Diagnostic radiology

What are the radiation risks associated with medical x-ray examinations?
What is a safe dose of medical x-rays?
Is any account taken of previous x-rays when ordering a new x-ray examination?
What is the maximum number of x-ray examinations that are advisable in one year?

Nuclear medicine

How do the doses and risks from nuclear medicine examinations compare with those from x-ray

What are the radiation risks associated with medical x-ray examinations?

The risks from medical x-ray examinations are generally extremely low, being in the nature of a very slight increase in the probability of cancer occurring many years or decades after the radiation exposure. Unfortunately, we all have a one in four chance of dying from cancer whatever the cause may be, so an x-ray examination will merely serve to increase this relatively large underlying risk by a very small amount. For the most common x-ray examinations, such as those of the arms, legs, chest or teeth, the radiation doses are very low and the increased risk is insignificant at less than the one in a million level. Even for high-dose examinations involving many x-ray films and fluoroscopy (for example, barium enemas) or computed tomography scans (CT scans) of a substantial part of the body, the risk is no more than about one in a few thousand. Typical lifetime additional risks for a number of common x-ray examinations are shown in the Patient Dose information table.

Another way of putting the doses of radiation from medical x-rays into perspective is to compare them with the natural background radiation that we are all exposed to all of the time. This background radiation comes from naturally occurring radioactive material in the ground and from cosmic rays irradiating the earth from outer space. Low-dose x-ray examinations of the chest, teeth and limbs involve similar doses of radiation to those received in a period of just a few days from natural background radiation. Relatively high-dose procedures such as barium enema examinations of the colon and CT body scans, on the other hand, involve doses equivalent to a few years of natural background radiation. Typical equivalent periods of natural background radiation for a number of common x-ray examinations are also shown in the table.

As it will take many years or even decades for a cancer to develop after exposure to radiation, the risks are reduced even further for people who are elderly at the time of exposure. For many people over sixty years of age there will simply not be sufficient time in their remaining lifespan for a radiation-induced cancer to develop. Conversely, children undergoing x-ray examinations may have up to twice the lifetime risk of a middle-aged person, from the same x-ray examination.

What is a safe dose of medical x-rays?

The doses of radiation used in medical diagnosis are relatively low and the risks associated with them are very small in comparison with the direct benefit to the patient from the improved diagnosis. The 'correct' dose for a particular x-ray examination is essentially the minimum that is required to make an accurate diagnosis. This will vary from patient to patient depending on their particular medical problem, on their physique and on the type of examination that is required. It is therefore not possible to specify 'correct' or 'safe' doses for examinations of different parts of the body which are generally applicable to all patients. Rather, an x-ray examination should first be justified by ensuring that there is a clear medical benefit to the patient and then the dose should be kept as low as reasonably practicable without compromising the diagnostic value.

If patients are concerned about the possible risks from having an x-ray examination they should seek reassurance from their doctor that the examination is indeed necessary and that it is likely to yield useful diagnostic information. All reasonable steps should be taken to minimise these risks and to be sure that the x-ray examination is justified in terms of an overriding health benefit to the patient. In most cases the risk to the patient's health from not obtaining an accurate diagnosis by foregoing or restricting the use of x-rays, will be much greater than the very small risks from the radiation.

Is any account taken of previous x-rays when ordering a new x-ray examination?

The doctor should be sure that the diagnostic information expected from the new x-ray examination is not already available from any previous examinations, even if they were requested by another doctor or undertaken in another hospital. If he or she is sure, the need for the new x-ray examination should be determined on its own merits, in relation to the patient's present medical condition. There is no need to consider previous medical exposures, as long as the new examination is fully justified and optimised so that the benefits to the patient outweigh the small radiation risks for this examination. The size of the additional risk (and the benefit) from this new examination is unaffected by previous exposures, even if they were taken only a short while ago.

What is the maximum number of x-ray examinations that are advisable in one year?

The appropriate number of x-rays for a particular patient depends entirely on that patient's medical condition. The doctor must, however, be sure that each x-ray examination he or she requests will provide useful information for the treatment or management of the patient so that there is a real benefit which will outweigh the very small radiation risk for each examination requested.

A large number of high-dose examinations (for example CT body scans) in one year may well be justified to diagnose and assess the treatment of a serious life-threatening condition.

How do the doses and risks from nuclear medicine examinations compare with those from x-ray examinations?

X-ray examinations involve exposing the patient to a well-defined beam of x-rays which passes through the part of the body under examination and is intercepted by an imaging device on the far side of the body. Nuclear medicine examinations, on the other hand, involve introducing a radioactively labelled drug into the body, usually by intravenous injection, which is preferentially taken up by the particular organ under investigation. The radioactive material emits gamma rays which pass out of the body to a gamma camera positioned over the organ of interest.

Whereas radiation is present during an x-ray examination only for the short period during which the x-ray machine is switched on, the radioactive material used in a nuclear medicine examination will continue to emit radiation, but at a rapidly decreasing rate, for many hours after the examination. The distribution of the radiation dose within the body is consequently quite different for x-ray and nuclear medicine examinations and the type of radiation is also different.

However it is possible to estimate the doses received by radiosensitive organs in the body from both of these types of examination and to combine the organ doses in a manner which takes account of their relative radiosensitivity. The combined organ dose is called the 'effective dose' and provides a measure of the total radiation risk from the non-uniform exposures typical of both x-ray and nuclear medicine examinations.

The lifetime risk of fatal cancer can be approximately estimated from the effective dose. Typical effective doses and fatal cancer risks for some common x-ray and nuclear medicine examinations are shown in the table. Equivalent periods of natural background radiation which will result in the same effective dose as the medical examinations are also shown for comparison.

The effective doses and risks for most of the common nuclear medicine procedures, particularly those involving the radionuclide technetium-99m (Tc-99m), lie in the middle of the range seen for x-ray examinations.