For example, alpha particles (see Ch. 18) penetrate only a few millimeters in tissue, lose all their energy and are totally absorbed, whereas X-rays penetrate much further, lose some of their energy and are only partially absorbed. The biological effect of a particular radiation-absorbed dose of alpha particles would be considerably more severe than a similar radiation-absorbed dose of X-rays.

By introducing a numerical value known as the radiation weighting factor (W_R) which represents the biological effects of different radiations on different tissues, the unit of equivalent dose (H_T) in a particular tissue provides a common unit allowing comparisons to be made between one type of radiation and another, for example:

X-rays, gamma rays and beta particles	W_R = 1
Fast neutrons (10 keV – 100 keV) and protons	W_R = 10
Alpha particles	W_R = 20

The equivalent dose (H_T) in a particular tissue is therefore calculated as follows:Equivalent dose (H_T) = radiation-absorbed dose (D) × radiation weighting factor (W_R) in a particular tissue

SI unit:	joules/kg (J/kg)
Special name:	Sievert (Sv)
Subunit names:	millisievert (mSv) (× 10⁻³)
	microsievert (µSv) (× 10⁻⁶)

For X-rays, the radiation weighting factor W_R = 1, therefore the equivalent dose (H_T) in a particular tissue, measured in Sieverts, is equal to the radiation-absorbed dose (D), measured in Grays.

Effective dose (E)

This measure allows doses from different investigations of different parts of the body to be compared, by converting all doses to an equivalent whole body dose. This is necessary because some parts of the body are more sensitive to radiation than others. The International Commission on Radiological Protection (ICRP) has allocated each tissue a numerical value, known as the tissue weighting factor (W_T), based on its radiosensitivity, i.e. the risk of the tissue being damaged by radiation – the greater the risk, the higher the tissue weighting factor. The sum of the individual tissue weighting factors represents the weighting factor for the whole body.

The tissue weighting factors recommended by the ICRP in 1990 and revised in 2007 are shown in Table 6.1.

Table 6.1

The tissue weighting factors (W_T) recommended by the ICRP in 1990 and revised in 2007

Tissue	1990 W_T	2007 W_T
Bone marrow	0.12	0.12
Breast	0.05	0.12
Colon	0.12	0.12
Lung	0.12	0.12
Stomach	0.12	0.12
Gonads	0.20	0.08
Bladder	0.05	0.04
Oesophagus	0.05	0.05
Liver	0.05	0.04
Thyroid	0.05	0.04
Bone surface	0.01	0.01
Brain	*	0.01
Kidneys	*	0.01
Salivary glands	–	0.01
Skin	0.01	0.01
Remainder tissues	0.05*	0.12^†

^*Adrenals, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus.

^†Adrenals, extrathoracic airways, gallbladder, heart wall, kidney, lymphatic nodes, muscle, pancreas, oral mucosa, prostate, small intestine wall, spleen, thymus and uterus/cervix.

The sum of the individual tissue weighting factors represents the weighting factor for the whole body. The effective dose (E) from an individual clinical radiograph, where the X-ray beam is absorbed by different tissues, is calculated as follows:

Effective dose (E) = ∑ Equivalent dose (H_T) in each tissue × relevant tissue weighting factor (W_T)

Special name:	sievert (Sv)
Subunit names:	millisievert (mSv) (× 10⁻³)
	microsievert (µSv) (× 10⁻⁶)

The 2007 revised tissue weighting factors include the salivary glands as an individual weighted tissue and the oral mucosa in the remainder tissues. Therefore the effective dose for dental radiographic examinations may be considerably higher when calculated using these revised tissue weighting factors.

One way of calculating the effective dose is by using a tissue equivalent anthropomorphic phantom with dosimeters placed in the most radiosensitive regions, as shown in Fig. 6.1.

Fig. 6.1 A An example of an anthropomorphic phantom used in dosimetry. B One slice of the phantom showing the positions where the individual dosimeters can be placed in specific radiosensitive tissues.

As individual doses are very small from any one examination, a number of radiographic exposures are repeatedly performed (e.g. ten times) using typical exposure factors (time, kV and mA), for that particular examination. The dosimeters are then ‘read’ to give a measurement of the radiation-absorbed dose (D) in the individual tissues. Using this data both the equivalent dose (H_T) and the effective dose (E) may be calculated, as shown in the simplified flow diagram shown in Fig. 6.2.

