16: Radiation protection

Radiation protection

16.1 Ionising radiation and its effects

The use of ionising radiation in medicine and dentistry is governed by the statutory requirements laid down in the Ionising Radiations Regulations 1999 (which deals with protection of workers and the public) and the Ionising Radiation (Medical Exposure) Regulations 2000 (which specifically addresses patient protection). Guidance notes on the use of radiation in general dental practice, summarising this legislation, are available to all dentists.

Ionising radiation includes X-rays, gamma rays and cosmic rays. These are all high-energy, short-wavelength, high-frequency electromagnetic radiations. They exist as tiny packets of energy called photons. While gamma rays are used in hospital practice in nuclear medicine, X-rays are the only usual concern of dentists.

X-rays are produced by an electrical process in an X-ray tube (Fig. 16.1). Electrons, released from a heated tungsten filament, are accelerated in a vacuum by the application of a high voltage (typically 50–75 kV (kilovolts)) and strike a positively charged target. The sudden halt of the electrons releases energy, mainly as heat but also as X-rays. The X-rays are a mixture of photons of different energies, but low-energy (dose-producing) photons predominate.

The X-rays are filtered, usually using aluminium, to remove the low-energy photons (Fig. 16.2). The photons are then shaped into an appropriately sized beam by collimation using steel diaphragms or cylinders (Fig. 16.3).

Somatic and genetic effects of X-rays

The irradiation of cells can result in somatic effects (i.e. those occurring in the irradiated somatic cells of an individual) or genetic effects (i.e. those occurring in the germ cells and transmitted to the offspring of the irradiated individual) because of gonadal exposure. In properly conducted dental radiography, genetic effects are not usually considered because the gonads should not be irradiated.

Somatic effects can be:

All tissue effects have threshold doses below which they do not occur. Above the threshold dose, the effect is certain to occur. In dental radiography, these thresholds should never be reached. The risk from dental radiography is for stochastic effects.

Doses and risks in dental radiography

‘Radiation dose’ is a measure of the energy imparted by X-ray exposure and its biological effect. At a simple level, we can measure the energy imparted per unit mass (joules/kg), but using established methods we can calculate ‘effective dose’. This is a ‘whole body equivalent’ value which can be directly related to stochastic effects. Doses and risks vary enormously according to the type of equipment used, so it is hard to give firm figures, but recent estimations of risk (and radiation dose) are given in Table 16.1.

Table 16.1

Estimates of dose and risk in dental radiography

Technique Effective dose (microsieverts) Risk of cancer (per million)
Intraoral (bitewing, periapical) <2 <0.1
Panoramic 3–24 0.1–1.3
Lateral cephalogram <6 <0.2
CBCT (‘dento-alveolar’) 11–674 (median = 61) 0.6–37 (median = 3.4)
CBCT (‘craniofacial’) 30–1073 (median = 87) 1.6–59 (median = 4.7)
Computed tomography (as used for dental implant planning) 280–1410 15–77

Cone beam computed tomography.

These risks are calculated for a 30-year-old adult, using the nominal risk coefficient for cancer of 5.5 × 10−2/Sv. Risks for children are two to three times greater, while for older patients risk falls until, at 80 years, they are virtually negligible.

16.2 Radiation protection

The aim of radiation protection is to ensure all exposures are kept as low as reasonably achievable (ALARA principle).

Protection of patients

In dental radiography, protection of patients is achieved by three main means:

Justification

There are legal and ethical requirements that no radiological examination should be used unless there is likely to be a benefit in terms of improved prognosis or management of the patient. This implies that no X-ray examination is ever ‘routine’ and that radiographic ‘screening’ is unacceptable. Instead, radiographs should be prescribed according to the clinical needs of the patient.

Selection of bitewing radiographs: The nearest we come to ‘routine’ radiography in dentistry is with the bitewing radiograph. For dentate patients who are new to the practice (and partially dentate patients where films can be supported in the mouth), most authorities agree that a posterior bitewing examination is justified. Thereafter, the intervals between bitewing examinations should be determined by assessment of caries risk. Current UK guidelines are shown in Table 16.2, although other guidelines with slightly different intervals are available and can be found internationally. Obviously bitewing frequency for an individual patient may change if the patient changes caries risk category.

