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The Biological Effect of Ionizing Radiation

The first level of the biological effect of the ionizing radiation is absorption radiations energy with the further interaction with the tissue matter that lasts for very short time, about a fraction of a second. As a result there develops a succession of biophysical, biochemical, functional and morphological changes that depending on specific conditions can take either several minutes, days or years. When radiation interacts with the matter the ionizing arises, atoms and molecules of the matter are excited and the heat is generated. By irradiation the process of ionizing and excitation arises only along the way of an ionizing particle.

As a result of the atom or molecule ionizing there arise two ions with positive and negative charges. Both of them are unstable, reactive and have strongly pronounced tendency to compound with central molecules. If the central molecules are excited the electronic configuration of the molecule can change, and as a result its molecular bond can be broken. Then decay products of the reacted molecules are also active and, in turn, enter into a reaction with the neutral molecules. Ionizing of a water molecule leads to its separation and formation of +, , 22, 2 that are considerably reactive and cause the oxidation of the water-soluble substances.

Thereby primary physical processes such as ionizing and excitation of atoms and molecules lead to chemical restructuring of irradiated molecules. There are direct and indirect effects in the primal mechanism of the biological effect. The direct effect is the changes that arise in cell molecules as a result of ionizing and excitation. The indirect effect is the one that unite all the chemical reactions, that deal with reactive but non-ionized products of the dissociation of ionized molecules.

The ionizing processes and excitations are starting mechanisms that define all the following changes in the irradiated tissues. The possibility of the ionizing depends on size of the molecule - the bigger is the molecule, the bigger is probability that it will interact with the ionizing molecule. All the most important molecules have big size. An example of that can be a molecule of DNA that participates in the process of inheriting, reproduction and cell metabolism control. Irradiation leads to molecular and DNA structure breaks. In the irradiated cell the regulation processes and function of its separate parts such as membrane and mitochondrion are interrupted. Cell death even when irradiated with high doze can last for very long time. There are two kinds of cell death as a result of irradiation: mitotic death and interphasic death. Mitotic death is one when cell interaction follows irradiation after first or second mitosis. Interphasic death is one before entering the mitosis phase.

Indirect radiation effect produce less gross violation, that are often reversible, but they cover more molecules in the tissue volume, that greatly exceed the radiation field size. An example of an indirect effect can be generalizes organism reaction, leucopenia, developing even in those cases when bone marrow is excluded from the irradiation zone.



The reaction intensity connected with direct and indirect mechanism effects of ionizing radiation depends on the initial organism condition as well as on plenty of physical and chemical factors.

Among all the chemical factors that influence the biological effect the most distinct is the influence of oxygen. When oxygen is present there arises a great number of reactive radicals and peroxides, that increase the oxidation process in the irradiated tissues. The primary radical longevity doesnt exceed fractions of a second, and newly generated oxidants exist over a long period of time. At the same time there can arise chain reactions and the higher is content of the oxygen, the longer are the chains. Oxygen can enter into a reaction with some ionized molecule and promote their change that could not exhibit under absence of oxygen. By growth of primary reaction intensity that develops under irradiation influence, oxygen increases cell radio-sensitivity. Therewith the this increase comes instantly after the oxygen content increase. Oxygen effect is expressed the most for electromagnetic radiation. It is higher when fractionate irradiation than single irradiation.

Introduction of oxygen in tissues after irradiation does not influence cell radio-sensitivity, quite the contrary, it promotes their faster restoration after radiation influence. Reverse action tissue radio-sensitivity reduction is supported by so-called protectors. They are the substances that bond oxygen and radical groups, and in such a way they depress progress of indirect effect reaction.

The changes of atom and molecular chemical structure under the influence of irradiation lead to progress of biochemical reactions in cells, that are not characteristic for them in normal state. These progressing biochemical changes are quite various, and their value for cells is different. The oxidative processes, protein, carbohydrate and lipometabolisms are destroyed, enzymes and ferments are inactivated.

Tissue and Organ Radio-sensitivity

Commonly normal tissues of various histological structure proliferative and non-proliferative are included in irradiation zone. All organs and tissues of a man are sensitive to ionizing radiation in different degree. Tissues and cells are differently affected under irradiation in the same organism and in the same organ. This property is traditionally called cell, tissue and organ radio-sensitivity.

Tissue radio-sensitivity is directly proportional to a degree of its proliferative activity and is inversely proportional to a degree of differentiation. The most sensitive to irradiation, i.e. the most prone to morphological change, is hematopoietic tissue, especially lymphocytes (its longevity 3-5 days); myeloid tissue (neutrophils longevity 5-7 days); epithelium of the small intestine (cell longevity 7 days); germinal epithelium (spermatozoon cycle 8-12 days, ovum cycle 30 days); dermal epithelium cell longevity 24-28 days). According to radio-sensitivity there are also endothelium, fibrous tissue, parenchyma of internal organs, cartilage, muscle and nervous tissue. But changes in the function of neural tissue occur rapidly and even at relatively low doses of radiation. Radio-sensitivity of tissues and cells is not constant and varies depending on the state of the organism, physical activity at the time of irradiation and the effect of external conditions (temperature, oxygen content, etc.).

Radio-sensitivity of normal tissues is quite high and often exceed that of the tumor. However, radiotherapy is also possible in these conditions, as well as recovery processes in normal tissues occurs more completely and more quickly than in the tumor.

Mean tolerant (tolerable) doses for normal tissues and organs are in the table.

 

Mean tolerant doses

Organs total radiation exposure Partial fractionation
fractional single
Lungs 18-20 Gy for 3 weeks (per one lung) 8 (3% pneumonitis); 9,3 (50%); 11 (80%) In standard fields ODS = 30-35 Gy (=30-35 )
Heart     40 Gy (5% of pericarditis)
Bowels 40-45 Gy for 30 fractions (during 70 days can course diarrhea) 10 - mortal 55 (28 fractions) ulcers, strictures - 1-5%
Liver 21-24 Gy (for 2 weeks usually do not course hepatitis)    
Hematopoietic system 1 Gy (up to 0,1 during 2-3 weeks) leukocyte and thrombocytopenia 0,5-1 (leukocyte and thrombocytopenia); 1,5 - mortal 30 Gy - aplasia of bone marrow
Crystalline lens 0,5-1 Gy - dimness (radiation cataract)    
Encephalon 40-45 Gy (up to 1,8Gy) - drowsiness, headaches, nausea 55 Gy- radionecrosis   65 Gy - may cause radionecrosis
Spinal cord (thoracic) ODS () must not be higher than 30-35   50 Gy probable radionecrosis
Ovaries 3 (older than 40 years) - persistent amenorrhea, 20 years - dysmenorrhoea    
Kidneys 25-30 (if only one kidney is irradiated)    
Bones 5 Gy to the growing bone - stop growth. 65 Gy with conventional fractionation (if bone growth stopped)

Date: 2015-12-24; view: 257


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