Avaliação da eficiência biológica relativa (RBE) de raios X de baixa energia

Abstract

Ionizing radiation has been used to obtain images for diagnosis, disease monitoring and cancer therapy, contributing to the life expectancy of the population. However, with the increased access to X-ray imaging the collective dose of exposure to ionizing radiation in the population has increased in the last decades, which is a pre-emptive factor, since the ionizing radiation is a genotoxic agent without the determination of a dose threshold at low dose and energy exposures, such as those occurring in diagnostic exams. Due to this, there is a concern that the radiation-induced lesions caused by diagnostic radiographic exposure accumulated during the course of life may induce the process of carcinogenesis. The breast is one of the most radiosensitive organs in humans, and is frequently inserted in the fields of exposure to diagnostic radiation, as well as the guideline for recurrent mammography exams in asymptomatic women, in order to detect early breast cancer. Mammography has specific parameters in the characterization of the X-ray beam that enable it in the formation of the image, however, this characterization of the beam has been related to an ionization pattern of the medium in a more dense form than by other more energetic X-rays beans. Consequently, interest arose in comparing the biological effects of exposure to diagnostic X-rays beams with different energy levels (28 and 150 kVp), in order to verify if the lower energy beam (28 kVp) would be able to compromise the cellular dynamic equilibrium more effectively than the highest energy beam (150 kVp). The evaluation of the biological effects of exposure to the 28 kVp X-rays beam, approximate to those used in mammography, compared to 150 kVp were performed by several biological assays in different experimental models. The assays performed evaluated the ability of X-ray beams to induce DNA damage by constructing survival curves, radiation-induced lesion repair capacity, and relative biological efficiency (RBE). In summary, our assessments show that exposure to the 28 kVp X-rays beam induced a moderate reduction in the survival fraction and increase in the induction of DNA damage in the experimental models used, in relation to the results presented by the 150 kVp X-ray beam. However, the lesions induced by both X-ray beans were almost effectively repaired within 24 hours after exposure to X-rays. The higher efficiency of the 28 kVp beam, compared to the 150 kVp beam, seems to be related to the formation of double breaks in the DNA, due to the attempt to repair the radiation-induced lesions next to each other, than those induced by the 150 kVp beam.