Quantitative evaluation of light distribution in skin tissue for short-pulsed laser treatment using Monte Carlo simulation combined with nonlinear absorption model of melanin

The treatment effect of pigmented lesions with picosecond and nanosecond lasers is produced mainly from optical absorption by melanosomes. Differences in the treatment effect have been evaluated based on the linear absorption of local fluence. However, nonlinear absorption by melanin inside melanosomes occurs during short-pulsed laser propagation in skin tissue owing to the high-power density. Our previous study demonstrated that a nonlinear absorption model based on sequential two-photon absorption and nonradiative decays was validated to describe nonlinear absorption by melanin. In this study, we comparatively evaluate light distributions in skin tissue with pigmented lesions irradiated with short-pulsed lasers using a Monte Carlo model combined with the nonlinear absorption model. Light distributions in a numerical model of human skin tissue with epidermal pigmented lesions were calculated for a single shot of 450-ps and 10-ns laser pulses at a wavelength of 532 nm, a fluence of 1 J/cm2, and a spot size of 3 mm. The computational simulations show that the penetration depth in the skin tissue was shorter for the picosecond laser. The simulation results suggested that picosecond lasers can reduce damage to surrounding tissue compared to nanosecond lasers. Although these results need to be validated by experiments, our simulations will provide a quantitative evaluation of the safety and efficacy of short-pulsed laser treatment of pigmented lesions.