Port wine stain (PWS) birthmarks are a congenital and progressive vascular malformation of the dermis, involving capillaries, which occurs in approximately 0.7% of children. The objective of laser surgery for this and similar conditions is to cause selective thermal damage, thrombosis, and, eventually, permanent photocoagulation in the PWS vessels. To achieve this, the radiated laser light is set at a specific wavelength, which is highly absorbed by the blood vessels’ hemoglobin (the major chromophore in blood). Unfortunately, the PWS vessels do not absorb all energy radiated — a significant amount is also absorbed by hemoglobin in the ectatic capillaries of the upper dermis. This unwanted absorption causes two problems: firstly, insufficient heat generation within the targeted vessels leads to poor clinical results, and, secondly, there is an increased risk of damage to the overlying epidermis.

In current PWS laser therapy, cryogen spray cooling (CSC) is used effectively to cool and protect selectively the epidermis (tens of micrometers thick) prior to the laser pulse, while minimally cooling the blood vessels. The thermal response of the system is characterized by time and/or temperature dependent boundary conditions. However, in many recent studies, the boundary conditions induced by CSC are regarded as constant. In the present work we study the effects of time-dependent boundary conditions on the overall epidermal thermal damage after PWS laser therapy. We use computer models to simulate the laser light distribution, heat diffusion, and tissue damage, and introduce experimentally determined time-dependent boundary conditions measured for custom-made and commercial atomizing nozzles. We show that time-dependent boundary conditions have a significant effect in the optimal laser dose required to induce photocoagulation of PWS blood vessels while preserving the epidermis.

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