If the dentist feels that a radiographic examination is of help in assessment of bone loss in periodontal disease, then bitewing radiographs will provide the necessary information in the premolar and molar regions, providing geometrically accurate images. Where periodontal probing depths exceed 5 mm, then vertical bitewing radiographs are appropriate.

Selection of periapical radiographs: Periapical radiographs are indicated in the following situations:

This list is not exhaustive. Where there is any localised dental or alveolar problem, a periapical radiograph may be appropriate.

Selection of panoramic radiographs: In terms of image quality, panoramic radiography is inferior to good intraoral radiographs. Consequently, for most dental diagnostic uses, it is a ‘second best’ imaging technique. Possible situations where it may be useful include:

Routine ‘screening’ of all new patients is never justifiable; research has shown that the majority of patients who receive a ‘screening’ panoramic radiograph receive no diagnostic benefit from the examination.

Selection of CBCT examinations: Cone beam computed tomography (CBCT) is a relatively new technology and its capabilities are changing as equipment is refined. Despite this, one general principle can be stated: that CBCT should only be used when the question for which imaging is required cannot be answered adequately by lower-dose conventional (traditional) radiography. This view reflects the status quo in which radiation doses and economic costs associated with CBCT are usually higher than with conventional radiography.

Artefact, arising from most metals (usually dental restorations) in the scan degrade the image quality significantly throughout that axial plane, producing radiating dark bands. This is one reason why CBCT should not be used as a method of caries detection, as the artefacts can mimic radiolucency. Similarly, metal posts in roots may produce the same effect. Detailed, evidence-based selection criteria for CBCT have been developed recently for Europe (see ‘Further reading’ section).

Dose limitation

Patient doses in dental radiography can be minimised by considering the following:

• Operating potential (kilovoltage): for intraoral radiography, a minimum of 50 kV is set and 65–70 kV is recommended. This is often fixed on dental intraoral X-ray sets but is usually used to control exposures on panoramic X-ray equipment.

• Tube current-exposure time product (mAs). The current (mA) is often fixed on dental intraoral X-ray sets, while the exposure time(s) is often fixed for panoramic and CBCT machines.

• AC/DC generation of X-rays: ‘DC’ (constant potential) generators lead to fewer low-energy (dose-producing) X-ray photons.

• Filtration: aluminium filters absorb low-energy X-ray photons.

• Collimation: on intraoral X-ray sets, the beam can be restricted to a rectangle of 4 cm by 3 cm, leading to a substantial dose reduction over the conventional 6-cm-diameter round beam; all new equipment should be fitted with rectangular collimation and it should be retrofitted on older equipment; on panoramic machines, selective field size collimation facilities may be available. For CBCT equipment, there should be a choice of fields of view and examinations must use the smallest that is compatible with the clinical situation if this provides less radiation dose to the patient.

• Image receptor speed: for intraoral radiography, digital systems may offer some reduction in mAs (and hence dose) compared with film. Where film is used, E- or F-speed films should be used. For digital panoramic equipment, this factor is out of the control of the operator, but for film-based panoramic radiography, a rare-earth screen/film combination should be used (a combination of ISO speed 400 or better is used). For CBCT equipment, the choice of image receptor is out of the control of the operator, although this will influence radiation dose.

• Lead shielding of patients: the only requirement to use a lead apron is when a pregnant woman is being examined using a technique involving a beam that would pass through the fetus. Thyroid shielding can be used if that organ lies in the primary beam of X-rays.

Quality assurance

A poor-quality image means that the patient receives reduced, or no, benefit from the risk of the X-ray examination. Even in the best hands, radiographs may be produced that are ‘rejects’. A quality standard of no greater than 10% of radiographs (5% for CBCT images) being non-diagnostic has been set for general dental practice. Good quality of radiographs can be addressed by attention to all of the criteria listed in Table 16.3.

Table 16.3

Methods of assuring good-quality radiographs

Area Improving methods
Radiographic technique Use of film-holding/beam-aiming devices for intraoral radiography
Careful positioning for panoramic radiography
Careful selection and instruction of patients
X-ray set Regular maintenance and servicing, as recommended by the manufacturer
Triennia/>

Only gold members can continue reading. Log In or Register to continue

Jan 9, 2015 | Posted by in Oral and Maxillofacial Pathology | Comments Off on 16: Radiation protection
Premium Wordpress Themes by UFO Themes