Radiation Protection: History & Fundamentals

Table of Contents

Introduction

Radiation is energy that travels through space in the form of electromagnetic waves or high-speed particles and has the ability to penetrate materials. It arises naturally through decay of radioactive materials but may also be man-made (artificial radiation). Radiation that has sufficient energy to displace electrons from the orbit of an atom, causing the atom to become charged, is called ionising radiation and has the potential to cause significant damage to human tissues and DNA.11 Non-ionising radiation poses less of a health risk. It contains less energy, sufficient only to cause atoms to vibrate, but not to displace electrons or break the chemical bonds holding molecules together. Examples of non-ionising radiation include radio waves and microwaves (figure 1). The discourse that follows focuses mainly on ionising radiation.

Figure 1. Types of radiation

Ionising radiation

There are four major types of ionising radiation (figure 2):

  1. Alpha particles
  2. Beta particles
  3. Neutrons
  4. Electromagnetic waves e.g., x-rays and gamma rays

All are derived from unstable atoms which, in order to achieve a stable state, must emit excess energy in the form of radiation.

Figure 2. Types of ionising radiation

Alpha particles

Alpha particles are made up from two protons and two neutrons. Their size prevents them from being able to travel more than a few centimetres in air and they can easily be stopped by a sheet of thin paper. Alpha particles do not present an external hazard as they are unable to penetrate our outer layer of dead skin cells. They are, however, dangerous when ingested. The 2006 poisoning of Alexander Litvinenko with Polonium-210 provides a stark reminder of how harmful internal emission of alpha particles can be.2

Beta particles

These take the form of either electrons or positrons (positively charged particles similar in size and mass to electrons). Beta particles can travel a few feet in air and can penetrate skin by a few centimetres, potentially causing burns. Similar to alpha particles, they cause more damage when emitted from ingested material.

Gamma radiation

These are photons of energy that have no mass or charge. They form when atomic nuclei transition from a high-energy state to a lower-energy state. Gamma rays can only be stopped by a thick layer of a material like lead or iron. Internally, they do less damage than alpha or beta particles. Gamma rays are used in medicine for both diagnostic and therapeutic purposes. Positron emission tomography (PET) is an example of the former; therapeutically, they are used in the field of oncology to treat malignant tumours during a process called gamma knife surgery.3 Gamma rays are also used to sterilise medical equipment.

X-rays

Like gamma rays, x-rays are photons of energy without mass or charge. It is the process by which these two forms of radiation arise that distinguishes them. When electrons transition from a high energy state to a lower energy state, x-rays are formed. Within the healthcare setting, the use of x-rays is commonplace; plain radiography is by far the most common imaging modality used. In 2019/20, of the 44.9 million imaging tests reported in England, just over half were plain radiographs (51.7%). Computerised axial tomography scans accounted for 13.4 % of the total.4 The history of radiation and radiation protection owes much to the discovery of x-rays. (Figure 3)5,6,7,8

Neutrons

Neutrons are uncharged particles that are emitted as a consequence of spontaneous or induced nuclear fission. They can travel thousands of metres in air, stopped only by a thick block of concrete or lead, and are more damaging to cells than gamma rays or x-rays. Neutrons are the only type of radiation that can turn other materials radioactive.

Figure 3. Radiation and radiation protection – a timeline5, 6, 7, 8

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Radiation exposure

Natural radiation, in particular cosmic radiation and natural radionuclides found in rocks and soil, is the main contributor to worldwide total radiation exposure and represents an annual average worldwide dose of 2.4 mSv (range 1-13 mSv).9 The annual average radiation dose in the UK is 2.7 mSv.10 Radon gas, a source of alpha particles, is formed from the decay of natural uranium in the earth’s crust and seeps into our homes and workplaces. It accounts for approximately half of the annual average human exposure to natural radiation and, for most people, is the largest source of radiation exposure throughout their lifetime.

Diagnostic and therapeutic medical use accounts for 98% of artificial radiation exposure, representing 20% of the total annual average radiation dose (16% in the UK). The annual average worldwide effective dose is 0.62 mSv (table 1).

We consume radioactive substances all the time. Radiation is present in the food we eat, the fluids we drink and the air we breathe. Similarly, we emit radiation, although this is more so in the infrared region of the electromagnetic spectrum.

Table 1. Annual average doses and ranges of ionising radiation by source.9

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Measures of Radiation

Radiation dose, the amount of radiation energy deposited in living tissue, is a complicated topic. It can be measured in a number of different ways (table 211, figure 3).

Table 2. Measures of radiation.11

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*Organ sensitivity correlates with the relative sensitivity of the cells from which they are composed. Blood-forming cells are highly radiosensitive due to their rapid regeneration rate, thus blood-forming organs are similarly radiosensitive. Conversely, muscle and nerve cells are relatively insensitive to radiation.
Doses to organs and tissues are normally averaged over their volume for estimating effective and equivalent doses. For skin, the dose equivalent is averaged over an area no greater than 1cm2

Absorbed dose and equivalent dose can be used to assess short-term risk to tissues i.e., weeks to months. Effective dose allows for simple comparisons of long-term risks.

