National reference doses for common radiographic, fluoroscopic and dental X-ray examinations in the UK.

The National Patient Dose Database (NPDD) is maintained by the Radiation Protection Division of the Health Protection Agency. The latest review of the database analysed the data collected from 316 hospitals over a 5-year period to the end of 2005. The information supplied amounted to a total of 23 000 entrance surface dose measurements and 57 000 dose-area product measurements for single radiographs, and 208 000 dose-area product measurements along with 187 000 fluoroscopy times for diagnostic examinations or interventional procedures. In addition, patient dose data for dental X-ray examinations were included for the first time in the series of 5-yearly reviews. This article presents a summary of a key output from the NPDD - national reference doses. These are based on the third quartile values of the dose distributions for 30 types of diagnostic X-ray examination and 8 types of interventional procedure on adults, and for 4 types of X-ray examination on children. The reference doses are approximately 16% lower than the corresponding values in the previous (2000) review, and are typically less than half the values of the original UK national reference doses that were derived from a survey in the mid-1980s. This commentary suggests that two of the national reference doses from the 2000 review be retained as diagnostic reference levels because the older sample size was larger than for the 2005 review. No clear evidence could be found for the use of digital imaging equipment having a significant effect on dose.

What are the risks from medical X-rays and other low dose radiation?

The magnitude of the risks from low doses of radiation is one of the central questions in radiological protection. It is particularly relevant when discussing the justification and optimization of diagnostic medical exposures. Medical X-rays can undoubtedly confer substantial benefits in the healthcare of patients, but not without exposing them to effective doses ranging from a few microsieverts to a few tens of millisieverts. Do we have any evidence that these levels of exposure result in significant health risks to patients? The current consensus held by national and international radiological protection organizations is that, for these comparatively low doses, the most appropriate risk model is one in which the risk of radiation-induced cancer and hereditary disease is assumed to increase linearly with increasing radiation dose, with no threshold (the so-called linear no threshold (LNT) model). However, the LNT hypothesis has been challenged both by those who believe that low doses of radiation are more damaging than the hypothesis predicts and by those who believe that they are less harmful, and possibly even beneficial (often referred to as hormesis). This article reviews the evidence for and against both the LNT hypothesis and hormesis, and explains why the general scientific consensus is currently in favour of the LNT model as the most appropriate dose-response relationship for radiation protection purposes at low doses. Finally, the impact of the LNT model on the assessment of the risks from medical X-rays and how this affects the justification and optimization of such exposures is discussed.

Implementation of DRLs in the UK.

This article describes the system of regulation and practical guidance that has been developed in the UK for implementing the requirement in the EC Medical Exposure Directive that all Member States shall promote the establishment and use of diagnostic reference levels (DRLs) for medical X-ray examinations. In particular, it describes the role of two national patient dose databases maintained by NRPB, which provide important sources of information on which formally adopted numerical values for 'national DRLs' will be based. One database deals with radiographic and fluoroscopic examinations and the recommended 'national reference doses' from the latest review of this database are discussed. The other database deals specifically with computed tomography (CT) examinations, which now account for 50% of the collective dose to the UK population from all medical X rays and are consequently of particular radiation protection concern. The first analysis of this CT database is still underway, but some encouraging indications of a reduction in patient dose for some CT examinations are reported. Progress in formally adopting numerical values for 'national DRLs', as required by the UK regulations, and the provision of authoritative guidance on the implementation of DRLs at the local level, are also discussed.

Radiation protection dosimetry for diagnostic radiology patients.

The radiation protection of patients undergoing medical X-ray examinations is governed by the principles of justification and optimisation. Radiation dosimetry is required to inform medical practitioners of the levels of exposure and hence the risks from the diagnostic procedures that they have to justify and to assist the operators of X-ray imaging equipment to determine whether their procedures are optimised. This paper describes the main dosimetric methods that have been developed to meet these requirements. Suitable radiation risk projection models are used to predict the risks to patients in the UK from computed tomography examinations, as a function of age at exposure and sex, and show that the lifetime risk of fatal cancer can reach 1 in 1000 for children. The concept of 'diagnostic reference levels' as an aid to the optimisation of medical exposures is described, and progress in implementing them in the UK is reported.

UK population dose from medical X-ray examinations.

