Optimising radiation dose

The use of radiological investigations is an accepted part of medical practice justified in terms of clear clinical benefits to the patient, which should far outweigh the small radiation risks. However, even small radiation doses are not entirely without risk. A small fraction of the genetic mutations and malignant diseases occurring in the population can be attributed to natural  background radiation. Diagnostic medical exposures, being the major source of man-made radiation exposure of the population, add about one-sixth to the population dose from background radiation.

Statutory Regulations [IR(ME)R 2000] require all concerned to reduce unnecessary exposure of patients to radiation. Responsible organisations and individuals using ionising radiation must comply with these regulations. One important way of reducing radiation dose is to avoid undertaking investigations unnecessarily (especially repeat examinations).

The effective dose for a radiological investigation is the weighted sum of the doses to a number of body tissues, where the weighting factor for each tissue depends upon its relative sensitivity to radiation-induced cancer or severe hereditary effects. It thus provides a single dose estimate related to the total radiation risk, no matter how the radiation dose is distributed around the body [See Table 1].

Table I Typical effective doses from diagnostic

medical exposure in the 2000s [21]

Diagnostic procedure

Typical effective dose (mSv)

Equiv. no. of chest x-rays

Approx. equiv. period of natural background radiation¹

Radiographic examinations:

<0.5

1

3

0.7

35

50

20

35

35

2.4

120

75

130

150

360

100

8

400

500

Radionuclide studies:

Lung ventilation (Xe-133)

Typical effective doses for some common diagnostic radiology procedures range over a factor of about 1000 from the equivalent of a day or two of natural background radiation (e.g. 0.02 mSv for a chest radiograph) to 4.5 years (e.g. for CT of the abdomen).  The doses for conventional x-ray examinations are based on results compiled by the NRPB from patient dose measurements made in a large sample of hospitals throughout the UK from 1990 to 2000 [21].  They are mostly lower than those given in the first to third editions of this booklet, which were based on data from the early 1980s, indicating a gratifying trend towards improved patient protection.  The doses for CT examinations and radionuclide studies are based on national surveys conducted by the NRPB and the British Nuclear Medicine Society (BNMS) and are unlikely to have changed significantly since then. 

Low-dose examinations of the limbs and chest are among the most common radiological investigations, but relatively infrequent high-dose examinations such as body CT and barium studies make the major contribution to the collective population dose.  The doses from some CT examinations are particularly high and show no sign of decreasing.  The use of CT is still rising.  CT now probably contributes almost half of the collective dose from all x-ray examinations.  It is thus particularly important that requests for CT are thoroughly justified and that techniques are adopted which minimise dose while retaining essential diagnostic information.  Indeed, some authorities estimate the additional lifetime risk of fatal cancer from an abdominal CT examination in an adult is around 1 in 2,000 (compared with the risk from a chest radiograph at 1 in a million) [22].  However, the overall risk of cancer in the general population is nearly 1 in 3, and in comparison to this the excess risk of a CT scan is very small and should be more than offset by the gain from a CT scan.

In these referral Guidelines the doses have been grouped into broad bands to help the referrer understand the order of magnitude of radiation dose of the various investigations.

Table 2 Band classification of the typical effective doses of ionising radiation from common imaging procedures

*The average annual background dose in most parts of Europe falls in band II.

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