Optimizing Combined Laser Treatment for the Removal of Port Wine Stains and Cryogen Spray Cooling to Reduce Thermal Heating at the Skin Surface

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Port wine stains (PWS) are birthmarks caused by the presence of dilated blood vessels, typically 15-55μm in diameter, located in the upper dermis of the skin. Currently, lasers in conjunction with cryogen cooling are the preferred treatment for PWS removal because they can selectively target PWS blood vessels while leaving the surrounding tissue unharmed. In this project, we compared the effectiveness of various combinations of laser, pre-heating and cooling methods for PWS removal. In COMSOL, we implemented pulsed dye laser heating using a finite element model of light diffusion coupled with heat transfer. Our geometry was based on a two-dimensional histological cross-section from a PWS punch biopsy in order to more accurately mimic the vascular anatomy of a PWS. We compared two types of cooling methods, cryogen spray cooling and water contact cooling. In addition, we implemented a preheating step to achieve higher temperatures in deeper blood vessels. We determined effectiveness of the treatment using an Arrhenius thermal damage equation to calculate injury values over the course of the treatment. For a single 2 ms laser pulse without cooling, the blood vessels reached a maximum temperature of 89°C. However, the skin surface temperature reached 65°C indicating that we would need to implement a cooling method in parallel with laser heating. We compared cryogen spray cooling with cold water therapy and found the cryogen spray to be more effective. Cryogen cooling for 100 ms before and during the laser treatment kept the post-laser epidermal temperature below 26°C, while water cooling only brought the post-laser temperature down to 34°C. Because cryogen was the more effective treatment, we used it as our preferred method of cooling for the remainder of our study. We then implemented this cooling method with a ten laser pulse treatment scheme, which elevated temperatures in the blood vessels but did not achieve coagulation temperatures in the deeper blood vessels. The addition of a 40 second preheating step at 60°C effectively increased the temperatures in the deeper blood vessels to desired levels, while keeping damage to the epidermal and dermal layers at a minimum. Our model of light transport and heat generation in the epidermis, dermis and blood vessels verified that it is possible to target blood vessels with laser therapy while inflicting minimal damage to surrounding tissue. Current methods for treating PWS using pulsed dye lasers are limited in that they cannot target blood vessels deeper in the tissue. In our design, we showed that including a preheating step and multiple laser pulses can effectively target blood vessels deeper in the dermis. Our model can be used to select process parameters and different treatment combinations prior to experiments and clinical trials. Simulating laser treatments as well as various pre-cooling and pre-heating methods in COMSOL reduces the need for excess experimentation and potentially decreases the time before new designs are approved for use.

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