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Radiation is an essential part of our life

Radiation is an essential part of daily life. From birth, we are exposed to radiation from cosmic rays in our surroundings and from food and drink that may contain traces of radioactivity. In fact, even the human body contains small amounts of radioactivity (in the form of radioisotopes of potassium, caesium and radium). The typical adult body emits about 24,000 gamma rays per minute, a very tiny amount of radiation. There are many different types of radiation that can act as a source of exposure for patients. We will focus on one type, ionizing radiation, throughout most of this discussion. Radiation is called ‘ionizing radiation’ when it is powerful enough to break molecular bonds. These bonds can be broken in materials such as water or even DNA, the building blocks of human life (see question 3). Because there is evidence that ionizing radiation can cause these changes in the human body, it is important that the various sources of radiation be understood. Non-ionizing radiation used in cell phones and microwave ovens, ultra-violet radiation, infrared radiation and radiofrequency waves used in TV and radio will not be discussed here.

Not only is radiation an essential part of human life, it can also be used to improve human life. It is the role of health care providers to use ionizing radiation in medicine efficiently so as to maximize benefit and minimize risk. The International Atomic Energy Agency (IAEA) continuously works towards making radiation use safer. More information about the uses of radiation and the IAEA’s work in improving radiation protection may be found on the IAEA website (www.iaea.org). However, potential benefits from radiation use come with a risk as many other benefits of human life do. For example, when a person gets into a car to go to the market to purchase food, he/she takes a small risk of being in a motor vehicle collision. Nevertheless, the benefit we get from going to the store to purchase food far outweighs the small risk of a collision. Radiation as talked about in the above paragraph provides benefits, but also has a small risk associated with its use. The important thing is keeping the benefits overwhelming the risks.

What is the reason that scientists and medical professionals talk about ionizing radiation nowadays? And what is the reason that we are discussing this topic? The number of medical tests that use ionizing radiation is increasing worldwide. The United Nations Scientific Committee on Effects of Atomic Radiation [UNSCEAR 2008] estimates that nearly 3.6 billion X ray examinations are performed worldwide every year. We would like to explain the small risk that is present when radiation is used to diagnose or treat medical illness. X rays generated by machines that help us see inside the human body provide a source of radiation exposure that is in addition to that from the ionizing radiation present all around us. These machines produce X ray beam of various energies and intensities. Depending on the energy and intensity of the X rays used, the age and the sex of the person having the medical test, the part of the body exposed and other factors such as the person’s family history of cancer, the risk will vary. Ionizing radiation has been shown to be harmful in high doses. It is not certain if there is direct harm from the much smaller doses of ionizing radiation that are used in diagnostic medical tests in controlled situations. It is important to understand that the risk that radiation will cause harm will vary depending upon its dose. However, it has been shown that risk varies for different groups of people. For the elderly, the risk is relatively small. For children and young females, the risk is likely slightly higher. Scientists also believe that radiation risk is cumulative. That means that the more times a person has a test that uses ionizing radiation, the higher the risk. It should be remembered that as long as the person will get a benefit from the test (in other words, the test is justified), the benefits most likely far outweigh the risks.

Radiation exposure may vary depending on the source of radiation, the equipment used and the type of medical test being performed. Simply stated, the amount of radiation that comes from a piece of equipment may not be related to the total dose a patient gets. For example, in medical practice, even though a fluoroscopy machine may emit up to 50 mGy/min (3000 mGy/hr), if the machine is turned on for less than a few seconds, the total radiation dose to the patient may be very low. One can control the length of exposure and thus the total exposure. When the machine is turned off, the radiation level goes to zero. For medical staff who work around this equipment daily, steps can be taken to minimize their exposure. For example, the operator can control his or her distance from the radiation source (the farther the distance from the source, the less radiation exposure) and can wear protective clothing such as a lead apron. Thus the dose rate or the radiation levels emitted by these machines should not be used alone to determine the level of harm or the potential for harm.

Similarly, radioactive substances (so called radiopharmaceuticals) used for medical tests and which also evaluate the function of a human organ in the body are administered in small amounts that vary from 1 to 20 mCi (37 to 740 MBq). Because doctors and scientists have studied the amount of radiopharmaceuticals that are needed, the radiation dose to the patient can be controlled.

The problems that arise from the use of ionizing radiation in medical imaging are not very common. However, these problems are reported in the newspapers and television and may seem to be more common than they are. We will review a few facts about the effects of ionizing radiation. A human receiving a very large amount of ionizing radiation in medical examination is not a common occurrence unless there are multiple exposures to high dose examinations and procedures. Accidents may also be the reason that high doses are received by patients.