OBJECTIVE: To assess the annual per caput and collective effective dose to the United Kingdom population from medical and dental X-ray examinations. METHOD: The results of a detailed survey of the frequency of X-ray examinations during the financial year 1997/1998 were combined with contemporary data on the effective doses typically received by patients. The resulting per caput and collective dose for 1997/1998 was updated to 2001/2002 by using annual statistics on the total numbers of computed tomography (CT), interventional and conventional examinations collected by the English Department of Health. RESULTS: The annual per caput effective dose for the UK in 2001/2002 was estimated at 0.38 mSv. Over the last 10 years CT has more than doubled its contribution and is now responsible for 47% of the collective dose from medical X-rays. The contribution from conventional radiographic and fluoroscopic examinations has nearly halved to about 34%. Interventional and angiographic procedures together contribute the remaining 19%. CONCLUSIONS: The annual per caput effective dose of 0.38 mSv is low in comparison with other countries having similarly developed systems of health-care. This is due to both a lower frequency of X-ray examinations per head of population and generally lower doses in the UK than in other developed countries.

Influence of patient age on normalized effective doses calculated for CT examinations.

Monte Carlo simulations of CT examinations have been performed to estimate effective doses, normalized to axial air kerma, for six mathematical phantoms representing ages from newborn to adult, and for three CT scanner models covering a range of designs. Organ doses were calculated for CT exposures of contiguous, 1 cm wide, transverse slices in each phantom and summed to give normalized effective doses for scans of four regions of the trunk and head. In all cases an inverse trend is observed between normalized effective dose and phantom age, with the dose to the newborn from head and neck scans being 2.2-2.5 times higher than that to the adult, depending on scanner model. Corresponding increases for scans of the trunk region are more variable between scanners and range from a factor of 1.3 to 2.4. If typical clinical exposure conditions for adults are also utilized for children, then, for example, the effective dose to the newborn from a chest scan could be above 15 mSv. It is concluded that CT has the potential to deliver significantly greater radiation doses to children than to adults and in view of their greater susceptibility to radiation effects, special efforts should be made in clinical practice to reduce doses to children by the use of size-specific scan protocols.

Diagnostic medical exposures in the U.K.

Surveys in the U.K. have identified the patterns of patient exposure from diagnostic medical radiological procedures and have led to the development of structured advice to promote optimisation of patient protection, including diagnostic reference levels for some common conventional X-ray examinations. Trends for reductions in individual patient and collective doses from these particular procedures have been offset by increasing application of computed tomography. Practice in diagnostic nuclear medicine is conducted by authorized physicians on the basis of recommended maximum usual activities of radiopharmaceutical for specific procedures.

Technical note: potentially higher patient radiation doses using digital equipment for barium studies.

Digital fluorography offers several advantages over conventional radiographic systems. The general expectation of digital equipment has been of a lower radiation dose to the patient. A series of dose-area product measurements on two digital sets during barium meal and barium enema examinations has shown that this expectation is not necessarily correct, particularly when the dose from the fluoroscopic part of the examination is considered. Users of digital fluorography sets are advised to check the performance of their equipment by suitable dose measurements.

Patient radiation doses from enteroclysis examinations.

Data relating to patient dose have been acquired for enteroclysis examinations (small bowel enemas) performed at the John Radcliffe Hospital, Oxford, on 23 adult patients. Dose-area products, fluoroscopy times and the number of radiographs taken are used to compare the examination procedure at the John Radcliffe Hospital with enteroclysis and barium follow-throughs performed elsewhere. The mean dose-area product for the 23 examinations was 6.8 Gy cm2 and the mean effective dose was estimated to be 1.5 mSv. These doses are intermediate between those arising from barium meals and barium enemas performed in the same room.

Estimation of effective dose from dose-area product measurements for barium meals and barium enemas.

The International Commission on Radiological Protection has recommended the use of a new dose quantity "effective dose". The doses to 22 organs of the human body are required for a formal calculation of this quantity. This paper shows that a quick estimate of the effective dose received by a patient from a barium meal or a barium enema can be made by multiplying the measured dose-area product for the complete examination by an appropriate conversion coefficient. Despite the varied techniques used for barium examinations, such an estimate is unlikely to be in error by more than 25%.

A comparison of diagnostic radiology practice and patient exposure in Britain, France and Italy.

Surveys have been conducted in Britain, France and Italy, using essentially the same techniques, to establish the level of provision of diagnostic radiology services, the frequency of X-ray examinations and examples of the radiation doses delivered to patients in each country. Different national strategies for conducting some types of X-ray examination and marked differences in the general availability of this aspect of health care indicate that the justification and optimization of medical exposures is not interpreted in the same way in these countries.