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Numerical modeling of spray cooling-assisted dermatologic laser surgery for treatment of port wine stains

Cryogen spray cooling (CSC) provides thermal protection to the epidermis during dermatologic laser surgery (DLS) for removal of port wine stain (PWS) birthmarks. The objectives of this study are: to improve the thermal modeling of skin undergoing CSC-assisted DLS for PWS treatment; and, to address the effect of temporal and lateral variations in surface heat transfer during CSC on epidermal protection. The finite element method is used to solve the light and heat diffusion equations in a skin-cross section composed by epidermis, dermis and two blood vessels. Thermal conductivities of each biological structure are modeled as temperature dependent functions. The model accounts for the latent heat of fusion and vaporization, and temporal and spatial thermal variations---due to the inherent non-homogeneous nature of sprays---in surface cooling. Thermal damage due to laser irradiation is evaluated by an Arrhenius integral model. For a 60 ms cryogen spurt, temperature maps of epidermis show that at the end of the spurt there are significant temperature differences, which resulted in epidermal damage after a 5 J/cm2 0.45 ms laser pulse at 585 nm on light color skin type. A 60 ms delay between end of spurt and laser onset produced a relative more homogeneous temperature distribution at the epidermis, and, subsequently, a more effective CSC-DLS for which only the blood vessels were thermally damaged. Temporal and lateral variations in surface cooling must be taken into account to guarantee that enough epidermal protection is provided.