2: RADIATION DOSE AND RISK
2.1: X-rays
X-rays
are a type of electromagnetic (EM) radiation. EM radiation also includes
visible light, radio waves, microwaves, cosmic radiation, and several other
varieties of “rays”. All can be considered as “packets” of energy, called
photons, which have wave properties, most importantly a wavelength and
frequency. EM radiation can vary in wavelength from 10-13 to 103 m with X-rays
having a small wavelength of 10-9 to 10-13m. The importance of this is that
small wavelengths mean high energy, deeper penetration though matter and high
energy transfer to the matter. When X-rays hit atoms this energy can be
transferred, producing ionisation of atoms. Other examples of ionising
radiation are alpha, beta and gamma radiation, which are mostly associated with
the decay of radioactive materials. All ionising radiations have the capability
cause harm to the organs and tissues of the body of exposed persons.
2.2: Radiation damage
When
patients undergo X-ray examinations, millions of photons pass through their
bodies. These can damage any molecule by ionisation, but damage to the DNA in
the chromosomes is of particular importance. Most DNA damage is repaired
immediately, but rarely a portion of a chromosome may be permanently altered (a
mutation). This may lead ultimately to the formation of a tumour. The latent
period between exposure to X-rays and the clinical diagnosis of a tumour may be
many years. The risk of a tumour being produced by a particular X-ray dose can
be estimated; therefore, knowledge of the doses received by radiological
techniques is important. While doses and risks for dental radiology are small,
a number of epidemiological studies have provided some limited evidence of an
increased risk of brain (Longstreth et al, 1993; Preston-Martin & White,
1990), salivary gland (Preston-Martin & White, 1990; Horn-Ross et al, 1997)
and thyroid (Hallquist et al, 1994; Wingren et al, 1997; Memon et al, 2010)
tumours for dental radiography.
The
effects described above are believed to have no threshold radiation dose below
which they will not occur (European Commission, 2001). They can be considered
as “chance” (stochastic) effects, where the magnitude of the risk, though not
the severity of the effect, is proportional to the radiation dose. There are
other known damaging effects of radiation, such as cataract formation, skin
erythema and effects on fertility, which definitely have threshold doses below
which they will not occur. These threshold doses vary in size, but all are of a
magnitude far greater than those given in dental radiography. Thus, except in
extraordinary circumstances, these deterministic effects are given no further
consideration.
2.3: Radiation dose
The terms “dose” and “exposure” are
widely used but often misunderstood. “Doses”‟ may be measured for particular
tissues or organs (e.g. skin, eye, bone marrow) or for the whole body, while
“exposure” usually refers to equipment settings (time, mA, kV). A commonly used
measure of dose in surveys is “entrance dose”, measured in milliGrays
(mGy).This has an advantage of being fairly easily measured by placing
dosimeters on the patient‟s skin. Diagnostic reference levels (DRLs), based
upon surveys of entrance dose or other easily measured quantities, may be set
as standards against which X-ray equipment and their operation by clinicians
can be assessed as part of quality assurance.
In these Guidelines, however, radiation
dose is expressed as effective dose, measured in units of energy absorption
per unit mass (joules / kg) called the Sievert (more usually the microSievert,
μSv, representing one millionth of a Sievert). Effective dose is calculated for
any X-ray technique by measuring the energy absorption in a number of “key”
organs/tissues in the body. Each organ dose is multiplied by a weighting factor
that has been determined as a reflection of its radiosensitivity. These
weighted doses are added together, so that the final figure is a representation
of “whole body” detriment. While effective dose is an impossible quantity to
measure in vivo, it is possible to determine it from laboratory studies
or computer modelling. This can then be used to estimate radiation risk.
Effective dose permits a comparison of different types of examinations to
different anatomical regions.
Many studies have measured doses of
radiation for dental radiography, but only some have estimated effective dose.
Much published work on conventional dental radiographic techniques pre-dates
the recent revision of tissue weighting factors by the ICRP (ICRP 2007). This
revision altered the existing tissue weighting factors and specific weighting
factors were added for salivary glands, brain, gall/bladder, heart, lymphatic
nodes, oral mucosa and prostate. As salivary glands, brain and oral mucosa are
often irradiated during dental X-ray examinations, this means that studies
using old weighting factors will very likely give different results to those
using the new factors. Furthermore, variation in the technical parameters of the
X-ray equipment and image receptors used in studies means that care should be
taken when comparing dose estimations from different studies. Because it is a
relatively new technique, most dental CBCT dosimetry research has used the more
recent tissue weighting factors. Nonetheless, it is still important to
recognise that the doses reported for one dental CBCT machine may be quite
different to another and that ranges of dose are more appropriate to use than
absolute figures.
2.4: Radiation risk
Radiation detriment can be considered
as the total harm experienced by an irradiated individual. In terms of
stochastic effects, this includes the detriment-adjusted nominal risk of cancer
and hereditable effects. The detriment-adjusted risk factor for the whole population
is 5.7 x 10-2 Sv-1. Regarding cancer, radiation detriment
considers cancer incidence weighted for lethality and life impairment. Table
2.1 was taken from ICRP (2007) and it gives the breakdown of this summed figure
into its constituent elements. Hereditable effects are believed to be
negligible in dental radiography (White 1992) and this is also true for dental
CBCT.