Figure 3. Graphic showing the relationship between the different measures of radiation. CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)

Biological effects of radiation

Awareness of radiation harm has existed since the early days of its discovery. To date, the most important studies on radiation harm are probably those conducted on the survivors of the 1945 Hiroshima and Nagasaki atomic bombings. Almost 200,000 subjects (120,000 residents, 77,000 offspring) have been studied, over a period of more than 70 years, as part of the Life Span Study (LSS). Within that population, a broad range of ages, radiation dose exposures and health statuses are represented.12

The harmful effects of radiation exposure can be grouped into two categories:13

  • Deterministic effects
  • Stochastic effects

Deterministic effects are harmful tissue reactions arising from the killing/malfunction of cells exposed to high doses of radiation, typically in excess of 1-2 Gy. They occur only if a large proportion of cells are affected and the loss cannot be compensated by increased cellular proliferation. A threshold dose exists, below which the effect is not evident. The International Commission on Radiological Protection (ICRP) has determined this level to be 100 mGy,14 but in reality, the threshold value varies from one individual to another. Above the threshold, the severity of the effect increases with radiation dose. Examples of deterministic effects include skin burns, radiation-related eye lens opacities and radiation syndromes.

Stochastic effects are effects that occur by chance and are typically due to low-level exposure to ionising radiation. Somatic mutations, leading to cancer in the exposed individual, and germline mutations, resulting in hereditary disorders are the two main examples of this. These effects are commonly modelled using a dose-response curve known as the linear no-threshold model (LNT).15 In this model, it is assumed that a threshold dose does not exist; ionising radiation has the potential to cause harm at any dose level. The risk of a stochastic effect occurring increases in proportion to the dose of radiation, but its severity is independent of dose. Thus, a cancer caused by a small amount of radiation can be just as malignant as one caused by a high dose. Finally, the risk of a health effect is independent of the pattern of exposure. Several small exposures are as likely to result in a stochastic effect as a single larger exposure of equal dose. Stochastic effects are typically characterised by a latent period i.e., the effects of radiation exposure do not become clinically apparent until months, even years, after exposure.

The LNT model has been adopted worldwide as a means to set public health policy on radiation exposure limits. It is not without limitations, however.16 It overestimates the risk of radiation carcinogenesis at low radiation doses. At low levels, it is difficult to differentiate between natural background cancer rates and those caused by radiation. The model takes no account of how efficient the human body is at repairing DNA damage attributable to low-dose radiation exposure. Competing models include the Threshold Model,17 which assumes that very small exposures to radiation are harmless, and the Radiation Hormesis model,18 which purports the benefits of low-dose exposure. Scientists disagree on which model should be used due to the lack of conclusive data.

Ionising Radiation Regulations

The use and control of ionising radiation in the United Kingdom is legally governed by the Ionising Radiation Regulations (IRR) and approved by the Health and Safety Executive (HSE). The aim of the latter is to “establish a framework for ensuring that exposure to ionising radiation arising from work activities, whether man made or natural radiation and from external radiation or internal radiation, is kept as low as reasonably practicable (ALARP) and does not exceed dose limits specified for individuals.” The most recently published code of practice is the Ionising Radiation Regulations 2017 (IRR17)19 which came into force on 1 January 2018, replacing the previous regulations formulated in 1999 (IRR99). These delineate the dose limits put in place by the ICRP to protect individuals from excessive exposure to radiation in planned exposure situations (table 3). Exposures received by an individual while not at work will be subject to the limits applying to any other person.

Table 3. Occupational radiation dose limits.19

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*A higher effective dose of 50 mSV in a single year may be authorised, providing the total dose over a period of 5 consecutive years does not exceed 100 mSV.
*The effective dose should not exceed 1 mSv per year in a woman who is pregnant or breastfeeding.
 

Classified workers

Under the Ionising Radiations Regulations 1999 (IRR99), employees who routinely work with ionising radiation may be designated as classified workers if a risk assessment deems them likely to receive an effective dose in excess of 6mSv per year or an equivalent dose in excess of 3/10ths of any relevant dose limit whilst carrying out their work.20 Where this is the case, an employee must mandatorily:

  • Wear personal dosimetry
  • Have dose records maintained (dose on last assessment, current annual dose, and lifetime cumulative dose)
  • Undergo medical surveillance (i.e., an initial medical examination followed by annual reviews)

If an employee works for more than one ‘radiation employer’ (i.e., an employer who in the course of a trade, business or other undertaking carries out work with ionising radiation), cooperation is necessary to ensure that information, particularly with reference to radiation exposure, is shared. Dose and medical surveillance records must be kept for a period of 50 years after the date of last entry. A periodic review of the continuing need for classification should be undertaken. Most medical staff experience levels below the defined thresholds and would normally be designated as non-classified workers (table 4).21 The level of occupational radiation exposure seen within the healthcare industry can be contextualised by comparison with exposure from a range of different sources (table 5).22

