The calculation of internal irradiation of nano-, microand macro-biostructures by electrons, beta particles and quantum radiation of different energy for the development and research of new radiopharmaceuticals in nuclear medicine

The universal approach for calculations of absorbed doses of internal exposure of nano-, micro- and macro- biostructures by electrons, beta particles and quantum radiation in a wide energy range, almost completely covering the energy range of radiation from radionuclides used in experimental and clinical nuclear medicine, is developed. The polynomial functions that describe the distribution of absorbed energy in biological tissue around point isotropic sources of electrons and quantum radiation in a wide energy range: from 0.1 keV to 10 MeV for electrons and from 10 keV to 4 MeV for quantum radiation, are presented. Integration of these functions over the volume of biostructures-«sources» and «targets» allows to carry out calculations of distribution of absorbed energy in a case of internal exposure of nano-, micro- and macro-biostructures by electrons, beta particles, and gamma quanta, which emitted by different radionuclides used in experimental and clinical nuclear medicine. The developed method was applied in order to calculate the distribution of absorbed dose in the volume of transplantable Erlikh-carcinoma (radius of tumor is equal to 0.8 cm), which irradiated by 153Sm-albumin microspheres radiopharmaceutics. The significant non-uniformity of dose distribution within tumor's volume in case of location of the radiopharmaceutics in the center of the tumor was demonstrated. The possibility of reducing the dose variability was evaluated. The obtained data show, that proper choice of the localization of the radiopharmaceutics in the tumour's volume is essential in order to reach the maximum radiation exposure of all tumor cells. It was demonstrated in a case of 153Sm the distribution of the radiation source on the periphery of the tumor helps to reduce essentially the nonhomogeneity of dose distribution within the tumor in comparison with the case of the source location in the central part of the tumor. The developed method was applied for estimations of exposure of subcellular microstructures (nucleus, cytoplasm and cell membrane) in a case of internal irradiation by 51Cr, 67Ga, 111In, 123I, 125I, 77Br, which are cascade emitters of Auger electrons and Coster-Kronig electrons. These emitters are able to produce high local ionization density in biostructures of nanometric dimensions. As a result, the following Auger emitters were selected as promising radionuclides for the development of radiopharmaceuticals, which are able to produce selective radiation effects on the DNA of cells: 123I, 125I, 77Br, 111In. It was also shown that at each act of photoelectric effect on attached stable atoms of iodine or bromine to the DNA, as well as on native phosphorus of DNA, the cascade emission of low-energy electrons resulted in absorption of 0.3-0.8 keV a volume with a diameter of about 100 nanometers around the atom. This energy is comparable to the value of the absorbed energy in a case of radioactive 125I decay. Results of estimates of expected values of RBE at the photoelectric effect on the atoms of iodine or bromine attached to the DNA of cells of melanoma B16 provides the rationale for further studies of this phenomenon to develop the methods of binary radiotherapy, using the photoelectric effect on heavy atoms (such as iodine or bromine) attached to the DNA of tumor cells.