Australian Clinical Guidelines for Radiological Emergencies - September 2012

Overview of Radiation Injury

Page last updated: 07 December 2012

Background

Risk refers to the potential for a harmful event, a hazard, to have an adverse impact on health. Exposure to radiation poses a health hazard, but the amount of risk to a person depends on many individual and environmental factors. These may not be precisely known at the time of an uncontrolled radiation exposure, but an attempt should be made to assess the elements of radiological risk systematically.

In general an assessment of the risk posed by radiation exposure should comprise the following elements.
  • Identify the hazard: the level of exposure and/or contamination which has occurred.
  • Identify the risk: estimate the potential health impacts of the amount of exposure and/or contamination which has occurred.
  • Communicate the risk: effectively communicate the potential health impacts of the exposure and/or contamination which have occurred.
  • Manage the risk: where possible reduce the impacts of the exposure and/or contamination which have occurred.
Identifying the hazard will generally involve the expert advice of a radiation physicist or other personnel able to measure and characterise the dose of radiation released from a source. Translating this into risk requires an assessment of individual factors which influence the probability a person will suffer injury from a given exposure. These include the timing of exposure, whether a whole or partial body dose of radiation was received, the age of the person and underlying physical illness. Risk communication is complex because there is no simple relationship between the perception of harm and the objective measurement of a hazard. It is, however, essential to communicate risk if measures to mitigate harm like decontamination, administration of medical countermeasures or evacuation are to operate effectively. This is especially the case where large numbers of casualties are exposed to potentially harmful radiation and mass intervention is required en mass.

Identification of hazard

The hazard posed by irradiation is determined by several interacting factors:
  • Absorbed dose
Radiation can be absorbed by a person if they are exposed to a source without adequate protection, or if their body becomes contaminated with radioactive material. Absorbed dose is measured in Grays (Gy), where one Gray corresponds to one Joule of energy absorbed per kilo of tissue. One Gray is a large unit of radiation which may be associated with signs of acute radiation syndrome (ARS).
  • Type of radiation absorbed
The amount of damage caused by exposure to radiation depends on the efficiency with which it transfers energy into body tissues. Radiation comprised of particles with relatively high mass delivers a greater proportion of their energy into tissues than do electromagnetic radiation, such as x-rays and gamma-rays, which may pass through the body. Doses of different types of radiation are, therefore, converted into ‘equivalent doses’ using a weighting factor for each kind of radiation.
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Table 9.1 Weighting factors for different types of ionising radiation
RadiationEnergy transferWeighting factor
Alpha particleHigh20
NeutronHigh5 - 20
Beta particle, electronsLow1
Gamma ray, x-rayLow1

Reference: Recommendations for limiting exposure to ionizing radiation (1995) and national standard for limiting occupational exposure to ionizing radiation (republished 2002); Radiation Protection Series No. 1; ARPANSA.

Equivalent doses are measured in Sieverts (Sv), which is equal to the absorbed dose in Grays multiplied by the weighting factor. A dose of 1/100 Gray delivered entirely as alpha particles would, for example, equal 20/100 Sieverts.
  • Full or partial body irradiation
Only rarely will a person will be exposed to the same dose of radiation equally across their body. In general, accidental radiation exposures cause a person may receive the majority of a radiation dose to only part of their body. This may occur because of the orientation of the person in relation to the source, or because of partial shielding. Sometimes only one part of the body is sufficiently close to a radioactive source to be injured, as with a small radioactive fragment contaminating a wound or with inadvertent handling of an intact source injuring the fingers or hand. Alternately, radioactive material may be distributed to a particular part of the body if it is, for example, inhaled into the lungs or localises to bone. In the case of partial body exposure the relevant dose is that absorbed by exposed tissue, not the dose averaged across the whole body.
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  • Tissue susceptibility
Tissues differ in their susceptibility to radiation and a given absorbed. The “effective dose” is calculated by multiplying the absorbed dose by a tissue weighting which represents the sensitivity of each tissue to radiation.
    Table 9.2 Tissue weighting factors by organ
    OrganTissue weighting factor T
    Gonads0.20
    Colon0.12
    Bone marrow (red)0.12
    Lung0.12
    Stomach0.12
    Bladder0.05
    Chest0.05
    Liver0.05
    Thyroid gland0.05
    Oesophagus0.05
    Skin0.01
    Bone surface0.01
    Adrenals, brain, small intestine, kidney, muscle, pancreas, spleen, thymus, uterusthe weighting factor 0.05 is applied to the average dose of these organs

    Reference: Recommendations for limiting exposure to ionizing radiation (1995) and national standard for limiting occupational exposure to ionizing radiation (republished 2002); Radiation Protection Series No. 1; ARPANSA.

