Radioresistance

Radioresistance is the level of ionizing radiation that organisms are able to withstand.

Ionizing-radiation-resistant organisms (IRRO) were defined as organisms for which the dose of acute ionizing radiation (IR) required to achieve 90% reduction (D10) is greater than 1000 gray (Gy)

Radioresistance is surprisingly high in many organisms, in contrast to previously held views. For example, the study of environment, animals and plants around the Chernobyl disaster area has revealed an unexpected survival of many species, despite the high radiation levels. A Brazilian study in a hill in the state of Minas Gerais which has high natural radiation levels from uranium deposits, has also shown many radioresistant insects, worms and plants. Certain extremophiles, such as the bacteria Deinococcus radiodurans and the tardigrades can withstand acute doses of ionizing radiation on the order of 5,000 Gy.

Induced radioresistance

Radioresistance may be induced by exposure to small doses of ionizing radiation. Several studies have documented this effect in yeast, bacteria, protozoa, algae, plants, insects, as well as in in vitro mammalian and human cells and in animal models. Several cellular radioprotection mechanisms may be involved, such as alterations in the levels of some cytoplasmic and nuclear proteins and increased gene expression, DNA repair and other processes.

Many organisms have been found to possess a self-repair mechanism that can be activated by exposure to radiation in some cases. Two examples of this self-repair process in humans are described below.

Devair Alves Ferreira received a large dose (7.0 Gy) during the Goiânia accident, and lived, whereas his wife, who got a dose of 5.7 Gy, died. The most likely explanation is that his dose was fractionated into many smaller doses which were absorbed over a length of time while his wife stayed in the house more and was subjected to continuous irradiation without a break so giving the self repair mechanisms in her body less time to repair some of the damage done by the radiation. This resulted in her death. He also eventually died. In the same way some of the persons who worked in the basement of the wrecked Chernobyl have built up doses of 10 Gy, these workers received these doses in small fractions so the acute effects were avoided.

It has been found in radiation biology experiments that if a group of cells are irradiated then as the dose increases the number of cells which survive decrease. It has also been found that if a population of cells are given a dose before being set aside (without being irradiated) for a length of time before being irradiated again then the radiation has less of an ability to cause cell death. The human body contains many types of cells and a human can be killed by the loss of a single tissue in a vital organ. For many short term radiation deaths (3 days to 30 days) the loss of cells forming blood cells (bone marrow) and the cells in the digestive system (wall of the intestines) cause death.

In the graph below, a dose/survival curve for a hypothetical group of cells has been drawn with and without a rest time for the cells to recover. Other than the recovery time partway through the irradiation, the cells would have been treated identically.

This is a graph showing the effect of fractionation on the ability of gamma rays to cause cell death. The blue line is for cells which were not given a chance to recover while the red line is for cells which were allowed to stand for a time and recover.

Inheritance of radioresistance

There is strong evidence that radioresistance can be genetically determined and inherited, at least in some organisms. Heinrich Nöthel, a geneticist from the Freie Universität Berlin carried out the most extensive study about radioresistance mutations using the common fruit fly, Drosophila melanogaster, in a series of 14 publications.

Radioresistance in radiation oncology

Radioresistance is also a term sometimes used in medicine (oncology) for cancer cells which are difficult to treat with radiotherapy. Radioresistance of cancer cells may be intrinsic or induced by the radiation therapy itself.

Radioresistance comparison

The comparison below is meant to give an indication of radioresistance for different species. There are generally big differences in radioresistance between experiments due to small number of specimens, as well as being unable to control the testing environment (for example, the calculations for humans was determined from the Hiroshima and Nagasaki bombings of WWII).

Lethal radiation doses (Gray)

Organism	Lethal dose	LD50	LD100	Class/Kingdom
Dog		3.5 (LD50/30 days)		Mammals
Human	4-10	4.5	10	Mammals
Rat		7.5		Mammals
Mouse	4.5-12	8.6-9		Mammals
Rabbit		8 (LD50/30 days)		Mammals
Tortoise		15 (LD50/30 days)		Reptile
Goldfish		20 (LD50/30 days)		Fish
Escherichia coli	60		60	Bacteria
German cockroach		64		Insects
Shellfish		200 (LD50/30 days)		-
Fruit fly	640			Insects
C. elegans∗		160-200	≫ 500-800	Nematode
Amoeba		1,000 (LD50/30 days)		-
Braconidae	1,800			Insects
Milnesium tardigradum	5,000			Eutardigrade
Deinococcus radiodurans	15,000			Bacteria
Thermococcus gammatolerans	30,000			Archaea

∗ While an LD₅₀ has been reported for wild type C. elegans individuals, an upper lethal limit has not been established, rather "nearly all animals were alive with no indication of excess lethality up to 800 Gy, the highest dose... measured."

See also

• Ex-Rad a radioprotective drug studied for its ability to protect against acute radiation syndrome
• CBLB502 a similar radioprotective drug, that protects against acute radiation syndrome, during radiotherapy.
• Radiosensitivity
• Background radiation
• Radiation hormesis
• Radiotrophic fungus
• Kojic acid