Methodological notes on physical parameters of low-level laser irradiation. Part 2. Light dosage at laser therapeutic sessions

Purpose. The general goal of this methodological article, consisting of two parts, is to provide a unifying theoretical approach to the still debated problem on determining the depth of laser light penetration into biotissues and the dosage of laser therapeutic effect from the standpoint of modern medical physics. The purpose of the second part of the article is to demonstrate that calculation of the absorbed dosage at laser therapy sessions is similar to the calculation of classical doses in radiobiology and radiation therapy.Materials and methods. The authors reviewed current state of terms and definitions related to the calculation of doses in ionizing and non-ionizing radiation. The Monte Carlo method was used to simulate the soft tissue volume in which 95 % of radiation energy is absorbed. A classical absorbed dose measured in Grays was estimated. Numerical simulation of absorbed doses for various typical laser therapy procedures was performed.Results. It has been shown that the effective irradiated volume of tissues, despite of small variations in soft tissue density between patients, allows to calculate the absorbed radiation dose in Grays, similar to radiobiological doses. Comparative findings on a single local absorbed dose for various percutaneous therapeutic procedures do not contradict the known clinical data, and even more, make the relationship of different doses for different therapeutic purposes more clear. As it has been found, typical doses range from 0.7 Gy for intravascular blood irradiation to 106 Gy for destructive photodynamic therapy and UV therapy procedures in dermatology.Conclusion. The proposed methodological approach proposes a new look at both the problem of the depth of laser light penetration into biotissues and the problem of laser light doses during therapeutic and diagnostic procedures from a unified medical and physical standpoint.