Radiosensitivity
Encyclopedia
Radiosensitivity is the relative susceptibility of cells, tissues, organs or organisms to the harmful effect of ionizing radiation
. Cells are least sensitive when in the S phase
, then the G1 phase
, then G2 phase
and the most sensitive in the M phase
of the cell cycle
. This is described by the law of Bergonié and Tribondeau, formulated in 1906:
From their observation, they concluded that quickly dividing tumor cells are generally more sensitive than the majority of body cells. This is not always true. Tumor cells can be hypoxic
and therefore less sensitive to X-rays that mediate most of their effects through free radicals produced by ionizing oxygen.
Later it has been shown that the most sensitive cells are those that are undifferentiated, well nourished, divide quickly and are highly metabolically
active. Amongst the body cells, the most sensitive are spermatogonia
and erythroblasts, epidermal stem cell
s, gastrointestinal stem cells. The least sensitive are nerve cells and muscle fibers.
Very sensitive cells are also oocyte
s and lymphocyte
s, although they are resting cell
s and do not meet the criteria described above. The reasons for their sensitivity are not clear.
, deterministic and stochastic.
(a less pronounced form of disease), lethal from 2 to 5 Sv (a certain percent of population dies within 60 days), above 5 Sv the majority of people die within 60 days and above 6 to 7 all people die. Of course, these effect depend also on many other factors, like age, sex, health etc.
is the most important. It develops because radiation causes DNA
mutation
s directly and indirectly. Direct effects are those caused by ionizing particles and rays themselves, while the indirect are those that are caused by free radicals, generated especially in water
radiolysis
and oxygen
radiolysis. The genetic effects confer the predisposition to cancer to the offspring. The process is not well understood.
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
. Cells are least sensitive when in the S phase
S phase
S-phase is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this...
, then the G1 phase
G1 phase
The G1 phase is a period in the cell cycle during interphase, before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great...
, then G2 phase
G2 phase
G2 phase is the 3rd and final subphase of Interphase in the cell cycle directly preceding Mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated...
and the most sensitive in the M phase
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
of the cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
. This is described by the law of Bergonié and Tribondeau, formulated in 1906:
X-rays are more effective on cells which have a greater reproductive activity.
From their observation, they concluded that quickly dividing tumor cells are generally more sensitive than the majority of body cells. This is not always true. Tumor cells can be hypoxic
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
and therefore less sensitive to X-rays that mediate most of their effects through free radicals produced by ionizing oxygen.
Later it has been shown that the most sensitive cells are those that are undifferentiated, well nourished, divide quickly and are highly metabolically
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
active. Amongst the body cells, the most sensitive are spermatogonia
Spermatogonium
A spermatogonium is an intermediary male gametogonium in the production of spermatozoa.There are three subtypes:...
and erythroblasts, epidermal stem cell
Adult stem cell
Adult stem cells are undifferentiated cells, found throughout the body after embryonic development, that multiply by cell division to replenish dying cells and regenerate damaged tissues...
s, gastrointestinal stem cells. The least sensitive are nerve cells and muscle fibers.
Very sensitive cells are also oocyte
Oocyte
An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
s and lymphocyte
Lymphocyte
A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...
s, although they are resting cell
G0 phase
The G0 phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle...
s and do not meet the criteria described above. The reasons for their sensitivity are not clear.
Cell damage classification
The damage of the cell can be lethal (the cell dies) or sublethal (the cell can repair itself). The effects on cells can be, according to ICRPInternational Commission on Radiological Protection
The International Commission on Radiological Protection is an advisory body providing recommendations and guidance on radiation protection; It was founded in 1928 by the International Society of Radiology and was then called the ‘International X-ray and Radium Protection Committee’...
, deterministic and stochastic.
Deterministic effects
Deterministic effects have a threshold of irradiation under which they do not appear and are the necessary cosequence of irradiation. The damage they cause depends on the dose: they are sublethal from 0,25 to 2 SvSV
-Geography:* El Salvador, ISO 3166-1 country code* Schuylkill Valley, a school district in Pennsylvania, United States* Sierra Vista, a small city in the South-Eastern corner of Arizona, United States* South Vietnam* Silicon Valley-Organizations:...
(a less pronounced form of disease), lethal from 2 to 5 Sv (a certain percent of population dies within 60 days), above 5 Sv the majority of people die within 60 days and above 6 to 7 all people die. Of course, these effect depend also on many other factors, like age, sex, health etc.
Stochastic effects
Stochastic effects are coincidental and cannot be avoided. They don't have a threshold. These can be divided into somatic and genetic. Among the somatic effects, secondary cancerCancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
is the most important. It develops because radiation causes DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
s directly and indirectly. Direct effects are those caused by ionizing particles and rays themselves, while the indirect are those that are caused by free radicals, generated especially in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
radiolysis
Radiolysis
Radiolysis is the dissociation of molecules by nuclear radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
radiolysis. The genetic effects confer the predisposition to cancer to the offspring. The process is not well understood.
See also
- LNT model, Linear no-threshold response model for ionizing radiation
- Background radiationBackground radiationBackground radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....
- cell death
- lethal doseLethal doseA lethal dose is an indication of the lethality of a given substance or type of radiation. Because resistance varies from one individual to another, the 'lethal dose' represents a dose at which a given percentage of subjects will die...
, LD50LD50In toxicology, the median lethal dose, LD50 , LC50 or LCt50 of a toxin, radiation, or pathogen is the dose required to kill half the members of a tested population after a specified test duration...