Wavelength‐dependent threshold fluences for melanosome disruption to evaluate the treatment of pigmented lesions with 532‐, 730‐, 755‐, 785‐, and 1064‐nm picosecond lasers

Abstract Background and Objectives A threshold fluence for melanosome disruption has the potential to provide a robust numerical indicator for establishing clinical endpoints for pigmented lesion treatment using a picosecond laser. Although the thresholds for a 755‐nm picosecond laser were previously reported, the wavelength dependence has not been investigated. In this study, wavelength‐dependent threshold fluences for melanosome disruption were determined. Using a mathematical model based on the thresholds, irradiation parameters for 532‐, 730‐, 755‐, 785‐, and 1064‐nm picosecond laser treatments were evaluated quantitatively. Study Design/Materials and Methods A suspension of melanosomes extracted from porcine eyes was irradiated using picosecond lasers with varying fluence. The mean particle size of the irradiated melanosomes was measured by dynamic light scattering, and their disruption was observed by scanning electron microscopy to determine the disruption thresholds. A mathematical model was developed, combined with the threshold obtained and Monte Carlo light transport to calculate irradiation parameters required to disrupt melanosomes within the skin tissue. Results The threshold fluences were determined to be 0.95, 2.25, 2.75, and 6.50 J/cm² for 532‐, 730‐, 785‐, and 1064‐nm picosecond lasers, respectively. The numerical results quantitatively revealed the relationship between irradiation wavelength, incident fluence, and spot size required to disrupt melanosomes distributed at different depths in the skin tissue. The calculated irradiation parameters were consistent with clinical parameters that showed high efficacy with a low incidence of complications. Conclusion The wavelength‐dependent thresholds for melanosome disruption were determined. The results of the evaluation of irradiation parameters from the threshold‐based analysis provided numerical indicators for setting the clinical endpoints for 532‐, 730‐, 755‐, 785‐, and 1064‐nm picosecond lasers.