Direct action or damage

The X-ray photons, or high-energy ejected electrons, interact directly with, and ionize, vital biologic macromolecules such as DNA, RNA, proteins and enzymes, as shown in Fig. 7.1A. This ionization results in the breakage of the macromolecule’s chemical bonds, causing them to become abnormal structures, which may in turn lead to inappropriate chemical reactions. Rupture of one of the chemical bonds in a DNA macromolecule may sever one of the side chains of the ladder-like structure. This type of injury to DNA is called a point mutation. The subsequent chromosomal effects from direct damage could include:

If the radiation directly affects somatic cells, the effects on the DNA (and hence the chromosomes) could result in a radiation-induced malignancy. If the damage is to reproductive stem cells, the result could be a radiation-induced congenital abnormality.

What actually happens in the cell depends on several factors, including:

Indirect action or damage

This process, which is shown in Fig. 7.1B, involves the ionization of the water molecule producing both ions and free radicals which can combine to damage the vital biologic macromolecules such as DNA. The sequence of events involved is summarized in Fig. 7.2. The free radicals can recombine to form hydrogen peroxide, a cellular poison, and a hydroperoxyl radical, another toxic substance. Both of these substances are highly reactive and produce biological damage. By themselves, free radicals may transfer excess energy to other molecules, thereby breaking their chemical bonds and having an even greater effect. As about 80% of the body consists of water, the vast majority of the interactions with ionizing radiation are indirect.

Tissue reactions (deterministic effects)

These are the non-cancer damaging effects, to the body of the person exposed, that will definitely result from a specific high dose of radiation. The severity of the effect is proportional to the dose received, and in most cases a threshold dose exists below which there will be no effect. They were previously referred to as deterministic effects, but are now referred to as tissue reactions by the International Commission on Radiological Protection (ICRP) because it now recognizes that some of these effects are not determined solely at the time of irradiation but can be modified after radiation exposure. They are further subdivided into:

Stochastic effects

Stochastic effects are those that may develop. Their development is random and depends on the laws of chance or probability. These damaging effects may be induced when the body is exposed to any dose of radiation. Experimentally it has not been possible to establish a safe dose – i.e. a dose below which stochastic effects do not develop. It is therefore assumed that there is no threshold dose, and that every exposure to ionizing radiation carries with it the possibility of inducing a stochastic effect. The lower the radiation dose, the lower the probability of cell damage. However, the severity of the damage is not related to the size of the inducing dose. This is the underlying philosophy behind present radiation protection recommendations described later. Stochastic effects are further subdivided into:

Cancer induction

If a somatic (body) cell is irradiated a radiation-induced malignancy cancer may develop. Quantifying the risk is complex and controversial. Data from groups exposed to high doses of radiation have been analysed and the results used to provide an estimate of the risk from the low doses of radiation encountered in diagnostic radiology. The high-dose groups studied include:

• The survivors of the atomic explosions at Hiroshima and Nagasaki

• Patients receiving radiotherapy

• Radiation workers – people exposed to radiation in the course of their work

• The survivors of the nuclear disaster at Chernobyl.

The problem of quantifying the risk is compounded because cancer is a common disease, so in any group of individuals studied there is likely to be some incidence of cancer. In the groups listed above, that have been exposed to high doses of radiation, the incidence of cancer is likely to be increased and is referred to as the excess cancer incidence. From the data collected, it has been possible to construct dose–response curves (Fig. 7.3), showing the relationship between excess cancers and radiation dose. The graphs can be extrapolated to zero (the controversy on risk assessment revolves around exactly how this extrapolation should be done), and a risk factor for induction of cancer by low doses of radiation can be calculated.

After reviewing all the available evidence, the International Commission on Radiological Protection suggest there is a 1 in 20,000 chance of developing a fatal cancer for every 1 mSv of effective dose. Using this estimate, a broad estimate of risk from various X-ray examinations may be calculated and these are shown in Table 7.1.