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    • Rate at which dose is absorbed
    The body has some capacity to repair the cellular and genetic damage caused by radiation exposure. This means that the amount of injury evident from radiation exposure will be less if the same dose is received gradually over a period which allows some healing to occur rather than the dose being received rapidly.
    • Presence or absence of contamination
    A person may be irradiated by proximity to radioactive material with which they have no contact. In this case the person will cease to absorb radiation when they leave the vicinity of the radioactive material. If radioactive material enters the environment, however, this may contaminate the surface of a person’s body (clothes, hair, and skin) or be absorbed into the body through ingestion, dermal absorption or inhalation. This may be the situation in an accidental or deliberate release of radioactive material as liquid, explosive debris or smoke. In this case irradiation will continue either until the radioactive source is removed from the person’s body through decontamination and excretion, or the source decays. Contamination is, therefore, a hazard for ongoing exposure to radiation.

    If the type of radiation source, the location of a person while exposed, and the environment in which irradiation occurred are known then the dose of radiation a person is exposed to can be accurately reconstructed after the event. This is, however, only likely in controlled environments such as a nuclear reactor or laboratory and where small numbers of people are involved. If a deliberate release of radioactive material occurs, or large numbers of people are exposed to radiation, then reconstructing the dose received by each based on physics may be impossible. In this case the dose of radiation received may have to be estimated from the measurable effect of radiation after exposure has occurred. Dose assessment is discussed in the next chapter.

    Assessment of risk

    Risk refers to the potential for a radiation hazard to cause harm. Irradiation can cause two classes of harmful; deterministic and stochastic. Deterministic effects of radiation are those whose severity is dependent on the dose of radiation received. These effects can be acute, occurring within hours or days, or delayed for months or years. Stochastic radiation effects are those whose probability of occurring is related to dose, but whose severity when they do occur is not dependent on the initial dose of radiation. Cancer is highly unlikely to result from exposure to low-dose radiation, for example, but is a severe disease whenever it does occur. The main stochastic effect of concern is carcinogenesis.
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    The likelihood of a person suffering stochastic or deterministic effects of radiation exposure is modified by individual risk factors, including age, sex, exposure to other carcinogens, susceptibility to DNA damage, nutritional and hormonal status. Because the impact of these factors has not been quantified, absolute risk cannot be assessed with certainty for most radiation exposures.

    Deterministic effects of radiation

    Death

    Death from acute radiation exposure can result from several syndromes which may occur in isolation or combination.
    • Haematopoietic syndrome caused by cell death in bone marrow resulting in a failure to maintain circulating blood components.
    • Gastrointestinal syndrome caused by death of the gastrointestinal lining resulting in haemorrhage and sepsis.
    • Cerebrovascular syndrome in which CNS function is disturbed resulting in altered consciousness and coma.
    • Pulmonary syndrome caused by damage to the lung from alpha or beta emitters resulting in fibrosis, fluid leakage and reduced gas exchange.
    • Cutaneous syndrome in which exposure of skin results in burns, particularly from beta radiation (beta-burns) which is able to penetrate skin but delivers most of its energy into the dermal layer.
    Table 9.3 Acute radiation syndromes
    SyndromeAcute dose (Gy)Characteristics / sequelae
    Subclinical < 2Subclinical
    Haemopoietic2 - 4Neutropaenia, thrombocytopaenia, haemorrhage, infection, electrolyte imbalance
    Gastrointestinal6 - 10Lethargy, diarrhoea, dehydration, necrosis of bowel epithelium, death in 10 to 14 days
    Cerebrovascular / cardiovascular> 30Agitation, apathy, disorientation, disturbed equilibrium, vomiting, opisthotonus, convulsions, prostration, coma, death in 1 to 2 days
    Pulmonary> 10Radiation pneumonitis
    Cutaneous > 40Severe ulceration of the skin, necrosis, fibrosis, sepsis