High doses have also been received by individuals a number of times in the history of the world. The case of the atomic bomb during World War II, and accidents at nuclear power plants account for events in recent memory. Death from acute radiation exposure is rare and occurs only in severe accident situations. More deaths were caused in Hiroshima and Nagasaki (the sites of the World War II atomic bomb explosions) because of heat (thermal effects) than by radiation exposure. Radiation has few short term effects. Thermal or heat effects are more important right after an incident. Later, in the long term, the ionizing radiation effects become more of a concern.

This is not to downplay the importance of safety considerations that are needed in any instance where radiation exposure of human beings is concerned: lessons must be learned from devastating situations such as Chernobyl in 1986 and the recent accident at Fukushima Daiichi in March 2011. However, a reasonable balance should be achieved between our understanding about radiation and the accompanying risks. This is especially true during a radiation accident when the emotional response of the public may be high.

In summary, when discussing sources of radiation exposure of humans, all different types of considerations must be taken into account, including the benefit the patient will get from the medical test compared to the small risk of harmful effects of radiation, what the source of the radiation is and the time the person is exposed to the source of radiation. When a patient gets a medical test, the source of the radiation should be carefully controlled to give the patient the smallest amount necessary to get the important medical information needed to help with their medical condition. For more information about:

1. What is radiation dose and dose rate?

Ionizing radiation consists of energetic particles (photons, protons, electrons, alpha particles or heavy nuclei) that interact with cells in the human body and deposit part or all of their energy. This may cause changes in the tissue. The amount of energy deposited in biological tissue due to radiation can be quantified in a biologically meaningful way by defining the ratio of radiation energy imparted (in joules) to the mass of the body (in kilograms). This quantity is called the absorbed dose or simply the dose and is measured in joules per kilogram (J/kg) or gray (Gy). The dose rate is the pace at which radiation dose is delivered to a point or physical object and is measured in gray per second (Gy/s). However, more than one tissue is exposed in any situation, so a quantity that adds up the risks to different tissues and captures the overall biological effects is often used. This is called the effective dose. Effective dose is expressed in sievert (Sv), which is equal to again 1 J/kg, although it has been modified by multiplication factors for radiosensitivity for different tissues and radiation types. This kind of complexity, with added difficulty as radiation dose to any particular organ is also expressed in Sv (and is called equivalent dose), requires interpretation by an expert and often leads to difficulties in correct interpretation by the public. The effective dose represents the whole body dose that would give the same cancer risk as caused by the doses that were imparted to different organs in a specific part of the body. Effective dose offers a way to compare approximately the relative risk between different radiation procedures.

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2. Why are there many dose quantities?

The discussion of radiation dose is complex. Because radiation has different effects depending on the tissue it interacts with, different dose descriptors have been developed. While this may seem to complicate the discussion, it actually makes clear the aspect of radiation that is being discussed. One can talk about the power of a radioactive source or the power of an X ray tube, but that does not really provide a measure of the biological effects. The factors to be considered when discussing radiation sources are: the type of radiation being emitted by the radioactive source (gamma rays, beta or alpha particles), the energies involved, or the pattern and distribution of X ray energies from an X ray source. The X ray emission intensity is not fixed. It may be selected automatically by the equipment or manually by the operator, depending on the body part. The human body is also made of many types of tissues that vary with the sex of the patient, the age of the patient, and the body part or organ being studied. Not all tissues are equally sensitive to the same radiation exposure. This mean that there is a unit for the strength of the radiation source, a unit for the incident radiation on body, a unit for the energy absorbed by the body and units to quantify radiation effects. Although many of these units are expressed as Sv or mSv, the meaning is different when referred to the dose to the whole body (effective dose) or to the dose to a particular organ (equivalent dose). It is for this reason that one often needs an expert’s opinion in interpreting radiation dose and the significance to the patient.

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3. What is the mechanism by which ionizing radiation may harm a living organism?

Ionizing radiation owes its name to its ability to directly or indirectly break molecular bonds. This bond may be in a water molecule or in DNA. In its simplest form, ionization is the removal of an electron from an atom leaving behind a positively charged entity. Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. As already mentioned damage to DNA may be due to direct ionization of DNA or caused by chemical interaction between DNA and another ionized atom created by radiation. Most of the time damage to the DNA caused by radiation is repaired by specialized molecular mechanisms or the cell dies (apoptosis), but sometimes the affected cell may survive with a mutation in the genetic code. This mutated cell could possibly cause unregulated cell division, which could lead to a cancerous tumour. In case of irradiation of living cells with high radiation doses cell damage is too high. Cells die in large numbers and when a sufficient number of cells are killed, tissue reactions (such as erythema, hair loss, cataract, infertility, etc.)  may occur.

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4. What are the main biological effects that may be caused by radiation?

There are two main biological effects of radiation: tissue reactions (deterministic effects), which cause an immediate and very predictable change to tissue, and stochastic effects, which relate to the potential for future harm to the tissue and the body.