Fig. 6.2 Flow diagram illustrating a very simplified example of how the *effective dose* is calculated for a panoramic radiograph using only five tissues of the anthropomorphic phantom. (1) The *radiation-absorbed doses* for the five tissues. (2) The calculated *equivalent doses* (W_R = 1 for X-rays) for the five tissues. (3) The *effective dose* for the investigation, which is the sum of each equivalent dose (H_T) multiplied by the relevant tissue weighting factor (W_T), which in this example is 12.15 µSv. For a real dosimetry study **all** the radiosensitive tissues irradiated, with specific weighting factors, would be included in the calculations.

When the simple term dose is applied loosely, it is the effective dose (E) that is usually being described. Effective dose can thus be thought of as a broad indication of the risk to health from any exposure to ionizing radiation, irrespective of the type or energy of the radiation or the part of the body being irradiated. A comparison of effective doses from different investigations is shown in Table 6.2.

Table 6.2

Typical effective doses for a range of dental and routine medical examinations

X-ray examination	Effective dose (E) (mSv)
Bitewing/periapical radiograph	0.0003–0.022
Panoramic radiograph	0.0027–0.038
Upper standard occlusal	0.008
Lateral cephalometric radiograph	0.0022–0.0056
Skull radiograph (posteroanterior, PA)	0.02
Skull radiograph (lateral)	0.016
Chest (posteroanterior, PA)	0.014
Chest (lateral)	0.038
CT head	1.4
CT chest	6.6
CT abdomen	5.6
CT mandible and maxilla	0.25–1.4
Barium swallow	1.5
Barium enema	2.2
Dento-alveolar cone beam CT (CBCT)	0.01–0.67
Craniofacial cone beam CT (CBCT)	0.03–1.1

Based broadly on the 2011 HPA publication Frequency and Collective Dose for Medical and Dental X-ray Examinations in the UK, the 2011 SEDENTEXCT publication Radiation Protection: Cone Beam CT for Dental and Maxillofacial Radiology and the 2013 Selection Criteria for Dental Radiography.

Collective effective dose or collective dose

This measure is used when considering the total effective dose to a population, from a particular investigation or source of radiation, and is measured in man-sieverts (man-Sv).

< ?xml:namespace prefix = "mml" />Collective dose=,effective dose(E)×population

For example, in the UK in 2008, the collective dose to the population from all X-ray examinations was calculated by the Health Protection Agency to be 24,700 man-Sv.

Dose limits

In addition to setting tissue weighting factors the International Commission for Radiological Protection (ICRP) also sets maximum annual dose limits for radiation workers – those people who are exposed to radiation during the course of their work. This exposure carries no benefit only risk. The ICRP divides radiation workers into two subgroups depending on the level of occupational exposure:

• Classified workers

• Non-classified workers.

The dose limits for each group are based on the principle that the risk to any worker who receives the full dose limit will be such that the worker will be at no greater risk than a worker in another hazardous, but non-radioactive, environment. By way of example, Table 6.3 shows the previous and current annual dose limits in force in the UK.

Table 6.3

The previous annual dose limits and those currently in force in the UK

	Old dose limits	Current dose limits
Classified workers	50 mSv	20 mSv
Non-classified workers	15 mSv	6 mSv
General public	5 mSv	1 mSv

Dose rate

This is a measure of the dose per unit time, e.g. dose/hour, and is sometimes a more convenient, and measurable, figure than the total annual dose limits shown in Table 6.3.

SI unit:

microsievert/hour (µSv h^–1)

Estimated annual doses

Everyone is exposed to some form of ionizing radiation from the environment in which we live. Sources include:

• Natural background radiation

– Cosmic radiation from the earth’s atmosphere

– Gamma radiation from the rocks and soil in the earth’s crust

– Ingestion of radioisotopes, e.g. ⁴⁰K, in certain foods

– Inhalation of radon gas and its decay products, ²²²Rn is a gaseous decay product of uranium that is present naturally in granite. As a gas, radon diffuses readily from rocks through soil and can be trapped in poorly ventilated houses and then breathed into the lungs.

– Inhalation of thoron gas

• Artificial background radiation

– Fallout from nuclear explosions

– Radioactive waste discharged from nuclear establishments

• Medical and dental diagnostic radiation

• Radiation from occupational exposure.

