Theoretical Substantiation of the Possibility of Performing Non-Damaging UV Diagnostics of Biological Tissues In Vivo

Since UV radiation is capable of causing skin erythema, there is a risk of damage during in vivo UV spectroscopy of skin. In particular, the conventional estimation of radiation dose indicates the impossibility of conducting such studies when using fiber sources to deliver UVA and UVB radiation to the skin due to the rapid accumulation of the minimal erythema dose (MED). Using numerical simulations, we investigated the possibility of achieving MED when exposing the skin to UV light of diagnostic power and forming irradiation spots of different sizes. It has been shown that the conventional approach to calculating the dose as radiant exposure (J/cm2) turns out to be unsuitable in the case of irradiation spots of small area (which is the case when fiber sources are used) since it greatly overestimates the dose. This, in turn, results in a significant underestimation of the permissible duration of the diagnostic procedure. The reason for this is the failure to take into account the diffusion of radiation in biological tissue. We substantiated that for a more correct calculation of the dose taking into account diffusion, it is necessary to estimate the volumetric energy density (J/cm3) in biological tissue. In vivo experiments confirmed that this approach is more correct in determining the time to reach erythema compared to the conventional approach. The calculations showed that the minimum spot area of UVA/UVB irradiation on the skin surface, beyond which the calculation of the dose as radiant exposure does not introduce a significant error, is 1.5–3 mm2, which corresponds to diameters of 1.4–2 mm in the case of a round irradiation spot.