    Reference: NCRP Report No. 98

    top of pageFigure 9.1 Alteration in fatal outcome for different rates of radiation dose delivery
    Figure 9.1 Alteration in fatal outcome for different rates of radiation dose deliveryD

    Death is more likely to occur for a given dose if that dose is absorbed rapidly. The death rate can be reduced if high-level supportive care is available.
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    Table 9.4 LD50 values (Gy) as a function of dose rate and degree of medical treatment
    Gamma dose rate (Gy/h)Minimal medical treatmentSupportive medical treatment without growth factorsSupportive medical treatment with growth factors
    0.24.506.427.81
    13.725.767.29
    103.325.276.47
    1003.295.156.13

    Reference: Haskin et al (1997)

    Chronic radiation syndrome

    Chronic radiation syndrome (CRS) is a poorly defined syndrome which occurs in people exposed to whole body irradiation of more than 1 Gray over a period of at least 3 years. It has been reported in poorly regulated industrial exposure to highly radioactive material. A dose this high could result from malicious use of radioactive material if the exposure was covert, such as with a source that is hidden or where radioactivity is introduced into a food-supply. Removal from ongoing radiation exposure results in slow improvement of CRS but the completeness of recovery varies. Clinical symptoms of CRS are non-specific and include:
    • Sleep and appetite disturbance
    • Generalised weakness and fatigability
    • Cognitive changes: altered mood, poor memory, reduced concentration
    • Neurological signs: vertigo, ataxia, parasthesias
    • Headache
    • Syncopal episodes
    • Hot flashes or chills
    Laboratory findings include pancytopenia and bone dysplasia.

    Cataracts

    Cataracts are opacity of the ocular lens which impairs vision by reducing the amount of light that enters the eye. Cataracts occur naturally and the risk is increased by age, genetic factors and exposure to certain medicines. Cataracts have been observed to be more common in astronauts and airline pilots, who are exposed to relatively low doses of radiation for prolonged periods. The rate of cataract formation is estimated as 10% in people receiving a 2 Gray dose of radiation, 50% in people receiving 5 Gray radiation and 90% in people receiving 10 Gray.

    Impairment of fertility

    Ionising radiation has the capacity to reduce fertility by damaging spermatogenesis or reducing ovulation.
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    Table 9.5 Effects of acute exposure on ovarian function
    Ovarian dose (Gy)Effect
    0.6No effect
    1.5Some risk for ovulatory suppression in women > 40 years of age
    2.5 – 5.0In women aged 15 to 40 years, 60% may suffer permanent ovulatory suppression; the remainder may suffer temporary amenorrhoea. In women > 40 years, 100% may have permanent ovulatory suppression. Menopause may be artificially produced
    5 – 8 In women aged 15 to 40 years, 60% may suffer permanent ovulatory suppression; the remainder may suffer temporary amenorrhoea. There is no data for women > 40 years
    > 8100% ovulation suppression

    Reference: United States Nuclear Regulatory Commission (1989)

    top of pageTable 9.6 Effects of fractionated testicular irradiation on sperm count
    Testicular dose (Gy)Effect
    0.1 – 0.3Temporary oligospermia
    0.3 – 0.5100% temporary aspermia from 4 to 12 months post-exposure. Full recovery by 48 months
    0.5 – 1 100% temporary aspermia from 3 to 17 months post-exposure. Full recovery beginning 8 to 38 months
    1 – 2 100% temporary aspermia from 2 to 15 months post-exposure. Full recovery beginning 11 to 20 months
    2 – 3 100% temporary aspermia from 1 to 2 months post-exposure. No recovery observed up to 40 months

    Reference: United States Nuclear Regulatory Commission (1989)

    Stochastic effects of radiation

    Cancer

    Radiation can cause cellular damage which increases the risk of a person developing cancer. The risk of developing cancer increases with the dose of radiation received, but not all people will develop cancer even at high levels of irradiation.
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    Estimates of the cancer risk associated with radiation exposure are based on longitudinal cohort studies in two populations; survivors of the atomic bombing of Japan in 1945 and nuclear industry workers. Both have been exposed to low life-time doses of radiation although, in the atomic bomb, people received their entire dose in a matter of seconds. This data reflects risk associated with external, whole body radiation doses.

