On the Mechanism of Radiation Injuries

G. P. TARTANEEV and V. A. MOSKALEV

(Presented by the scientific seminar of the Faculty of Physics and Technology)

We shall not dwell in detail on a review of the existing theories on the mechanism of action of ionizing radiation, which at times have only historical significance, such as Dessauer’s heat theory, the target theory, and others; instead, we shall only briefly consider the viewpoint currently prevailing.

The question of the mechanism of the action of radiation consists of two independent but organically interconnected parts. The first of these is the question of the primary interaction of radiation with tissues; the second part is the pathways of realization, the mechanisms of the pathological process.

The theory of indirect action has gained the greatest popularity and recognition. Briefly, its essence is as follows.

As a result of ionization processes, a water molecule loses or gains an electron and undergoes further dissociation,

\[ \mathrm{H_2O} + e \to \mathrm{H_2O^-} \to \mathrm{H} + \mathrm{OH} \]

\[ \mathrm{H_2O} - e \to \mathrm{H_2O^+} \to \mathrm{H^+} + \mathrm{OH} \]

In this case the newly formed products may undergo various recombinations, especially in a medium containing oxygen. Thus:

\[ \mathrm{OH} + \mathrm{OH} \to \mathrm{H_2O_2}, \]

\[ \mathrm{H} + \mathrm{O_2} \to \mathrm{HO_2}, \]

\[ \mathrm{H_2O_2} + \mathrm{OH} \to \mathrm{H_2O} + \mathrm{HO_2} \]

\[ \mathrm{HO_2} + \mathrm{HO_2} \to \mathrm{H_2O_2} + \mathrm{O_2} \]

\[ \mathrm{H} + \mathrm{H} \to \mathrm{H_2}, \quad \text{etc.} \]

Ultimately, these processes lead to the formation of oxidizing radicals \([\mathrm{OH}, \mathrm{H}, \mathrm{HO_2}, \mathrm{H_2O_2}]\), which, acting on the sulfhydryl groups \((\mathrm{S—H})\) of enzymatic systems, convert them into disulfide groups \((\mathrm{S—S})\), thereby disrupting the activity of the indicated systems, which chiefly take part in processes of synthesis (in particular, of nucleic acids).

Setting forth such a point of view, a number of authors (A. P. Egorov, P. D. Gorizontov, G. M. Frank, and others) consider the possibility of ionization directly in the tissues of the organism to be unlikely. Thus, for example, P. D. Gorizontov, in one of his works from 1955, writes: “In the primary processes the possibility is not excluded of ionization acting directly on the protoplasm molecule, which may cause protein denaturation, its splitting, and even polymerization. However, such a possibility is unlikely, and in the overall biological effect the processes of ionization of water, as the principal medium of the organism, are of decisive importance. It is therefore accepted that the primary reactions are the result of the indirect (through water, consequently, through the solvent¹) action of ionizing radiation on the protein structures of the cell.”

In a number of domestic works, the theory of the action of radiation through water molecules has been further developed. According to the mechanism described above, injuries arise in all tissues, including the nervous system. As a result, the nervous system is subjected to a threefold action: the action of radiation directly, irritations from interoceptors, and irritations by newly formed substances that have entered the blood. Depending on the different combinations of these factors, radiation sickness may have different degrees of expression.

In principle, we agree that the products of the decomposition of water play a certain role, and possibly a considerable one, and that the nervous system is under a threefold action. However, we cannot agree that the process of ionization in tissues (precisely in tissues, and not in water molecules) is unlikely and does not influence the development of radiation lesions. This proposition cannot be considered valid if only because the content of dense substances in the body of an adult human is 35–42%; moreover, according to Bertrand, the elementary composition of the dense substances of the human body is as follows: C—48.43%, O—23.70%, N—12.85%, H—6.65%, Ca—3.45%, S—1.60%, P—1.58%, Na—0.65%, K—0.55%, Cl—0.45%, Mg—0.10%. The content of Fe, Cu, Mn, Co, Zn in the tissues is very small; they are therefore combined into the so-called group of microelements.