Table 7.1

A broad estimate of the risk of a standard 30-year-old adult patient developing a fatal radiation-induced malignancy from a variety of dental and medical X-ray examinations

X-ray examination	Estimated risk of fatal cancer
Bitewing/periapical radiograph (70 kV, round collimation, D-speed film)	1 in 1,000,000
Bitewing/periapical radiograph (70 kV, rectangular collimation, F-speed film)	1 in 10,000,000
Panoramic radiograph (average)	1 in 1,000,000
Upper standard occlusal	1 in 2,500,000
Lateral cephalometric radiograph	1 in 5,000,000
Skull radiograph (PA)	1 in 1,000,000
Skull radiograph (lateral)	1 in 1,250,000
Chest (PA)	1 in 1,430,000
Chest (lateral)	1 in 540,000
CT head	1 in 14,300
CT chest	1 in 3000
CT abdomen	1 in 3500
CT mandible and maxilla	1 in 80,000 to 1 in 14,300
Barium swallow	1 in 13,300
Barium enema	1 in 9100
Dento-alveolar cone beam CT	1 in 2,000,000 to 1 in 30,000
Craniofacial cone beam CT	1 in 670,000 to 1 in 18,200

Risk is age-dependent, being highest for the young and lowest for the elderly. The risks shown in Table 7.1 are for a 30-year-old adult. The 2004 European Guidelines on Radiation Protection in Dental Radiology recommend that these should be modified by the multiplication factors shown in Table 7.2, which represent averages for the two sexes. In fact, at all ages risks for females are slightly higher and risks for males slightly lower.

Table 7.2

The multiplication factor for risk for different age groups based on the 2004 European Guidelines on Radiation Protection in Dental Radiology

Age group (yr)	Multiplication factor for risk
<10	×3
10–20	×2
20–30	×1.5
30–50	×0.5
50–80	×0.3
80+	Negligible risk

This epidemiological information is being updated continually and recent reports suggest that the risk from low-dose radiation may be considerably greater than thought previously. However, the present figures at least provide an idea of the comparative order of magnitude of the risk involved from different investigations. Dental radiology employs low doses of radiation and hence the risk of stochastic cancer-induction is very small. However, the total number of intraoral and extraoral dental radiographs taken is very high – estimated at around 20 million per year in the UK alone. It is thought that diagnostic radiology (medical and dental) is responsible for about 700 cases of cancer per year in the UK of which about 10 cases are attributable to dental radiology – hence the need for the practical radiation measures outlined later.

Summary of the harmful effects important in dental radiology

In dentistry, the size of the doses used routinely are relatively small (see Ch. 6) and well below the threshold doses required to produce tissue reactions (deterministic effects). However, the stochastic effects can develop with any dose of ionizing radiation. Dental radiology does not usually involve irradiating the reproductive organs, thus in dentistry the heritable effects are of limited importance and the main concern is that of cancer induction.

Practical radiation protection of patients

The main radiation dose to patients comes from:

The main practical radiation protection measures can therefore be considered under three headings:

Clinical judgement

• All dentists must have received adequate training in dental radiology and should undertake continuing education and training after qualification to keep their knowledge and skills up to date, particularly in relation to the clinical applications of new technology, e.g. cone beam CT (CBCT) (see Ch. 16). This seems reasonable as it is the dentist who decides on the acceptability of the risk to which the patient is being subjected.

• Before an exposure can take place, it must be clinically justified by a dentist (i.e. assessed to ensure that it will lead to a change in the patient’s management and prognosis). Every exposure should be justified on the grounds of:

– The availability and/or findings of previous radiographs

– The specific objectives of the exposure in relation to the history and following clinical examination of the patient

– The total potential diagnostic benefit to the patient

– The radiation risk associated with the radiographic examination

– The efficacy, benefits and risks of alternative techniques having the same objective, but involving no or less exposure to ionizing radiation.