    It is not known whether there is a ‘threshold’ below which radiation causes no increase risk in risk of cancer. Cytogenetic abnormalities can be observed at doses as low as 100 mGy but there may be non-detectable damage at lower doses which is able to cause cancer in some individuals.

    Because cancer also occurs in people without excess exposure to radiation, the increase in risk resulting from irradiation is described in relative terms. The excess relative risk (ERR) is the ratio of the extra cases cancer observed in people exposed to radiation compared to the cases observed in people not exposed to additional radiation. If 15 cases of cancers were observed in every 1000 nuclear workers and 10 cases in every 1000 of the general population, the ERR would be (15-10)/10 or 0.5.

    A longitudinal study has followed survivors of the atomic bombing of Japan, the majority of whom were exposed to doses <100mSv over a period of seconds. This study estimated the excess relative risk (EER) of developing solid cancers as between 0.47 and 0.50 per Sievert of radiation dose. A person exposed to a dose of 100mSv would, for example, have an ERR of 0.05.

    An analysis of several large studies which have examined the risk of cancer in nuclear workers estimated the ERR for all solid tumours as 0.87/Sv.
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    The absolute risk of developing fatal cancer for anyone in the general population has been estimated at 20% over a lifetime. A person with an excess in relative risk (ERR) of 0.05 therefore has a 5% increase in this underlying 20% risk of developing a solid tumour, or 21%. This would mean that a population of 5000 people exposed to 100 mSv of radiation might expect an increase from 1000 cancers to 1050 cancers over the life of this population.

    Communication of risk

    In most radiological incidents the majority of people are likely to be exposed to doses which do not cause immediate and severe physical effects. Although moderate and low doses of radiation can cause illness in some people, there are limited options for intervening to reduce this risk once irradiation has occurred. The main objective in most people is, therefore, to manage the psychological consequences of this risk and effective communication is a key aspect of this. Providing information about the magnitude and severity of health risks will help reduce distress and the inappropriate use of medical interventions which are potentially harmful. If mass casualties occur then managing the anxiety of people is particularly important to allow triage for appropriate management and prevent medical facilities being overwhelmed.

    The general objectives of risk communication are:
    • To engender understanding of the probability and nature of adverse health effects faced by the person.
    • To produce an understanding of the limitations of medical intervention in reducing this risk.
    • To allow decisions to be made about the appropriate management of a person’s risk.
    • To reduce psychological distress by engendering trust in the validity of the risk assessment.

    Reduction of risk

    The only way to reduce the health risks which result from irradiation is to minimise the total dose of radiation a person receives. Depending on the situation this can be achieved by a range of means:
    • Evacuation from a contaminated site, terminating ongoing exposure
    • Providing sheltering to reducing contact with radioactive material or exposure to radiation
    • Surface decontamination (removal of clothes, washing) to remove radioactive material
    • Internal decontamination to increase the rate at which radionuclides are removed from the body, or block their uptake into the body
    The aim of risk reduction measures is to achieve an acceptable level of risk, not eliminate the risk from radiation entirely. It is assumed that there is a linear relationship between radiation dose and health risks and therefore there is no absolutely ‘safe’ dose of radiation which is free of adverse effects. Risk should be minimised by reducing the total radiation dose received by the public to a level As Low As Reasonably Achievable (ALARA). What is reasonably achievable depends on the availability of facilities for removing people from a contaminated site and providing decontamination. It is also reasonable to accept a higher dose of radiation if this is necessary to achieve a reduction in other risks faced by the person. In people who have suffered trauma in an explosion, for example, complete decontamination may be a lower priority than preventing death from these injuries. Similarly, the health risks posed by some medicines, such as DTPA, used for internal decontamination may exceed the risks from low-level radiation exposure.

    In defining ALARA it is useful to consider how this has been applied to the public or those working with radiation. An average exposure of 1 mSv per year is considered an acceptable risk for the general public and reflects environmental exposure to radiation. This may be exceeded in some years provided the 5 year average is 1mSv. Occupational exposure among nuclear industry workers/radiologists etc. is acceptable at 20mSv per year averaged over five years. A bone scan, a medical procedure which involves a relatively high dose of radiation, involves exposure to about 30 mSv of radiation, compared to a dental x-ray at 20 ÁSv. The medical use of radiation is a situation in which an increased risk from irradiation has been ‘traded off’ or justified against the benefit from performing an investigation.
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