Table 4.  Reported occupational whole-body exposures of UK medical workers in 2009/10.21

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Table 5. Comparison of radiation doses from a number of different sources.22

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References

  1. Borrego-Soto, G., Ortiz-López, R., & Rojas-Martínez, A. (2015). Ionizing radiation-induced DNA injury and damage detection in patients with breast cancer.Genetics and molecular biology, 38(4), 420–432. https://doi.org/10.1590/S1415-475738420150019
  2. Miller CW, Whitcomb RC, Ansari A, McCurley C, Nemhauser JB, Jones R. Murder by radiation poisoning: implications for public health. J Environ Health. 2012 Jun;74(10):8-13. PMID: 22708201.
  3. Bir SC, Ambekar S, Bollam P, Nanda A. Long-term outcome of gamma knife radiosurgery for metastatic brain tumors originating from lung cancer. Surg Neurol Int 05-Sep-2014;5:
  4. https://www.england.nhs.uk/ statistics/wp-content/uploads/ sites/2/2020/10/Annual-Statistical-Release-2019-20-PDF-1.4MB.pdf
  5. Mould, R F. “The early history of X-ray diagnosis with emphasis on the contributions of physics 1895-1915.” Physics in Medicine and Biology 40.11 (1995).
  6. Khare, P., Nair, P., Khare, A., Singh, V., & Chatterjee, R. (2014). The road to radiation protection: a rocky path.Journal of clinical and diagnostic research : JCDR, 8(12), ZE01–ZE4. https://doi.org/10.7860/JCDR/2014/ 5832.5223
  7. Brodsky, A., Kathren, R. L., & Willis, C. A. (1995). History of the Medical Uses of Radiation. Health Physics, 69(5), 783–823. https://doi.org/10.1097/00004032-199511000-00012
  8. Widder, Joachim. (2014). The origins of radiotherapy: Discovery of biological effects of X-rays by Freund in 1897, Kienböck’s crucial experiments in 1900, and still it is the dose. Radiotherapy and Oncology. 112. 10.1016/j.radonc.2014.06.010.
  9. UNSCEAR (2008). United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and Effects of Ionizing Radiation. Report to the General Assembly, with Scientific Annexes. Volume 1: Sources. New York, United Nations.
  10. https://www.phe-protectionservices.org.uk/ radiationandyou/
  11. UNEP (2016) Radiation: effects and sources, United Nations Environment Programme, 2016
  12. Douple, E. B., Mabuchi, K., Cullings, H. M., Preston, D. L., Kodama, K., Shimizu, Y., Fujiwara, S., & Shore, R. E. (2011). Long-term radiation-related health effects in a unique human population: lessons learned from the atomic bomb survivors of Hiroshima and Nagasaki.Disaster medicine and public health preparedness, 5 Suppl 1(0 1), S122–S133. https://doi.org/10.1001/dmp.2011.21
  13. Choudhary, S. Deterministic and Stochastic Effects of Radiation. Canc Therapy & Oncol Int J. 2018; 12(2): 555834. DOI: 10.19080/CTOIJ.2018.12.555834.
  14. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP publication 103. Ann ICRP. 2007;37(2-4):1-332. doi: 10.1016/j.icrp.2007.10.003. PMID: 18082557.
  15. Institute of Medicine (US) Committee for Review and Evaluation of the Medical Use Program of the Nuclear Regulatory Commission; Gottfried KLD, Penn G, editors. Radiation In Medicine: A Need For Regulatory Reform. Washington (DC): National Academies Press (US); 1996. K, The Linear, No-Threshold Model.Available from: https://www.ncbi.nlm.nih.gov/books/ NBK232710/
  16. Weber,W, Zanzonico, The Controversial Linear No-Threshold Model. Journal of Nuclear Medicine Jan 2017, 58 (1) 7-8; DOI: 10.2967/jnumed.116.182667
  17. Hoel DG, Li P. Threshold models in radiation carcinogenesis. Health Phys. 1998 Sep;75(3):241-50. doi: 10.1097/00004032-199809000-00002. PMID: 9721832.
  18. Jolly, D., Meyer, J. A brief review of radiation hormesis. Phys. Eng. Sci. Med. 32, 180–187 (2009). https://doi.org/10.1007/BF03179237
  19. https://www.legislation.gov.uk/uksi/ 2017/1075/pdfs/uksi_20171075_en.pdf
  20. https://www.legislation.gov.uk/uksi/ 1999/3232/contents/made?view=plain
  21. Oatway, WB, Jones, AL, Holmes, S, Watson, S, Cabianca, T. (2016). Ionising Radiation Exposure of the UK population: 2010 Review.
  22. https://www.gov.uk/government/ publications/ionising-radiation-dose-comparisons/ionising-radiation-dose-comparisons

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