• To assist with the justification process dentists should make use of published evidence-based selection criteria. For example, in 2013 the Faculty of Dental Practice (UK) of the Royal College of Surgeons of England published the 3^rd Edition of their booklet Selection Criteria for Dental Radiography. Their recommendations are graded on the quality of the evidence available using the following scale:

• A = based on evidence from at least one study with in vitro validation as part of the body of literature of overall good quality and consistency.

• B = based on evidence from well conducted clinical trials but with no specific in vitro validation studies.

• C = based on evidence from expert committee reports or opinions and/or clinical experience of respected authorities and indicates an absence of directly applicable studies of good quality.

• NSR = based on evidence from high-quality, non-systematic literature review.

Several of their recommendations are included later in Chapter 20, 21 and 22. A flow diagram showing how imaging is justified on the basis of history, clinical examination, different clinical signs and symptoms of disease and possible treatment plans – based broadly on the recommendations on the 2013 Selection Criteria is shown in Figure 7.4.

Radiographic technique

• All staff involved in X-raying patients should:

– Have received adequate training and should undertake continuing education and training after qualification to keep their knowledge and skills up to date.

– Undertake radiography accurately to avoid retakes, for example by using image receptor holders and beam-aiming devices for intraoral radiography as described in Chapters 9 and 10, or by using patient positioning aids during panoramic radiography as described in Chapter 15

– Use the minimum number of projections

– Optimize all exposure settings to ensure that all doses are kept as low as reasonably practicable (ALARP) consistent with the intended purpose

– Ensure all image processing (chemical or computer) (see Ch. 5) is carried to the highest standards so that images do not have to be retaken

– (Consider the use of lead protection).

Confusion and controversy still surround the use of lead protection for patients in different countries. In the UK current guidance suggests that patient protection is best achieved by implementation of the practical dose reduction measures outlined above in relation to clinical judgement, equipment and radiographic technique, and not by lead protection. The latest UK Guidance Notes state:

• There is no justification for the routine use of lead aprons for patients undergoing intraoral or panoramic radiography.

• Thyroid collars, as shown in Fig. 7.5, should be used in those few cases where the thyroid may be in the primary beam. (In the authors’ opinion, this can include maxillary occlusal radiography and CBCT, and thyroid protection is therefore shown in Chs 11 and 16.)

• Lead aprons do not protect against radiation scattered internally within the body.

• Protective aprons, having a lead equivalence of not less than 0.25 mm, should be provided for any adult who provides assistance by supporting a patient during radiography.

• When a lead apron is provided, it must be correctly stored (e.g. over a suitable hanger) and not folded. Its condition must be routinely checked including a visual inspection at annual intervals.

Practical radiation protection of the general public

This group includes everyone who is not receiving a radiation dose either as a patient or as a radiation worker, but who may be exposed inadvertently, for example, someone in a dental surgery waiting room, in other rooms in the building or passers-by. The general public is at risk from the primary beam, so specific consideration should be given to:

• The siting of X-ray equipment to ensure that the primary beam is not aimed directly into occupied rooms or corridors

• The thickness/material of partitioning walls

• Advice from a medical physicist on the siting of all X-ray equipment, surgery design and the placement of radiation warning signs, as shown in Fig. 7.6.

Practical radiation protection of radiation workers

The radiation dose to dentists and their staff can come from:

The main protective measures to limit the dose that workers might receive are therefore based mainly on a combination of common sense and the knowledge that ionizing radiation is attenuated by distance and obeys the inverse square law (see Fig. 7.7).

• The main practical radiation protection measures include:

– Ensuring all radiation workers (dental staff) know the risks to their own health created by exposure to X-rays and the safety precautions they need to take, including:

– Always standing outside the so-called controlled area – approximately 1.5 m from the X-ray machine and the patient (or behind appropriate lead screens/barriers) and never in the path of the main beam, as shown in Fig. 7.8

– Never holding an image receptor in a patient’s mouth

– Never holding the X-ray tubehead during an exposure

– Always using the X-ray equipment safely and in accordance with current guidance and good practice.