Tissue reactions happen when the dose exceeds a specific threshold. Cataract formation is an example of such tissue reactions.The severity of tissue reactions, rather than their probability of occurrence, is proportional to the dose imparted to the tissue.

Stochastic effects refer to the potential for cancer occurrence and they owe their name to the random (stochastic) nature of radiation interaction with matter. Stochastic effects are thought to have no dose threshold for occurrence (the ‘linear, non-threshold theory’). Theoretically a single mutation of the DNA may cause a carcinogenic effect. However, it is important to understand that many cells may undergo mutation and yet no cancer will result. In reality cellular repair mechanisms greatly reduce this possibility. However the probability of occurrence of stochastic effects is considered to be proportional to the imparted dose, no matter how low the dose might be. The probability of occurrence of stochastic effects is additive and is proportional to the dose, whereas the severity of the cancer does not depend on the amount of imparted dose. Ionizing radiation also has the potential to cause another type of stochastic effect called ‘hereditary anomalies’. However, such effects have so far not been observed to occur in humans, although they have been documented in non-human species.

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5. What are the sources of radiation exposure?

Radiation exposure falls into two main categories: exposure from natural sources and exposure from artificial (man-made) sources. Natural radiation can be of either cosmic or terrestrial origin. Cosmic radiation consists of high energy particles (heavy nuclei [0.6%], alpha particles [11.2%], protons [86.2%] and electrons [2%]) [UNSCEAR, 2000] bombarding the earth all the time. Radiation also comes from the earth itself, such as rocks and soil and naturally occurring radiation emitting isotopes. These sources can impart doses to human tissues by external irradiation, inhalation (mainly radon gas inside buildings) or ingestion (mainly 40K). The average radiation dose from all natural sources is around 2.4 mSv per year (as a global average) and corresponds to slightly less than 80% of the total average dose received by humans. The rest of the dose comes from artificial sources, with medical radiation use being responsible for just under 20% of the total average dose. The remaining 0.40% consists of dose contributions from nuclear weapons testing fall-out, occupational exposure, nuclear power plant discharges and radiation from the Chernobyl accident. The figure below shows all contributions to the average annual dose according to the UNSCEAR report of 2008.

Mammography

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6. Can radiation be measured easily?

Although radiation is colourless, odourless and generally not detectable by the human senses, radiation can easily be detected and measured by instruments. Ionizing radiation is easily detected by using one type of detector, called a dosimeter. Today it is possible for professionals working with radiation to protect themselves by carrying real-time radiation meters equipped with sound alarm systems, which will indicate when radiation levels exceed a specified level. In the medical setting there are many methods that make accurate radiation measurements possible.

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7. How is the risk of cancer connected with dose?

Studies on large numbers of humans or populations that have been exposed to relatively high radiation doses have been and continue to be conducted throughout the world. Such populations include the survivors of the atomic bombings in Japan during World War II, survivors of major radiological accidents, as well as patients who have undergone clinical procedures utilizing high radiation doses, such as radiotherapy. Life-long follow-up studies of these populations that have received high levels of known whole body doses or specific organ doses have revealed the correlation between radiation dose and cancer. International organizations have collected and continue to collect epidemiological data, from which radiation related cancer incidence coefficients are calculated. These coefficients refer to the risk for radiation-induced cancer to specific organs or in general for cancer occurrence anywhere in the human body. Effective dose relates to the assessment of the possibility of radiation-induced cancer anywhere in the body. UNSCEAR and ICRP are examples of international organizations that collect data on radiation risk for cancer.

It is important to note that cancer is a relatively common worldwide disease. Radiation may cause an increase in the number of cancers or ‘excess cancers’ above this baseline risk. When an estimate of cancer risk is given for a population, it means that there is a potential and in quantitative terms a probability (chance) that cancer will occur. One has to see this in the light of the quite high background rate of cancer, which ranges between 14% an 30.1% depending upon sex, the country and region [Global Cancer Statistics, 2011]. For example if one says that the probability of cancer from radiation exposure is 1 in 2000 at radiation dose of 10 mSv, that implies that if 2000 persons all receive a radiological examination that imparts 10 mSv to each person, there is probability that 1 amongst them may have cancer over and above the background cancer rate, which is about 280 to 602 out of the 2000 persons

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8. How much is the risk for cancer as a result of radiation exposure?

The risk of occurrence of radiation-induced cancer depends on the total dose that was received by the person radiated. The ICRP has published [ICRP, 1991] radiation-induced cancer estimates. The following table summarizes the risk coefficients for different population groups. It should be pointed out that these estimates relate to the risk of cancer induction in any part of the human body. In addition as noted above, they refer to a population and should not be applied to individuals. Radiosensitivity is not the same for all humans; rather it is different depending on age and gender. These estimates or coefficients should only be used for the comparison of the risk arising from different practices. In the light of new epidemiological data, cancer risk factors (called tissue weighting factors) were revised in 2007 [ICRP, 2007]. Some factors were decreased (for example, for the gonads, from 0.20 to 0.08), whereas the factor for the breast was increased from 0.05 to 0.12 on the basis of new data that became available in preceding decade. The table below shows the nominal risk coefficients for cancer and heritable effects in % per Sv.

