Incident Fluence Analysis for 755-nm Picosecond Laser Treatment of Pigmented Skin Lesions Based on Threshold Fluences for Melanosome Disruption

Abstract

Background and objectives: In this study, the threshold fluences for disrupting the melanosomes for pigmented skin lesion treatment were determined using a 755-nm picosecond laser for clinical use. Based on the melanosome disruption thresholds, incident fluences corresponding to the target lesion depths were evaluated in silico for different laser spot sizes.

Study design/materials and methods: Melanosome samples were isolated from porcine eyes as alternative samples for human cutaneous melanosomes. The isolated melanosomes were exposed to 755-nm picosecond laser pulses to measure the mean particle sizes by dynamic light scattering and confirm their disruption by scanning electron microscopy. The threshold fluences were statistically determined from the relationships between the irradiated fluences and the mean particle sizes. Incident fluences of picosecond laser pulses for the disruption of melanosomes located at different depths in skin tissue were calculated through a light transport simulation using the obtained thresholds.

Results: The threshold fluences of 550- and 750-picosecond laser pulses were determined to be 2.19 and 2.49 J/cm² , respectively. The numerical simulation indicated that appropriate incident fluences of picosecond laser pulses differ depending on the depth distribution of the melanosomes in the skin tissue, and large spot sizes are desirable for disrupting the melanosomes more deeply located within the skin tissue.

Conclusion: The threshold fluences of picosecond laser pulses for melanosome disruption were determined. The incident fluence analysis based on the thresholds for melanosome disruption provides valuable information for the selection of irradiation endpoints for picosecond laser treatment of pigmented skin lesions. Lasers Surg. Med. © 2021 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.

Keywords: irradiation condition; light transport simulation; melanosome disruption; picosecond laser pulse; pigmented lesions; skin tissue; threshold fluence.

Figure 1

Numerical model of human skin geometry consisting of the epidermis, dermis, subcutaneous fat, and three different sized blood vessels (capillary vessels, upper dermis vessels, and deep dermis vessels).

Figure 2

Mean melanosome sizes after (a) 550‐ and (b) 750‐picosecond laser pulse irradiations with varying irradiation fluences measured using a particle size analyzer (n = 3). Error bars represent the standard deviation (SD) of the mean melanosome sizes. The differences in the mean particle sizes between irradiated (crosses and triangles) and unirradiated (diamonds) melanosomes were statistically evaluated. Despite no significant differences in the crosses, there are significant differences in the triangles (P < 0.01). The threshold fluences of 550‐ and 750‐picosecond laser pulses disrupting the melanosomes were determined as 2.19 and 2.49 J/cm², respectively.

Figure 3

(a) Melanosome disruption was confirmed based on comparison of scanning electron micrographs of (i) unirradiated melanosomes and (ii) melanosomes irradiated using a picosecond laser. The 755‐nm picosecond laser pulses were irradiated at a pulse width of 550 picoseconds and a fluence of 5.25 J/cm². (b) The melanosomes were shown with greater magnification. The long axes of the melanosomes shown as the pink lines were measured using the ImageJ. Scale bars in (a) and (b) indicate 2 and 1 μm, respectively.

Figure 4

Light‐propagation efficiencies α produced through Monte Carlo simulation for spot sizes of (a) 2, (b) 3, and (c) 4 mm. (i) Spatial distributions on zx‐plane with y = 0 and (ii) depth profiles alongz‐axis with x = y = 0. Broken lines at 0.15 and 1.65 mm exhibit boundaries between the epidermis and dermis and between the dermis and subcutaneous fat, respectively.

Figure 5

Incident fluences F of (a) 550‐ and (b) 750‐picosecond laser pulses for disruption of melanosomes with different depth distributions in skin tissue calculated at spot sizes of (i) 2, (ii) 3, and (iii) 4 mm. Broken line at 0.15 mm exhibits boundaries between the epidermis and dermis. The depth range of subcutaneous fat was not shown since pigmented skin lesions are located in the epidermis/dermis.