Monitoring

Dental staff are almost always designated as non-classified workers by the International Commission on Radiological Protection (ICRP). As explained in Chapter 6, the ICRP sets annual dose limits for different categories of radiation workers and for non-classified workers the current limit is 6 mSv per year. In the UK, the Health Protection Agency regards an annual limit of 1 mSv as more appropriate for dental staff working in general dental practice. The amount of radiation received by individuals can be monitored and measured using a variety of different monitoring devices and can include:

• Film badges

• Thermoluminescent dosimeters (TLD)

– Badge

– Extremity monitor

• Optically stimulated luminescence dosimeters (OSLD)

• Personal electronic dosimeters (PED).

These devices do not protect against radiation. They merely provide data as to the amount of radiation that has been received over a period of time. More immediate information that the wearer is being irradiated can be provided by personal electronic dosimeters, including sophisticated systems such as the recently developed Unfors Alert dosimeter. A selection of various dosimeters is shown in Fig. 7.9.

Film badges

The main features of film badges are:

• They consist of a plastic frame (usually blue or white) containing a variety of different metal filters and a small radiographic film which reacts to radiation.

• They are worn on the outside of the clothes, usually at the level of the reproductive organs, for 1–3 months before being processed.

• They are the most common form of personal monitoring device currently in use.

Advantages

• Provide a permanent record of dose received

• May be checked and reassessed at a later date

• Can measure the type and energy of radiation encountered

• Simple, robust and relatively inexpensive

Disadvantages

• No immediate indication of exposure – all information is retrospective.

• Processing is required which may lead to errors.

• The badges are prone to filter loss.

Thermoluminescent dosimeters (TLDs)

The main features of TLDs are:

• They are used for personal monitoring of the whole body and/or the extremities, as well as measuring the skin dose from particular investigations.

• They contain materials such as lithium fluoride (LiF) which absorb radiation and then release the energy in the form of light when heated.

• The intensity of the emitted light is proportional to the radiation energy absorbed originally.

• Personal monitors consist of a yellow or orange plastic holder, worn like the film badge for 1–3 months.

Advantages

• The lithium fluoride is re-usable

• Read-out measurements are easily automated and rapidly produced

• Suitable for a wide variety of dose measurements.

Disadvantages

• Read-out is destructive, giving no permanent record, results cannot be checked or reassessed

• Only limited information is provided on the type and energy of the radiation

• Dose gradients are not detectable

• Relatively expensive.

Optically stimulated luminescence dosimeters (OSLDs)

The main features of OSLDs are:

• They are used for personal monitoring of the whole body.

• They consist of a badge containing an aluminium oxide detector and metal and plastic filters.

• The detector is read by exposing it to a light source, which releases the stored radiation energy in the aluminium oxide as blue light (luminescence).

• The radiation exposure can be calculated from the amount and intensity of blue light released.

Advantages

• Quick non-destructive readout

• Multiple readouts possible so radiation doses can be verified if required

• Good sensitivity and respond to a wide range of energies

• Relatively robust.

Disadvantages

• No immediate indication of exposure – all information is retrospective.

Personal electronic dosimeters (PEDs)

The main features of PEDs are:

• They are battery operated devices normally worn in the operator’s pocket for personal monitoring of the whole body.

• They are usually based on an energy-activated silicon diode (a solid-state detector) to measure radiation dose.

• Some also measure the dose rate and have an audible alarm to indicate radiation as it is received.

Advantages

• Direct digital read-out gives immediate information

• Once purchased, no ongoing running costs

• Relatively robust.

Disadvantages

• Initial cost can be expensive

• Only sophisticated devices give a permanent record of exposure

• No indication given of the type or energy of the radiation

• Not very sensitive to low-energy radiation, and so may not be reliable in the energy range encountered in dental radiography.