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Biological dosimetry following radiation exposure
- Biological dosimetry is an internationally approved method to quantify the exposure and to perform a dose assessment following a suspected radiation overexposure.
- Biological dose estimation can supplement physical dosimetry. In case physical dosimetry is not available, it can also be used as an independent method.
- The dicentric chromosome and micronucleus assays in peripheral blood lymphocytes are the methods of choice used for estimating acute radiation overexposure.
- In the case of past radiation exposure (several years ago) so-called symmetrical chromosomal translocations are used as radiation markers. They can be visualised as bicoloured chromosomes using the “FISH” (fluorescence in situ hybridisation) technique.
- In the cytogenetic laboratory of the BfS in Munich/Neuherberg, individuals who have or may have been exposed to an increased radiation dose have the possibility to use the biological dosimetry service for dose assessment under specific requirements.
Biological dosimetry is an internationally approved method to quantify the exposure and to perform a dose assessment following a suspected radiation overexposure. In contrast to physical methods, not the dose itself is recorded but the effects of the dose at the cellular level are investigated. Biological dosimetry takes into account the inter-individual variations in radiation sensitivity when assessing the effects of radiation in humans.
Biological dose estimation can supplement physical dosimetry. In case physical dosimetry is not available, it can also be used as an independent method. For this purpose, scientists use certain biological "markers" which can be detected like fingerprints in blood cells after exposure to ionising radiation. Damages induced in chromosomes of the cell nucleus, so-called cytogenetic markers are especially suitable.
The dicentric chromosome and micronucleus assays in peripheral blood lymphocytes are the methods of choice used for estimating acute radiation overexposure. Dicentric chromosomes result from the misrepair of breaks in two different chromosomes. They bear two centromeres instead of one centromere in undamaged chromosomes. Micronuclei include whole chromosomes or chromosome fragments not incorporated into the daughter nuclei during cell division. They are enveloped by a nuclear membrane.
In the case of past radiation exposure (several years ago) so-called symmetrical chromosomal translocations are used as radiation markers. Just like undamaged chromosomes, they only have one centromere. Symmetrical translocations can be visualised as bicoloured chromosomes using the “FISH” (fluorescence in situ hybridisation) technique.
Conventional chromosome analysis
The dicentric chromosome is currently regarded as the most reliable and sensitive biological indicator in case of an acute, recent exposure (weeks – months) to ionising radiation.
Dicentric chromosomes very rarely occur spontaneously (around 1 dicentric chromosome in 1000 cells) and are highly characteristic of ionising radiation. In order to use the method for dose assessment, the first step is to determine the frequency of dicentric chromosomes in the blood sample of a potentially exposed person. The dose is then estimated by means of dose-response curves, also referred to as calibration curves. This method can be used in the dose range from 0.1 to about 5 Gy.
At the BfS usually 1000 cells are analysed for an individual dose reconstruction. The method is internationally considered the gold standard for biological dosimetry. As about half of the cells with dicentric chromosomes are lost during cell division, this marker is not suitable for determining the dose of past radiation exposures.
Just like dicentric chromosomes, micronuclei can be used in the case of acute radiation exposure. However, as this method is less radiation specific and less sensitive than the conventional chromosome analysis, it is not applied as a routine method at the BfS for dose estimation after radiation accidents.
The advantages of the method are the relatively uncomplicated analysis and the easy-to-learn technique. The micronucleus assay can be used in the dose range between about 0.3 to 5 Gy.
The disadvantage of the method – in contrast to conventional chromosome analysis – is the relatively large intra- and inter-individual variability with regard to the frequency of the micronuclei. This means that the number of micronuclei varies distinctly from person to person and even in the same person at different time points even without the influence of radiation exposure. In addition, micronuclei are less specific for radiation, since chemical pollutants can influence the frequency. The spontaneous frequency of micronuclei is higher as well and thus has an influence on the sensitivity of the method.
FISH assay (fluorescence in situ hybridization)
The analysis of symmetrical translocations is less radiation specific and less sensitive than the conventional analysis of dicentric chromosomes. However, this technique allows detection of radiation exposure still years after, provided that the blood forming tissue has been exposed. The reason for this is that cells with symmetrical translocations can often be replicated by the blood-forming system with a minor cell loss of in contrast to cells with dicentric chromosomes. They can thus be detected as radiation markers still years after exposure and can be used for dose estimation. This also applies to chronic radiation exposure extending over a long period of time. However there is a considerable increase of symmetrical translocations with increasing age and also lifestyle (e.g. smoking) appears to be related to a higher frequency of symmetrical translocations.
