Calculation of spatial distributions of absorbed energy of ionizing radiation in biological tissue for sources of different shapes and sizes at internal irradiation by alpha-particles, electrons and photons with various energies: a unified approach for use in nuclear medicine

Aim of the study - develop a unified approach for calculating spatial distributions of absorbed energy of ionizing radiation within and outside of biostructures with various shapes and sizes at internal irradiation by corpuscular and photon radiation in order to simplify procedures and to reduce the volume of calculations for dosimetrical support to use of radiopharmaceuticals in nuclear medicine. The data bases, containing information on spatial distributions in biological tissue of absorbed energy, emitted by point isotropic sources of corpuscular and photon radiation, were used. Numerical data characterizing absorbed fractions of energy inside and outside the volumes of biological structures modeled by spheres and layers of different sizes at internal irradiation by alpha particles, electrons and photons in the energy ranges of 0.01-10 MeV, 0.0005-10 MeV and 0.015-4.0 MeV, respectively, were obtained. This almost completely covers the energy spectra of all radionuclides used in nuclear medicine. It has been established that numerical sequences of calculated values of absorbed fractions of energy, expressed as dependences of fractions of energy on the ratios of biological structures' sizes or distances around them to the so-called “radii of 99% absorption of energy around point sources”, becomes a unified form applicable to biostructures of various sizes and shapes at internal irradiation by corpuscular and photon radiation with different energies. The proposed approach is useful tool for calculating absorbed doses inside and outside the volumes of organs and tumor formations, at the level of their microstructures, as well as at the cellular and subcellular levels.