Radiotoxicity

Radiotoxicity ( Latin radius ' ray 'and ancient Greek τοξικότητα , from toxikón (phármakon) , arrow (poison)' from toxa 'bow and arrow') is an influence of radionuclides on living beings . It is a measure of how harmful an ionizing radiation ( gamma radiation , particle radiation , electron emission ) or a nuclide is due to the radiation emitted by it when absorbed into the body or during exposure. The degree of radiotoxicity of a radionuclide is influenced, among other things, by the type of radiation emitted and the radiation energy, as well as absorption in the organism and the length of time it remains in the body ( biological half-life ).

Relationship between radioactivity and radiotoxicity

Radioactivity is the property of certain elements or some of their isotopes to transform the atomic nucleus without any apparent cause . These elements are called radioactive . During these transformations, ionizing radiation is released.

However, the term radiotoxicity usually refers to radionuclides and only to the harmful effects of the radiation they emit. This does not mean any additional toxic effects due to chemical reactions, for example in the sense of lead poisoning when radioactive lead is absorbed . Radiotoxicity can lead to radiation sickness .

Influencing factors

Absorbed dose

The extent of possible damage depends on the amount of radiation absorbed in the tissue. The physical unit of this absorbed dose is gray . The decisive factor for the biological effect is not so much the physically easily measurable amount of radiation as, in addition to the type of radiation, also the different tissue sensitivity, the fact whether a whole-body or partial-body exposure is present and also the duration of the radiation exposure due to the possibility of repairing radiation damage by the organism. Therefore, the absorbed dose must be weighted both with regard to the type of radiation and the type of tissue in order to be able to make statements about the radiotoxicity. For this purpose, radiation weighting factors and tissue weighting factors were introduced.

Type of radiation

Alpha radiation has only a short range and low penetration depth . When it is absorbed into the body, however, a very high dose occurs in the cells, which is expressed with a radiation weighting factor of 20. Alpha radiation has a low harmful potential when irradiated from the outside, but when absorbed into the body, the probability of damage is very high. Polonium 210, an alpha emitter, when introduced into the body, is the most dangerous radioactive material of all.

Also, beta radiation has a short range and depth of penetration. But it has a very high ionization capacity . It is precisely the low depth of penetration of the radiation that, when absorbed into the body, ensures that this ionization capacity affects a small area and can thus cause severe damage.

Gamma radiation , on the other hand, plays a role above all in the case of external exposure, as it decreases quadratically with the distance and thus also acts over a greater distance with strong radiation sources. In addition, shielding against gamma radiation can only be achieved through greater effort, e.g. B. with lead aprons for X-ray staff or lead disguise around X-ray systems is possible. In addition to the type of radiation, the ionization density of the radiation and thus the linear energy transfer also play a role. With soft gamma radiation, the proportion of absorbed energy is greater than with hard gamma radiation. The relative biological effectiveness takes this into account.

Tissue type

In addition, the type of tissue is also important for the biological effect, which is why the concept of the effective dose equivalent was introduced. By multiplying the absorbed dose by a dimensionless weighting factor, the unit Sievert was introduced for the dose equivalent (or Rem , where 1 Sv = 100 rem).

So the same physical radiation dose z. B. less harmful to bone or nerve tissue than z. B. for rapidly dividing tissues such as bone marrow or the intestinal mucosa. In the case of the gonads , in addition to the acute or chronic toxicity for the individual, there is also the effect of the germline mutation with possible effects on the offspring.

metabolism

The metabolic behavior of radionuclides influences the extent of their radiotoxicity. Isotopes of iodine accumulate in the thyroid gland and cause a much higher organ dose there than in other organs. ¹³¹ Iodine causes radiation exposure of the thyroid gland 5000 times higher than that of the bone marrow or the gonads . Strontium, on the other hand, is built into the bone like calcium and accordingly causes a high dose of organs in the bone marrow.

Biological half-life

In the case of incorporated radionuclides, in addition to the amount and type of radiation, the speed with which the substance is removed from the body also plays an important role in terms of toxicity. The biological half-life is the period of time after which the initial amount of the substance in the body is halved. In contrast, the radioactive half-life describes the time in which half the amount of a radionuclide has decayed. The effective half- life is calculated from the biological and physical half -life .

Other factors

The age of the affected individual plays a role. The tissues of children and especially of fetuses are considered to be particularly sensitive to radiation because they have a higher rate of proliferation .

Different species have different radiation sensitivities. Therefore, the results of animal experiments on radiotoxicity can only be applied to humans to a limited extent . Compared to mammals, birds and cold-blooded animals are particularly less sensitive to radiation. The bacterium Deinococcus radiodurans is considered to be the most insensitive to radiation .

Classification

When determining the radiotoxicity of a nuclide, the various possible damage are weighted into account. According to the degree of damage, the radiotoxicity of nuclides is divided into four groups:

1. Group: Radionuclides with very high toxicity, e.g. B. ²¹⁰ Pb, ²²⁶ Ra. Lead  210 is a beta emitter with a half-life of 22 years, Radium  226 is an alpha emitter with a half-life of 1602 years.
2. Group: Radionuclides with high toxicity, e.g. B. ¹²⁴ I, ²²⁴ Ra. Iodine  124 has a half-life of approx. 4 days and is a positron emitter (β ⁺ decay), Radium 224 is an alpha emitter with a half-life of 3.6 days.
3. Group: Radionuclides with medium toxicity, e.g. B. ¹⁴ C , ¹⁸ F . Both beta emitters: carbon  14 β ^- , fluorine  18 β ⁺ .
4. Group: Radionuclides with low toxicity, e.g. B. ¹²⁵ I , ^99m Tc . Iodine and technetium are low energy gamma emitters that can be shielded well.

Web links

Radiotoxicity. Lexicon entry. On: kernfragen.de.

Individual evidence

1. ↑ EM Minicucci, GN da Silva, DM Salvadori: Relationship between head and neck cancer therapy and some genetic endpoints. In: World journal of clinical oncology. Volume 5, Number 2, May 2014, pp. 93-102, ISSN  2218-4333 . doi : 10.5306 / wjco.v5.i2.93 . PMID 24829856 . PMC 4014801 (free full text).
2. ↑ W. Koelzer: Lexicon for nuclear energy. Updated version 2011. Forschungszentrum Karlsruhe GmbH. P. 126.
3. ↑ Polonium-210 - a brief information. (PDF; 474 kB). On: springer.com.
4. ↑ Radiotoxicity and biological half-life. ( Memento of the original from January 2, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. On: uni.frankfurt.de. 
5. ↑ Harald Schicha, Otmar Schober: Nuclear medicine: basic knowledge and clinical application. ISBN 3-7945-2889-1 . P. 68.