The Radiation Protection Division of the Health Protection Agency has estimated the annual doses from these various sources in the UK as illustrated in Table 6.4. As shown, an individual’s average dose from natural background radiation is estimated at about 2.23 mSv per year (84%) with and additional 0.423 mSv (16%) from artificial sources (total average dose approximately 2.7 mSv) including dental radiography. In the USA natural background radiation is estimated at approximately 3.2 mSv per year with an additional 3.0 mSv from artificial sources (total average dose 6.2 mSv). These figures are useful to remember when considering the magnitude of the doses associated with various diagnostic procedures, as shown previously in Table 6.2.

Table 6.4

Health Protection Agency estimated average annual doses to the UK population from various sources of ionizing radiation in 2005

Radiation source	Average annual dose (µSv)	Approximate %
Natural sources
Cosmic radiation	330
External exposure from earth’s crust	350
Internal radiation from certain foodstuffs	250
Exposure to radon/thoron and their decay products	1,300
Total from natural sources	2,230	84
Artificial sources
Fallout	6
Radioactive waste	1
Medical and dental diagnostic radiation	410
Occupational exposure	6
Total from artificial sources	423	16
Total from all sources	~2.7 mSv	100

Dose limitation

As mentioned previously, the International Commission on Radiological Protection (ICRP) regularly publishes data not only on radiation dose but also general recommendations on dose limits and dose limitation based on the following general principles:

• No practice shall be adopted unless its introduction produces a positive net benefit (Justification)

• All exposures shall be kept as low as reasonably practicable (ALARP), taking economic and social factors into account (Optimization)

• The dose equivalent to individuals shall not exceed the limits recommended by the ICRP (Limitation).

For the purposes of dose limitation, the ICRP has divided the population into three groups:

• Patients

• Radiation workers (classified and non-classified)

• General public.

In the UK the 2001 Guidance Notes emphasize that the need for, and the usefulness of, these examinations should be critically assessed when deciding whether they are justified. They also recommend that these types of examinations should be requested only by medical/dental practitioners and that the patient’s consent should be obtained.

Examinations for medical research

• There are no set dose limits.

• All research projects should be approved on the advice of an appropriate expert group or Ethics Committee and subject to Local Rules and regulations.

• All volunteers should have a full understanding of the risks involved and give their consent.

Radiation workers

As described earlier, the ICRP further divides workers who are exposed to radiation during the course of their work. into two subgroups depending on the level of occupational exposure:

• Classified workers

• Non-classified workers.

The ICRP maximum dose limits for each group as shown in Table 6.3 is based on the principle that the risk to any worker who receives the full dose limit will be such that the worker will be at no greater risk than a worker in another hazardous, but non-radioactive, environment.

The main features of each group of radiation workers are summarized below:

Classified workers

• Receive high levels of exposure to radiation at work (if Local Rules are observed this is highly unlikely in dental practice).

• Require compulsory personal monitoring.

• Require compulsory annual health checks.

Non-classified workers (most dental staff)

• Receive low levels of exposure to radiation at work (as in the dental surgery).

• The annual dose limits are 3/10 of the classified workers’ limits. Provided the Local Rules are observed, all dental staff should receive an annual effective dose of considerably less than the limit of 6 mSv. Hence, the regulations in the UK suggest the setting of ‘Dose Constraints’. These represent the upper level of individual dose that should not be exceeded in a well-managed practice, and for dental radiography the following recommendations are made:

1 mSv	for employees directly involved with the radiography (operators)
0.3 mSv	for employees not directly involved with the radiography and for members of the general public.

In addition to the above dose limits, the legal person must ensure that the dose to the fetus of any pregnant member of staff is unlikely to exceed 1 mSv during the declared term of the pregnancy.

• Personal monitoring (see Chapter 7) is not compulsory in the UK, although it is recommended if the risk assessment indicates that individual doses could exceed 1 mSv per year. The 2001 Guidance Notes state that in practice this should be considered for those staff whose weekly workload exceeds 100 intraoral or 50 panoramic images, or a pro-rata combination of each type of examination.

• Annual health checks are not required.

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

• The primary beam, if they stand in its path

• Scattered radiation from the patient

• Radiation leakage from the tubehead.

The main practical radiation protection measures to limit the dose that workers receive are described in Chapter 7.