For an individual dose reconstruction usually 3000 cells are analysed at the BfS. The detection limit for this method is about 0.3 to 5 Gy due to the confounders mentioned above.
Biological dosimetry in the cytogenetic laboratory
In the cytogenetic laboratory of the BfS in Munich/Neuherberg, individuals who have or may have been exposed to an increased radiation dose have the possibility to use the biological dosimetry service for dose assessment under specific requirements. BfS staff will apply appropriate methods like the "chromosome aberration assay" or the "micronucleus" assay. These test methods are performed in lymphocytes of circulating blood and have proved successful for routine application.
Lymphocytes in peripheral blood are at a significant advantage to other cell systems: in contrast to many other tissue cells, they circulate in the whole body and are all at the same stage of the cell cycle at the time of the radiation exposure as well as at the time of the blood draw, i.e. they have the same amount of DNA. This results in the same type of damage in the chromosomes.
The frequency of cytogenetic damage barely changes within the first weeks after radiation exposure. Cytogenetic investigations of blood cells from venipuncture used for chromosome analysis will generally reflect the radiation exposure of the whole body at the day of blood sampling. Moreover, collecting and shipment of blood samples under normal conditions are unproblematic. In the case of past radiation exposure, a dose estimation can be performed under specific circumstances using stable chromosomal aberrations.
When the frequency of the observed cytogenetic damage in the peripheral blood is statistically significantly higher than the control value (spontaneous frequency), dose-response curves are used for biological dose estimation. By means of these curves the frequency of a radiation marker can be quantified and converted to a radiation dose. The curve progression depends on the radiation quality, the biological endpoint used, here dicentric chromosomes, symmetrical translocations or micronuclei, and it also depends on whether the whole body or only a part of the body has been exposed to radiation. Therefore for a precise determination of the dose detailed information concerning the accident situation is necessary. The results for biological indicators which are age-dependent, as known for micronuclei or symmetrical translocations, have to be compared with those of people of the same age.
The BfS has dose-effect curves for different radiation qualities. For the spontaneous frequency of various types of chromosome damage in the population, the so-called control rate, extensive data is available as well.
What can biological dosimetry achieve?
Depending on the accident situation, different numbers of people may have been exposed to increased radiation. Accordingly, the methods used for dose estimation differ and are adopted. Here, a distinction is made between small-scale and large-scale radiation accidents.
Small-scale radiation accident
In order to estimate the individual dose after an acute, recent exposure, usually between 500 and 1000 cells are analysed and the number of dicentric chromosomes is determined. In this case, the lower detection limit for homogeneous whole body irradiation is 0.1 Gy for chromosome analysis and 0.3 Gy for the micronucleus assay. The analysis of symmetrical translocations is not used for acute, recent radiation exposure in contrast.
Large-scale radiation accident
In case of a large-scale radiation accident with several hundreds of people, biological dosimetry can also give a valuable contribution to dose estimation. In this case a rapid and preliminary dose estimation is initially performed to distinguish highly irradiated people, who require medical treatment from those who have not or only sparsely been irradiated and do not need immediate medical care.
Experience has shown that analysing 30 to 50 cells ("triage mode") is sufficient for a rapid classification of potentially exposed persons. In this context, it is also possible to determine whether effects such as nausea, dizziness and hair loss have been caused by radiation or other reasons like for instance psychological stress. If necessary, the number of analysed cells can later be increased and the dose can be estimated more precisely in individual cases.
When should biological dosimetry be performed?
Prior to blood taking the relevant person or the attending physician should definitely contact the BfS. It has to be clarified what caused the suspected increased radiation exposure and whether chromosome assay is practicable and reasonable from a scientific point of view . Following points should be considered in the process:
- What are the indications for the suspected radiation exposure?
- What is known about the type of radiation exposure (for example radiation source, dose rate, distance to the source, duration of exposure and so on)?
- Is a whole body exposure higher than 0.1 gray expected?
- Would other methods (incorporation measurement or excretion analysis) be more suitable for detecting the exposure?
- How long ago did the radiation exposure occur?
State of 2018.04.04