What is ‘radiation’?

There are many forms of radiation, some are harmful, while others are not. Visible light, microwaves and mobile phone signals are all types of radiation called electromagnetic radiation. X-rays and gamma rays are also forms of electromagnetic radiation and can be thought of as being different colours of light we cannot see. Other types of radiation are not electromagnetic. These include alpha, beta and neutron radiation.

Sources of radiation:

All radiation needs a source. Some radiation comes from radioactive atoms. Most of these ‘radioisotopes’ occur naturally and cannot be avoided. We are exposed to radiation from naturally occurring radioactive materials in the air we breathe, the food we eat and the buildings we occupy. Most electromagnetic radiation does not come from radioactive material, however. In fact, all matter ‘glows’ electromagnetic radiation, or light. At room temperature, this light corresponds to the infrared portion of the electromagnetic spectrum and is outside the sensitivity range of our eyes. If an object is heated up sufficiently, the wavelength of light decreases until it is within the sensitivity range of our eyes, appearing as a red glow. X-ray machines, CT scanners and most radiotherapy machines produce radiation but do not contain any radioactive substances. Instead, x-rays are produced using electricity in a similar way to a lightbulb. This allows x-ray exposures to be precisely controlled and switched on and off when needed.  

Is all radiation harmful?

No. Radiation may be divided into two types: ionising and non-ionising. In the case of ionising radiation, the energy is sufficient to be able to break chemical bonds, potentially leading to DNA damage. X-rays, gamma rays and alpha and beta particles are ionising radiation, while visible light, microwaves and mobile phone signals are not. This means that visible light and mobile phone signals have a very limited, or non-existent ability to cause biological damage.

Medical radiation:

By far the largest man-made source of ionising radiation is healthcare. X-rays, CT scans, fluoroscopy and nuclear medicine use ionising radiation to produce images, but MRI and ultrasound do not. Radiotherapy also uses ionising radiation, though in much larger doses. Medical radiation is vital in the diagnosis, treatment and follow-up of many diseases and injuries. Over recent decades, the availability and use of medical radiation has increased dramatically. Although the overall impact of this, in terms of patient care, has been very positive, concerns have been raised about the doses patients could be receiving.

What are the consequences of DNA damage?

When DNA is damaged by ionising radiation, two harmful consequences are possible: the damaged cell can die, or it can survive but be left in a mutated state. The cell killing effects of ionising radiation are only important if a sufficient number of cells is killed to result in a physiologically noticeable effect, known as a ‘tissue reaction’. The most well-known type of tissue reaction is sunburn. In the early days of x-rays, in the late 1890s and early 20^th century, many patients and x-ray workers received sunburn-like skin injuries. Due to vastly improved radiation protection practices and much lower doses, skin injuries are now extremely unlikely in diagnostic imaging. As many forms of ionising radiation can penetrate through the body, tissue reactions are not restricted only to the skin.

The consequences of cell mutation are different. Theoretically, a single mutated cell could lead to the development of cancer. This means there is no minimum exposure required for there to be a risk of cancer. Cancer may be an unlikely consequence of radiation exposure, but it is theoretically possible.

What do we know about the cancer risks from ionising radiation?

It depends on the dose. Radiation doses are measured in units of gray (Gy). The localised dose from radiotherapy (i.e. the dose to the tumour) is typically 10-70 Gy. We know beyond doubt that such high doses increase the risk of developing cancer. This risk is small, however. The majority of cancer survivors treated with radiotherapy do not develop a further cancer.

The radiation doses from diagnostic x-ray exposures are thousands of times smaller than the doses from radiotherapy (usually <0.01 Gy, or 10 mGy). In theory, this means the risks are also thousands of times lower. For example, a chest x-ray is associated with an estimated lifetime risk of fatal cancer of 1 in 1 million (0.0001%). A typical CT scan would carry an estimated risk of around 1 in 10,000  (0.001%) (these risk estimates are averaged across all ages, male and female). We cannot prove these risks, however, as current studies lack sufficient statistical power. What this means is that we cannot tell if the observed difference in the number of cancer cases following low dose exposures are simply due to random fluctuations, rather than a real effect. Most of what we know about the risks from low-moderate doses of ionising radiation comes from studies of the survivors of the atomic bombings of Hiroshima and Nagasaki. These suggest that cancer risks from doses typical of diagnostic x-ray procedures and CT scans are likely to be very low. However, due to the statistical uncertainty at low doses, we cannot rule out the possibility that the risks are zero.

My child has had an x-ray examination or CT scan. What are the risks?

If you or your child has had an x-ray examination, you may be concerned about the potential risks. This is a very difficult question to answer, firstly because of the large uncertainties in risk at low doses, but also because of the large variation in dose from one procedure to another. By law, x-ray examinations can only be requested if there is sufficient net benefit to the patient. In other words, the benefits of the information provided by the x-ray exposure must outweigh the risks.

As a very approximate guide, most CT examinations will result in an estimated lifetime risk of cancer of 1 in 1000-10,000 (0.1% - 0.01%) for an average person. The estimated risk for most normal x-ray examinations (general radiography) is less than 1 in 10,000 (0.01%), with many below 1 in 1 million (0.0001%). The examinations with the lowest doses, and hence lowest risks, are chest x-rays, extremity x-rays (hands, wrists, feet and ankles) and bitewing dental x-rays. Risks for many types of cancer are higher for people exposed at a younger age and tend to be somewhat higher for females.

How can radiation both cure and cause cancer?

As described above, radiation can either kill cells or leave them in a mutated state. These two outcomes are mutually exclusive. In the case of radiotherapy, the aim is to irradiate the tumour with a sufficiently high dose to kill most, or all cancer cells. Few cells in the treatment volume are left alive and thus prone to become mutated. Healthy tissues outside the treatment volume will also receive a radiation dose, especially in the beam bordering area. In this case, the cell killing effect is much smaller and some damaged cells may survive but be left in a mutated state. Radiotherapy is an effective cancer treatment, but undoubtedly increases the risk of developing a further cancer in years to come.

Why does this matter?

The assumption that small doses of ionising radiation are harmful, even if the risks are likely to be very low, has important consequences. It means clinicians need to be careful with how many x-ray examinations they request. It means needing to weigh up the advantages of providing diagnostic information with the potential for increased cancer risks. It also means a trade-off between image quality and radiation dose. All of these decisions need to be made in the context of large uncertainties in the magnitude of risks. If the risks are being underestimated, it could mean diagnostic x-ray imaging is being overused, resulting in an unacceptable cancer burden on society. If risks are being overestimated, patients may be being wrongly denied x-ray examinations, potentially resulting in missed diagnoses.