15.3 Radiation Protection
Key Takeaways
- Dental X-rays use low doses, but the principle is ALARA/ALARP: justification, optimisation (collimation, fastest film/sensor, lead apron where indicated), and limitation — no dose without clinical benefit.
- Rectangular collimation reduces dose to the patient substantially compared with round collimation and is the recommended standard.
- Patient protection: lead apron (modern guidance does not require one routinely for dental intra-oral but still recommended for panoramic/thyroid where relevant), thyroid collar, and pregnancy — radiation avoided especially in first trimester.
- Staff protection: distance, time, and shielding (the inverse square law; stand behind a barrier, at least 1.5–2 m or outside the controlled area, and never in the primary beam).
- Quality assurance — equipment checks, film processing control (if applicable), and staff training/IRMER responsibilities — underpin a safe, auditable radiation service.
The Basis of Protection — ALARA/ALARP and the Three Principles
Dental radiography uses low doses of ionising radiation, but any dose carries a stochastic (cancer) risk; the philosophy is ALARA (as low as reasonably achievable) — equivalent to ALARP under UK regulation. Three principles govern practice:
- Justification — there must be a clinical benefit that outweighs the risk; every exposure must have a defined indication (selection criteria).
- Optimisation — doses must be kept as low as reasonably practicable while obtaining the diagnostic information needed (collimation, fastest receptor, correct technique).
- Limitation — doses to individuals are limited; staff exposures controlled by working practice.
Doses and Units
- Absorbed dose (gray, Gy) — energy deposited per unit mass.
- Equivalent dose (sievert, Sv) — absorbed dose weighted by radiation type; X-rays weight factor 1.
- Effective dose (Sv) — equivalent dose weighted by tissue sensitivity; a whole-body risk estimate.
Typical dental effective doses:
| Examination | Approximate effective dose |
|---|---|
| Intra-oral periapical/bitewing | 1–8 µSv |
| Panoramic | 10–25 µSv |
| Lateral cephalometric | 2–5 µSv |
| CBCT (small field) | 10–80 µSv; large field higher |
Natural background radiation is about 2.4 mSv/year (~2,400 µSv) — a useful comparator for patients anxious about dose.
Patient Dose Reduction
| Measure | Effect |
|---|---|
| Rectangular collimation | Reduces skin dose ~60–70% vs round collimation; limits beam to the receptor — the single biggest dose reduction |
| Fastest film/sensor consistent with image quality | 'E'/'F'-speed film or digital — lower dose than 'D'-speed |
| Digital receptors | ~50% dose reduction vs film |
| Proper technique | Avoid repeats — repeats double the dose |
| Lead apron and thyroid collar | Reduce scattered dose to reproductive organs and thyroid; modern guidance questions routine intra-oral apron use but recommends for panoramic and where appropriate |
| Avoid unnecessary exposures | Apply selection criteria |
Rectangular collimation is the standard of best practice in the UK.
Special Patient Groups
Pregnancy
- Avoid dental radiographs during pregnancy, especially in the first trimester (organogenesis), unless strictly necessary.
- If imaging is essential (trauma, acute infection), use a lead apron and thyroid shield, the lowest-dose technique, and justify each exposure explicitly.
- Most elective radiography is deferred to the second trimester or after delivery.
Children
- Greater lifetime cancer risk per unit dose (longer to express, more sensitive tissue).
- Use the smallest volume, lowest-dose technique, lead apron/thyroid collar, and strict selection criteria.
Staff Protection — Time, Distance, Shielding
Staff are protected by three measures:
- Distance — the inverse square law (dose ∝ 1/distance²) means moving away dramatically reduces dose. Stand at least 1.5–2 metres from the patient and tube, ideally at 90° to the primary beam and behind a barrier.
- Time — minimise the number of exposures; stand outside the controlled area during exposure.
- Shielding — lead-lined walls/doors; the controlled area is defined (typically 1.5–2 m radius from the tube) and marked.
- Never hold the film or the tube for a patient during exposure; use a holder.
- If assistance is essential (disabled/child patient), a parent/carerr wearing a lead apron holds the film; never a staff member.
Regulations
- IRR17 (Ionising Radiations Regulations 2017) — protects staff and the public; defines controlled areas, dose limits, and employer duties.
- IRMER (Ionising Radiation (Medical Exposure) Regulations 2017/2023 amendments) — protects patients; defines roles (referrer, practitioner, operator) and justification/optimisation duties.
- Local rules — written by the radiation protection adviser/supervisor (RPA/RPS); govern day-to-day safe use.
Dose limits (IRR17):
| Person | Annual effective dose limit |
|---|---|
| Employee (classified) | 20 mSv |
| Trainee / non-classified employee | 6 mSv |
| Member of the public | 1 mSv |
Quality Assurance
QA underpins a safe, auditable service:
- Equipment: regular critical examinations and routine performance tests (kVp, exposure timer, beam alignment, filtration) by a qualified person.
- Image receptors/processing: darkroom fog test, film processing control (if film used), digital sensor calibration.
- Patient dose audits: dose reference levels (DRLs) for common examinations.
- Staff training: IRMER training for referrers, practitioners, and operators; ongoing CPD.
- Repeat analysis: monitor reasons for repeat films and act on trends.
Records of all checks and incidents must be kept for audit and inspection.
CBCT-Specific Dose Optimisation
Cone-beam CT delivers a higher dose than conventional dental radiography and is justified only when it will change management that 2D imaging cannot. Optimisation follows a graded approach: use the smallest field of view that answers the question (a localised volume for a single impacted tooth, a full arch for orthodontic planning), the lowest mA and kVp consistent with diagnostic image quality, the shortest rotation or pulsed acquisition, and pixel or voxel resolution matched to the task rather than the maximum available. Children require particular care because of their greater lifetime stochastic risk. Every CBCT exposure needs a written justification by an IRMER practitioner, a recorded dose where the equipment provides it, and a report by a trained examiner; incidental findings outside the region of interest must be reported and acted on. A CBCT should never be a substitute for a good intra-oral radiograph when the latter answers the clinical question.
Which single measure most reduces the patient's skin dose from an intra-oral radiograph while still fully exposing the receptor?
Under the inverse square law, if a staff member moves from 1 metre to 2 metres from the X-ray source, the dose is reduced by a factor of approximately:
A pregnant patient requires an urgent periapical radiograph for an acute periapical infection in the second trimester. Which approach is most appropriate?
Under IRR17, what is the annual effective dose limit for a member of the public from ionising radiation?