Radiosensitivity

Radiosensitivity - the susceptibility of cells, tissues, organs or organisms to the effects of ionizing radiation (for the molecules use the term radioactivity ). A measure of radiosensitivity is the dose of radiation that causes a certain level of death of irradiated objects:

• for inactivation of cells - an indicator of D ₃₇ or D ₀ on the survival curve;
• for organisms - the dose that causes the death of 50% of individuals over a certain observation period ( LD ₅₀ ).
  The use of radioprotectors or radio sensitizers (including oxygen ) modifies the radiosensitivity of healthy or tumor cells.

A quantitative characteristic of any radiomodifying effect is the “dose change factor” ( PID ), which is calculated as the ratio of equally effective radiation doses in the presence and absence of a radiomodifying agent. In this case, regardless of the direction of the modifying effect (i.e., strengthening or weakening of the radiation effect), the ratio of the larger dose to the smaller is taken. In the case of the oxygen effect , the PID value is often called the Oxygen Enhancement Ratio (OER). 9.

With general irradiation of animals, the stepwise nature of their death in certain dose ranges is noted, due to the failure of certain critical organs or systems responsible for survival in these dose ranges, which manifests itself in the form of three main radiation syndromes - bone marrow, intestinal and cerebral.

The development of radiation syndromes is determined by the cytokinetic parameters of the corresponding self-renewing cellular systems - hematopoiesis, small intestine and central nervous system (CNS).

Bone marrow and intestines are typical examples of actively renewed radiosensitive cell systems, and the central nervous system, on the contrary, are the least dividing (stationary) radioresistant organs.

The radiosensitivity of the body is most often determined by bone marrow damage, since hematopoiesis is the critical system responsible for survival at doses up to 10 Gy. The critical organ in the next range from 10 to 100 Gy is the small intestine.

Cell determinants that determine the degree of radiation damage to both critical self-renewing systems are stem cells of the bone marrow and intestines.

The clinically significant radiation reactions that develop in the short term after irradiation in certain (threshold) doses, associated with the cellular emptying of actively proliferating self-renewal systems, are combined by the term deterministic effects (or tissue reactions) .

Temporary, easily replaceable cellular losses at lower doses, not causing clinically significant reactions of the body, are quasi-determined effects .

Tissue radiosensitivity is a relative concept. In radioresistant stationary or weakly proliferating organs and tissues, under the influence of irradiation, latent typical radiation injuries occur (are preserved, preserved), in particular chromosomal aberrations, which can be detected under conditions of activation of cell division, for example, during post-traumatic regeneration.

Radiation injuries that develop in the long term after irradiation due to the death of functional cells of weakly proliferating tissues, such as vessels, bones and nerves, are late deterministic effects.

Organ radiosensitivity depends on the radiosensitivity of the tissues that form this organ.

Criteria:

• body mass decreases
• decreased functional activity (in acute radiation sickness, muscle weakness)
• devastation of the organ by specific cells (when the lungs are irradiated with 60 Gy, pneumosclerosis occurs at the site of the tumor).

Classification of organs by radiosensitivity.

• the most radiosensitive (lymphoid organs, red bone marrow, gonads, small intestine)
• medium degree of radiosensitivity (skin, endocrine glands)
• radioresistant (liver, kidneys, brain)