tested: seated and supine.
The tumor damage was mostly influenced by the laser absorption coefficients of the tumor and the choroid. These results support the clinical observation that the TTT efficacy is directly related to the tumor melanotic characteristics and the laser absorption. The specific heat and thermal conductivity of the iris and the choroid had negligible effects in the results. In eyes with liquified VH, heat transfer can be influenced by the patient’s position. The temperatures and the TD of the tumor were smaller for a patient in the supine position as opposed to seated.
The laser coefficient absorption of the tumor has a significant effect on tumor destruction. The VH viscosity may influence the TD especially when applied to a patient in the supine position. With an age-reduced VH viscosity, increased laser exposure times or stronger laser power outputs may be warranted in order to achieve optimal tumor destruction.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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