What reference doses should we use?

Key Points

• A wide variation is observed in patient doses from dental radiography. This indicates a significant potential for dose reduction.

• The concept of a reference dose is a useful aid to help identify where action is most urgently needed.

• Reference doses of 4 mGy (patient entrance dose) for an adult mandibular molar intra-oral radiograph and 65 mGy mm (dose-width product) for a standard adult panoramic radiograph are recommended.

Abstract

Objective

To establish reference doses for use within dental radiography.

Design

Retrospective analysis, single centre.

Setting

UK General Dental Practice, 1995–1998.

Method

A statistical analysis was performed on the results from NRPB evaluations of dental x-ray equipment within general practice. The third quartile patient entrance dose was determined from 6,344 assessments of intra-oral x-ray equipment. The third quartile dose-width product was determined from 387 assessments of panoramic x-ray equipment.

Results

The third quartile patient entrance dose for an adult mandibular molar intra-oral radiograph is 3.9 mGy. The third quartile dose-width product for a standard adult panoramic radiograph is 66.7 mGy mm.

Conclusion

NRPB recommends the adoption of reference doses of 4 mGy for an adult mandibular molar intra-oral radiograph and 65 mGy mm for a standard adult panoramic radiograph. These reference values can be used as a guide to accepted clinical practice. Where radiography is carried out using doses above these reference values, a thorough review of radiographic practice should be made to either improve techniques, or justify keeping the current techniques. However, attainment of doses at or below the reference values cannot be construed as achievement of optimum performance; further dose reductions below the reference value are still practicable.

Main

Reference doses for dental radiography Napier I D Br Dent J 1999; 186: 392–396

Comment

Many dental practitioners would find it helpful to know that their radiation protection measures are effective and to see how typical radiation doses delivered in their surgeries compare with national patterns. Napier presents the results of an analysis of the patient entrance doses for two commonly taken dental projections, and enables the practitioner to make this comparison. His extensive data has been gathered from the x-ray equipment postal pack surveys offered by the NRPB's Dental X-Ray Protection Service.

Dental radiographs account for around 25% of all radiological investigations undertaken in the UK, making them one of the most common radiographic examinations.¹ Although dental radiographs contribute only a small amount to the collective radiation dose of the UK population, when such high numbers of radiographs are involved there is always scope for dose reduction.

There are two approaches to dose reduction; by reducing the numbers of radiographs taken and by reducing dosages from individual radiographs. The Royal College of Radiologists, taking the former approach, published guidelines for doctors on prescribing radiographic investigations,² which set out to reduce the number of unnecessary and unproductive examinations. Recently similar guidelines on selection criteria in dental radiography have been published by the Faculty of General Dental Practitioners,³ suggesting timing and indications for dental radiographs in a variety of clinical situations. Both publications categorise recommendations by the level of evidence that exists thus making these, as far as possible, evidence-based.

Dose reduction represents the alternative approach. Both legislation and published guidelines have addressed this issue.^1,4 Despite the existence of these, this paper usefully highlights the fact that there is still wide variation in the doses received by patients undergoing similar dental radiographic examinations. Dose measurement is a complex area. Skin entrance doses measured here are a simple measure of the radiation reaching the patient but give an indication of the radiobiological harm. Napier is justified in suggesting that some doses — which may be as much as 13 times higher than the average — should be brought into line with those to which most patients are exposed. This is not only a problem in dentistry — the NRPB have identified a similar problem in general x-ray departments. Here they have recommended the introduction of 'reference doses' for common examinations,⁶ suggesting dose reduction targets at or below this level.

Napier extends the concept of 'reference doses' to dental radiography, and, as elsewhere, sets this at the third quartile. Thus 75% of practitioners already deliver less that the reference dose. It is primarily the 25% who expose patients to doses in excess of this (and he shows that some are far in excess of this) that are recommended to implement urgent dose control. A further target, the 'achievable dose', is set at the mean national dose.

The aim of a reference dose is to set thresholds above which immediate action is needed to reduce the dose. Practitioners producing exposures below the reference dose should now be looking at how they may continue to work toward doses as low as reasonably practicable.