Nominal risk coefficients for cancer and heritable effects (% per Sv).
Exposed Population Cancer Heritable effects Total

ICRP 1990

ICRP 2007

ICRP 1990

ICRP 2007

ICRP 1990

ICRP 2007

Whole population

6.0

5.5

1.3

0.2

7.3

5.7

Adults

4.8

4.1

0.8

0.1

5.6

4.2

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9. What is radiation sickness?

Radiation sickness or acute radiation syndrome is an immediate or acute reaction of the human body to radiation when the dose exceeds a specific level or threshold. A whole body dose of 1 Sv is sufficient to cause radiation sickness. The severity of radiation sickness depends on the dose received. Different systems of the human body show malfunction at different doses. At relatively lower doses (3-5 Sv) haematological effects connected with reduced blood count (pancytopenia) such as infections may occur. At higher doses (5-15 Sv) the gastrointestinal system is severely affected. Nausea and vomiting may be observed in very short time after irradiation. Doses exceeding 10 Sv lead to shock and are usually deadly (lethal). At doses exceeding 15 Sv, death is unavoidable. The central nervous system breaks down and patients develop seizures and inability to control their body muscles (ataxia). Death occurs in a matter of hours to 1-2 days.

Putting the seriousness of radiation sickness into perspective, 50% of people uniformly irradiated with 3-5 Sv are expected to die within the first two months if they do not receive medical treatment. However acute radiation syndrome is very rare. It has mainly been observed following the Hiroshima and Nagasaki bombings and the 1986 Chernobyl accident, among people who were on site during the accident or in the first moments of recovery operations, when dose rates were very high. In everyday life the regulatory system adopted by governments does not allow workers and the public to be exposed to doses that might cause radiation sickness in normal circumstances. It can only occur in accident situations.

When radiation sickness occurs the treatments generally include blood transfusions and antibiotics to manage the haematopoietic effects. Anti-diarrheal and anti-vomiting drugs may also be used to relieve gastrointestinal symptoms. However the damage to the nervous system due to high doses is not treatable.

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10. How has radiation use benefitted humankind?

Since the discovery of X rays in 1895 by Roentgen and radioactivity by Becquerel in 1896, radiation has been used in many diverse sectors of human life. Starting with its use in medicine, millions of lives have been saved. Advances in technology generally make the techniques and equipment used in medicine safer but usage pattern contributes to increased radiation dose in many situations. Newer equipment is capable of providing much more medical information than equipment used in the past. For the same amount of information, the radiation dose will normally be reduced when newer technology equipment is used. However, acquiring more information normally requires a higher radiation dose. There is also the question of increased usage as a result of convenience. With digital photography cameras people tend to take more pictures and delete those that are not so good. Similarly, newer imaging equipment provides a high degree of convenience such that in just one breath hold, the full chest can be scanned by a modern CT scanner. As a result there is tendency to scan a larger area of the body or even the chest and abdomen together, which leads to a higher radiation dose.

Nuclear power is an efficient and cost effective source of energy. Knowledge gained from past experience has improved plant, fuel and waste management safety in such a manner that today the effect of the nuclear power industry on human health is undetectable and probably negligible in normal circumstances (please see question 5).

Radiological methods are also used for the sterilization of medical equipment and evenfood in numerous developing countries. Some branches of science utilize radiological methods for basic research and development of non-invasive testing methods.

Radiation has also been used by humans in detrimental ways. The nuclear bombs of Hiroshima and Nagasaki will always remind us of the dangerous side of radiation. The IAEA is continuously working along with its Member States so that such situations will not happen again and the proliferation of nuclear weapons on the planet is halted.

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11. How can radiation use be made safer?

Radiation use has proven to be safe and beneficial when radiation protection is practiced. Informing professionals involved in radiological procedures and the public about radiation protection is highly important and will keep risks at acceptable levels. Advances in technology, accident reporting, increasing quality assurance programmes and the spread of a safety culture will increase the levels of safety in all sectors where radiation is used. In the age of the internet, low cost user friendly global communication facilitates the delivery of and access to useful information on radiation safety through websites such as this. Such actions prompt the global flow of knowledge, which is not limited by national boundaries and is accessible to even those countries where fewer resources are available. Nevertheless, improving radiation safety remains an on-going endeavour and is full of challenges as technology evolves.

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References


X rays: What patients need to know
 
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