Laser treatment has gained popularity for the treatment of skin blemishes. To improve the efficacy and reduce complications from laser treatment, a research group led by Osaka Metropolitan University developed an index of the threshold energy density, or fluence, for melanosome disruption, corresponding to the dependent wavelengths of the picosecond (ps) lasers used for treatment. The wavelength-specific irradiation index will help clinicians determine the optimal endpoint for pigmented lesion treatment based on numerical indicators. Picosecond lasers remove lesions from the skin by delivering energy beams in pulses that last for about a trillionth of a second. The lasers target melanosomes, which produce, store, and transport the melanin responsible for pigment. Scanning electron micrographs of melanosomes. Unirradiated melanosomes show a smooth surface (left). Irradiation by picosecond (ps) lasers with a pulse width of 450 ps at a wavelength of 1064 nm and a fluence of 8.50 J/cm² results in the disruption of the surface structure of the melanosomes (right). Courtesy of Osaka Metropolitan University. In the experiment, the researchers irradiated a solution of melanosomes extracted from porcine eyes using ps lasers with varying fluence. They measured the mean particle size of the irradiated melanosomes through dynamic light scattering. To observe melanosome disruption, they used scanning electron microscopy to determine the disruption thresholds. They developed a mathematical model and combined the model with each threshold obtained and with Monte Carlo light transport to calculate the irradiation parameters required to disrupt melanosomes within the skin tissue at varying thresholds. The researchers quantitatively evaluated irradiation parameters for 532-, 730-, 755-, 785-, and 1064-nm ps laser treatments and determined the wavelength-dependent thresholds for melanosome disruption. They established the threshold fluences at 0.95, 2.25, 2.75, and 6.50 J/cm², respectively. The evaluation of irradiation parameters from a threshold-based analysis provided numerical indicators for setting the clinical endpoints for ps laser treatment. The indicators quantitatively revealed the relationship between the irradiation wavelength, incident fluence, and spot size required to disrupt melanosomes that were distributed at different depths in the skin tissue. When the researchers compared these results to previously reported clinical studies, they were able to confirm that the calculated irradiation parameters were consistent with clinical parameters that showed high efficacy and a low incidence of complications. The wavelength-dependent numerical indicators for melanosome disruption could contribute to the effective evaluation and treatment of pigmented lesions. “The use of this indicator is expected to play an important part in setting irradiation conditions in clinical practice,” researcher Yu Shimojo said. “In addition, the implementation of picosecond laser therapy based on scientific evidence, rather than relying solely on physicians’ experience, is expected to improve the safety and effectiveness of the treatment.” According to the team, this is the first ps laser index to be created for each of the wavelengths used in clinical practice for the treatment of pigmented lesions. Although the thresholds for a 755-nm ps laser were previously established, wavelength dependence has not been investigated until now. The research was published in Lasers in Surgery and Medicine (www.doi.org/10.1002/lsm.23773).