The ultrahard radiation of the betatron can interact with any of these elements, causing excitation of their atoms, ionization, or photonuclear transformations. The denial of the possibility of ionization directly in the tissues is, in our opinion, erroneous. We believe that, alongside the indirect, mediated action of ionizing radiation on the organism, there exists a direct, immediate action of radiation on all chemical elements without exception that make up the animal organism, irrespective of the place occupied by the element in the organism. As a result of such an interaction of radiation with the physiological medium, various combinations of tissue molecules may arise, which will be reflected first of all in the functions of cells and tissues, and not in their microstructure.

The disturbances described above arise simultaneously in all systems, but functionally the most sensitive is the nervous system. Thus, the development of the pathological process is influenced not only by those changes that arise during irradiation as a result of direct and indirect action in the peripheral tissues and fluids, but also by changes arising as a result of disturbances in the central nervous system itself.

As indicated, betatron rays can cause nuclear transformations in tissue. These transformations arise when the energy of the radiation exceeds the threshold energy value of the photonuclear process for the given element. Nuclear reactions are also induced by neutrons, which are present in fairly large quantities in the beam of betatron rays (especially—

observed at ray energies of 20 MeV and higher). In the interaction of high-energy radiation quanta and neutrons, on the one hand, with molecules of tissues and fluids of the organism, on the other hand, nuclear reactions arise, as a result of which individual elements that are part of the organism move to a new place in D. I. Mendeleev’s table, i.e., new elements appear in the organism that are not characteristic of the given tissue site; this, of course, cannot fail to affect the chemical structure of the molecules and the functional activity of the damaged tissues. Moreover, if in ionization there is a real possibility of complete restoration of the structure of a molecule under the condition of rapid capture of a free electron, then in the decay of a nucleus restoration of the molecular structure is impossible, since for complete restoration of the structure it would be necessary to replace the newly appeared element by the original one. For example, \(C^{12}(\gamma n)C^{11}\), \(C^{11}\rightarrow B^{11}+\beta^{+}+\nu\).

The formation in the organism of artificial radioactive isotopes not only disrupts molecular systems, but also causes additional irradiation of the surrounding tissues. In the present work experiments were carried out which showed the presence of activation of dense tissues when they are irradiated by betatron rays. These experiments make it possible to assert that ionization processes are observed not only in water molecules, but also in all other molecular groups of the organism. Betatron radiation, possessing a very high penetrating power, produces ionization at all points of the irradiated volume; moreover, bone is ionized with approximately the same intensity as soft tissues. In addition, the presence of activation means the presence of additional (though short-lived) ionization of the surrounding tissues and fluids, caused by radioactive decay of the nuclei of unstable isotopes of elements.

Our observations were carried out on guinea pigs that were irradiated on a betatron with an energy of 10–11 MeV.

Examination of the pigs for induced activity by means of a type “B” apparatus with a \(\beta\)-counter showed its presence in the pigs both during life and after death. The induced activity persisted for a number of days. The average excess over the natural background when using a B-2 type beta counter amounted in individual cases to about 50%. Study of the fecal masses of these animals allowed us to observe their weak radioactivity for many days, caused by \(\beta\)-emitters.

Analysis of the induced activity showed that decay does not occur in water molecules, since two radioactive isotopes of oxygen are known: \(O^{15}\) with a half-life of 126 sec and \(O^{19}\) with a half-life of 31 sec. Hydrogen has one radioactive isotope—\(H^{3}\) with a half-life of 12.1 years. In our observations these isotopes were excluded, since the threshold energies required for the formation of the radioactive isotopes \(O^{15}\) and \(O^{19}\) lie within 12–15 MeV, whereas we were dealing with an energy of 10–11 MeV.

The very first irradiation of bone, dried to constant weight, and of a guinea pig on a betatron with an energy of 20 MeV gave a high level of induced activity, especially in the first minutes.

Conclusion

As a result of irradiation of animals by betatron rays with energies of 10–20 MeV, photodisintegration of the nuclei of elements that are components of the tissues and fluids of the organism is observed. The newly formed radioactive nuclei cause additional irradiation of the tissues adjacent to these nuclei, and the dose of this irradiation will be highest immediately after the cessation of external irradiation.

1. Italics by the authors. ↩