Estudo das distribuições de LET, QF e RBE para feixes de fótons de baixa energia em meios biológicos usando o método de Monte Carlo

Abstract

In this work we investigate if LET (Linear Energy Transfer) distributions of photons with energies typically used in diagnostic radiology suffer changes with depth, with the detachment of the field of the incident beam and inhomogeneity of the medium. And what is the influence of these changes on the RBE (Reletive Biological Effectiveness) and the quality factor, QF, of these radiations. In this way, through computer simulations by the Monte Carlo method, we obtained LET distributions of secondary electrons released by low energy photons in homogeneous and non-homogeneous medium to the spectra of 30, 50 and 200 kVp (reference radiation) and the resulting RBE. Considering the effectiveness of radiation proportional to linear energy transfer on average dose, LD, noted that even at the point where the ionization density of the studied X-rays is maximum relative to the reference radiation, X-rays of 200 kVp, the RBE and the quality factor does not exceed the value of 2.0. These variations in the pattern of deposition of energy between the Spectra considered occur more steeply to low LET's, contributing to the increase of RBE in this region. It is therefore concluded that the variations in the patterns of energy deposition, for this energy range, can do the RBE move away from the unit, but not as some radiobiological studies have reported, 4 to 8 factors times the unit. More detailed studies of geometry and density are needed in order to provide more data about the physics of energy deposition in that range. This work is important to improve the existing recommendations radiation protection as well as considerations of justification for the X-ray diagnostic procedures in pediatrics, since we couldn't find such studies about these beam qualities.