EPR in Retrospective Dosimetry

Unfortunately today there is a worrying, but real, potential for exposure to ionising radiation. This may arise, for example, as the result of malfunction at a nuclear plant, warfare or an act of terrorism. Individuals in the vicinity of such an event would be at risk of developing serious clinical effects, which may present immediately or within days or weeks depending on how much of the radiation was absorbed. The regions of the body most vulnerable to damage by high-energy radiation are the lining of the intestinal tract and the blood cell-producing cells of bone marrow.

Damage to the body caused by a large dose of radiation gives rise to radiation sickness. Symptoms include nausea, vomiting, diarrhoea, dizziness, hair loss, and fatigue of varying severity depending on the level of exposure. In the longer term radiation exposure significantly increases the risk of developing leukaemia or other cancer.

It is likely that a large number of people will be in the area at the time of a radiation incident making it a considerable challenge to provide them all with immediate medical attention. A rapid triage to identify those people most at risk would thus be needed. Radiation-mitigating agents are available, but they must be administered as soon as possible after exposure to be effective1,2. However, these agents are associated with side effects and so should not be used unless the risk of radiation sickness is high, ie, a high level of radiation has been absorbed. The use of radiation-mitigating agents in people at low risk of radiation damage may do more harm than good. It is therefore imperative to know the degree of radiation exposure before commencing treatment.

The level of radiation absorbed can be estimated from the position of an individual relative to the source of the radiation. However, amidst the panic likely to ensue after a radiation incident it is probable that individuals would not be able to recall their exact location. A method for rapidly determining the extent of radiation exposure of an individual is thus desirable.

Evaluating radiation exposure

There are many techniques available for establishing an individual's level of exposure to ionising radiation. As previously mentioned, methods of approximation, such as reconstruction of the dose patterns in the exposed environment, and evaluation of the onset and severity of clinical signs and symptoms, may not have the accuracy required. It is also possible to determine the extent of radiation damage from blood samples, eg, measurement of radiation-induced changes in chromosomes. This will necessitate the taking of samples and sending them to a laboratory for analysis so, although helpful in the long-term management of a patient, will not be able to inform acute treatment decisions.

In vivo EPR measurements of radiation-induced changes in the enamel of teeth provides the means to rapidly differentiate between the levels of radiation received allowing individuals requiring immediate medical attention to be readily identified.

EPR dosimetry

EPR (electron paramagnetic resonance) is a spectroscopic technique that detects species containing unpaired electrons. It has been used in a range of dosimetric applications, including dating of geological minerals3 and for efficacy testing in food irradiation4.

When human tooth enamel is exposed to ionizing radiation free radicals are formed from the carbonate impurities in the hydroxyapatite of enamel, which can be detected using EPR analysis. EPR has thus also been effectively used in vivo for dosimetry of individuals accidentally exposed to radiation5,6. EPR dosimetry using tooth enamel is based on the correlation between the intensity or amplitude of some of the radiation-induced signals with the dose absorbed in the enamel. It can give an indication of the amount of radiation absorbed that is accurate to within 50 cGy7.

However, in some retrospective evaluations of radiation incidents there may not be any human tooth enamel available for analysis. For this reason, EPR dosimetry has been evaluated using tooth enamel from cow and goat8. The radiation-induced EPR signals obtained were very similar to those seen in human enamel, both in terms of sensitivity and linearity. This thus opens up the possibility for using enamel from cows and goats when human evaluation is not possible.

Most recently, EPR dosimetry has been investigated using the enamel from camel molars to assess whether camel enamel could be used in place of human enamel in EPR applications9. Enamel from camel molars was irradiated with gamma doses of 1 Gy up to 100 kGy. The irradiated enamel was then subjected to EPR analysis at room temperature using a Bruker, EMX spectrometer.

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The radiation-induced EPR signal increased linearly with gamma dose up to about 15 kGy and then levelled off. The EPR spectra were similar to those obtained from the enamel of human molars, but differed from cow and goat enamel9.

The authors concluded that, although the response of camel enamel to gamma radiation was 36% lower than that of human enamel, camel teeth are suitable for retrospective gamma dosimetry.

References

  1. Medical response to a radiologic/nuclear event: integrated plan from the Office of the Assistant Secretary for Preparedness and Response, Department of Health and Human Services.
  2. Coleman CN, et al. Ann Emerg Med. 2009; 53(2):213‑222.
  3. Thompson J, Schwarcz HP.  Radiat Meas 2008;43:1219–1225.
  4. Sanyal B, et al. Radiat Meas 2010;45:899–905.
  5. Skvortsov VG, et al. Appl Radiat Isot 2000;52:1275–1282.
  6. Swartz HM, et al. Radiat Meas 2007;42:1075–1084.
  7. Chumak V,  et al. Appl Radiat Isot 1996;47:1281–1286.
  8. Jiao L, et al. J Radiat Res 2014;55:1101–1106.
  9. El-Faramawy NA, et al. Radiat Environ Biophys 2017. DOI 10.1007/s00411-017-0718-1

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Last updated: May 2, 2019 at 8:32 AM

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