Voxel-Based Ray-Tracing Modeling of 635 nm Optical Energy Deposition in Skin with Different Pigmentation Levels.

Energy absorption in the skin caused by photobiomodulation (PBM) therapy is highly dependent on the presence of epidermal melanin pigmentation. Accurate determination of the spatial energy distribution is vital for dosimetric purposes based on pigmentation.
Methods: A voxel-based numerical model that utilizes the ASAP software package was created to model light scattering and absorption in the skin, comprised of a three-layer system (stratum corneum, epidermis, dermis). The absorbed power density (W/mm³) was determined from the voxel-based absorbed energy for an input laser power of 2-20 mW at a wavelength of 635 nm. Low and high pigmentation models were considered using epidermal absorption coefficients selected to represent conditions associated with Fitzpatrick skin types I-II and V-VI, respectively. 
Results: Power density was shown to have pronounced depth dependence, with maximum power absorption observed at a depth of about 0.02-0.04 mm in the basal epidermis. High pigmentation was accompanied by higher absorption within the superficial epidermal region, with the increased absorption remaining concentrated near the epidermis. The energy-balance check showed agreement between total absorbed and incident power within 5-8% under the modeled conditions.
Conclusion: Skin pigmentation considerably affects spatial power deposition in 635 nm irradiation. The proposed approach represents a physically consistent framework to study effects related to skin pigmentation in optical dosimetry, while its physiological implications need experimental verification.&#xD.