Most patients are aware that there is some radiation risk involved in diagnostic imaging procedures. However, their knowledge is often negatively biased by media reports which tend to highlight mainly the negative aspects of radiation. So what are some of the risks of diagnostic radiation to patients and how real are these risks?

Effects of radiation

The biological changes in cells and tissues following radiation usually occur only after a period of time that may range from hours (e.g., after accidental exposure) to years (e.g., in the case of radiation-induced cancer) after the exposure. The length of the latency period and the effects of radiation depend largely on the type of cell and the amount of radiation involved.

At doses above about 50 rad, humans cells may be destroyed by radiation but smaller doses of radiation may result in non-lethal changes that can cause mutations or cancer. This is probably the main concern of most patients subjected to diagnostic radiation although there are some lesser known effects such as induction of cataracts.

In general, much of what we know of the effects of radiation in humans is the result of studies on survivors of the atomic bomb or other major radiation disasters such as the Chernobyl incident. Where cancers are concerned, there is increased incidence of malignancies of the skin, lung and bone as well as blood dyscrasias among the survivors.

For instance, the incidence of leukemia among survivors of the atomic bomb is estimated to be 3 to 5 times higher than in the unexposed population. High incidence of thyroid carcinomas has also been reported in children living in areas with high radioactive contamination from the Chernobyl accident.

These long-term effects such as the induction of leukaemia and cancers are not perfectly understood and there are debates and controversies regarding the data, much of which is projected or estimated.

However, from what is currently known of the dose effects of radiation, the National Radiation Protection Board (NRPB), UK and the International Commission on Radiological Protection (ICRP) have issued guidelines on radiation dose limits for both radiation workers and the public (Table 1).

Table 1 Radiation dose limits for radiation workers and the public

With regard to patients however, there is no dose limit set as it is a balance of the benefits derived from the examination versus the potential risks.

Radiation doses of diagnostic procedures

In general, although the radiation doses involved in diagnostic imaging are quite low, they have been classified by the IRCP as having negligible, minimal, very low or low risk. Some examinations (for example chest X ray : dose ~ 0.03 mSv) have doses that are similar to what a person would receive during a 7 hour transatlantic flight.

Another simple way to view the doses involved is to see them in the light of the equivalent period of background radiation which we are naturally subjected to. This is summarised in Table 2.

Table 2 : Radiation dose of various diagnostic procedures and estimated equivalent period of background radiation

*UK average = 2.2 mSv per year: Regional averages range from 1.5 to 7.5 mSv per year

In general, it is estimated that the increased risk of cancer from diagnostic radiation doses ranges from 1 in a few million to 1 in a few hundred thousand depending on the type and frequency of examination performed.

Radiation Protection

Notwithstanding the low risks involved, physicians adopt a number of radiation protection measures that further decrease the risk of adverse effects of radiation. These include:-

a) Justification of procedure – ensure there is a genuine need for the examination before requesting for it i.e. ensure that the benefits outweigh the risks

b) Judicious choice of examination – use examinations which do not involve ionising radiation where possible if these can provide the required information e.g. ultrasound or MRI instead of CT scans in some cases.

c) Optimisation of protection - optimise the procedure to administer a radiation dose which is as low as reasonably achievable (ALARA principle) and use of protective shield where applicable.

Conclusion

Imaging plays a vital role in the diagnostic work-up of many patients nowadays. It is also used for follow-up of pathology as well as a screening tool. Thus the number of imaging studies that are performed worldwide has also increased proportionally.

While the radiation dose from these examinations is not completely innocuous, patients can rest assured that their clinicians and radiologists are well-trained to cope with the situation and as with patient’s medication, only the appropriate examination and the “correct dose” will be given.

Dr Balaji Ravikanth
Clinical Associate

Quek Swee Tian
Senior Consultant

Department of Oncologic Imaging Natio
nal Cancer Centre Singapore