Monte Carlo simulations of optical propagation in human skin using experimentally measured laser parameters

To simulate light propagation in human skin irradiated with laser sources emitting at 660 nm, 830 nm, and 904 nm, using different beam diameters and divergences, in order to characterize the internal fluence rate distribution profiles within the tissue. Monte Carlo simulations were performed using the GPU-accelerated MCX platform. Optical properties (absorption and reduced scattering coefficients) were experimentally obtained from human skin samples and incorporated into a three-dimensional voxel-based model. Laser parameters, including output power, beam radius, and divergence, were experimentally characterized and used as input for simulations. The effects of wavelength, beam divergence, and source radius on axial and spatial fluence rate distribution were quantitatively evaluated under controlled power conditions. All wavelengths exhibited exponential attenuation with comparable depth-dependent behavior within the first millimeters of tissue. Beam radius significantly modulated surface peak fluence rate and central-axis magnitude at depth under constant total power. In contrast, real divergence values typical of commercial devices produced minimal changes in axial fluence under contact-mode conditions, whereas large theoretical divergence angles markedly reduced fluence retention and increased lateral spread. Among the investigated variables, beam radius exerted the strongest influence on subsurface fluence magnitude under constant power, whereas wavelengths within the therapeutic window (660-904 nm) showed comparable attenuation behavior in superficial tissue. Realistic divergence values typical of commercial devices minimally affected axial fluence under contact-mode conditions, while larger angular spreads significantly reduced central-axis fluence retention.