Absorption of Short-Pulsed Laser Radiation in Superficial Human Tissues: Transient Vs Quasi-Steady Radiative Transfer

Transient radiative transfer effects are pertinent to thermal treatment of superficial cancer via short-pulsed laser irradiation. The transient effects become particularly important due to relatively strong scattering and long attenuation path of radiation in human tissues in the therapeutic window until the complete absorption. Our analysis is based on transport approximation for scattering phase function and the Monte Carlo method for radiative transfer. One-dimensional radiative transfer problem is considered, which was proved to be applicable for simulation of heat transfer and thermal destruction of tumors in superficial human tissues in the case of indirect heating strategy. A series of Monte Carlo calculations enables us to find the threshold of the steady-state approach applicability. In the biomedical problem under consideration, the steady-state solution for absorbed radiation power is sufficiently accurate at duration of laser pulse more than about 10 ps. The calculations for human tissues with embedded gold nanoshells, which are used to increase the local volumetric absorption of the radiation, showed that overheating of the nanoshells with respect to the ambient biological tissue is strongly dependent of the laser pulse duration. This effect is quantified for short pulses by solving the unsteady radiative transfer problem

Effects of short-pulsed laser radiation on transient heating of superficial human tissues

Transient radiative transfer effects are pertinent to thermal treatment of superficial cancer via short-pulsed laser irradiation. Importance of the transient effects arise from relatively strong scattering and long attenuation path of radiation in human tissues in the therapeutic window until the complete absorption. Our analysis is based on transport approximation for scattering phase function and the Monte Carlo method for solving the three-dimensional radiative transfer problem. Monte Carlo simulations are used to study applicability of the quasi-steady radiative transfer approach, and demonstrate that in the biomedical problem under consideration, the quasi-steady solution for absorbed radiation power is sufficiently accurate for laser pulse duration longer than 10 ps. The simulations for superficial tissues with embedded gold nanoshells, used to increase the local volumetric absorption, show that overheating of the nanoshells with respect to the ambient biological tissue is strongly dependent on the laser pulse duration. This effect becomes considerable for laser pulse duration shorter than 1-2 ns. The quasi-steady approach for radiative transfer results in significantly underestimated temperatures of human tissues for short laser radiation pulses. The latter is explained by a relatively strong reflection of the short pulsed radiation by highly scattering human tissues