Risk is age-dependent, being highest
for the young and least for the elderly. Here, risks are given for the adult
patient at 30 years of age. These should be modified using the multiplication
factors given in Table 2.2 (derived from ICRP 1990). These represent averages
for the two sexes; at all ages risks for females are slightly higher and those
for males slightly lower.
Beyond 80 years of age, the risk
becomes negligible because the latent period between X-ray exposure and the
clinical presentation of a tumour will probably exceed the life span of a
patient. In contrast, the tissues of younger people are more radiosensitive and
their prospective life span is likely to exceed the latent period.
Table 2.1: Detriment-adjusted
nominal risk coefficients for stochastic effects
Detriment (10-2Sv-1)
|
|
Cancer
|
5.5
|
Hereditable effects
|
0.2
|
Total
|
5.7
|
Table 2.2: Risk in relation to
age. These data are derived from (ICRP 1990) and represent relative
attributable lifetime risk based upon a relative risk of 1 at age 30
(population average risk). It assumes the multiplicative risk projection model,
averaged for the two sexes. In fact, risk for females is always relatively
higher than for males.
Age group (years)
|
Multiplication factor for risk
|
<10
|
x 3
|
10-20
|
x 2
|
20-30
|
x 1.5
|
30-50
|
x 0.5
|
50-80
|
x 0.3
|
80+
|
Negligible risk
|
2.5: Doses and risks with CBCT
The literature review conducted by the
SEDENTEXCT project included 13 studies in which dosimetry for dental CBCT was
performed and in which effective dose was calculated either using the ICRP (2007)
tissue weighting factors or using the ICRP (1990) tissue weighting factors with
the radiosensitivity of the salivary glands and brain taken into account. Two
further studies from the SEDENTEXCT Consortium were also included (Pauwels et
al, in press; Theodorakou et al, in press). Table 2.3a shows the
reported effective doses for a range of dental CBCT units collated from the
studies reviewed, all of which used “adult” phantoms. Table 2.3b provides
equivalent data using paediatric phantoms conducted as part of the SEDENTEXCT
project by Theodorakou et al (in press). The more restricted dose range
seen for paediatric phantom studies reflects the relatively limited range of
equipment studied by Theodorakou et al (in press) and the exclusion of
the higher dose equipment included in Table 2.3a.
Pauwels et al (in press)
presented data on average relative contribution of organ doses to effective
dose in dental CBCT (Fig.2.1). The bulk of the contribution comes from
remainder organs, salivary glands, thyroid gland and red bone marrow. For the
paediatric phantom, the remainder organs, the salivary glands and the thyroid
contribute equally and for the adolescent phantom the remainder organs and the
salivary glands gave the highest contribution (Theodorakou et al, in press).
Fig 2.1: Average contribution of organ
doses to effective dose calculations for CBCT, adapted from Pauwels et al (in
press).
Table
2.3c presents the reported effective doses for conventional imaging and
multislice CT (MSCT) imaging to act as a comparison with dental CBCT data. The
majority of studies were based on thermoluminescent dosimetry (TLD) techniques
using anthropomorphic phantoms. They showed significant variation in
methodology, especially with respect to the type of phantom used and TLD number
and positioning. The effect of the number and position of the TLD dosimeters on
the accuracy of the assessment has been assessed in the SEDENTEXCT project by
Pauwels et al (in press). They recalculated their organ dose data using
a limited number of selected TLDs and found significant variability in organ
dose depending on the number and position of TLDs, with the largest deviations
seen for small FOV protocols and for thyroid and remainder tissues. This
emphasises the importance of using sufficient TLDs in effective dose
calculation for dental CBCT.
Looking
at the median values and the ranges for dento-alveolar and craniofacial dental
CBCT effective dose in Tables 2.3a and 2.3b, the reported data are markedly
skewed, with high doses being reported in a small number of studies for
particular equipment. What is suggested from this is that some dental CBCT equipment
is associated with effective doses that are not as low as reasonably
achievable.
Table 2.3a: The range of effective dose and the median
values in parentheses from dental CBCT in Sv. Studies are divided into
“dento-alveolar” (small and medium FOV) and “craniofacial” (large FOV). The
height of the dento-alveolar FOVs is smaller than 10cm allowing imaging of the
lower and upper jaws. For the craniofacial FOVs, the height is greater than
10cm allowing maxillofacial imaging.
CBCT unit type
|
Effective dose (μSv)
|
References
|
Dento-alveolar
|
11-674 (61)
|
Ludlow et al 2003
Ludlow and Ivanovic 2008
Lofthag-Hansen et al 2008
Hirsch et al 2008
Okano et al 2009
Loubele et al 2009
Roberts et al 2009
Suomalainen et al 2009
Qu et al 2010
Pauwels et al, in press
|
Craniofacial
|
30-1073 (87)
|
Ludlow et al 2003
Tsiklakis et al 2005
Ludlow et al 2006
Ludlow and Ivanovic 2008
Garcia Silva et al 2008a
Okano et al 2009
Faccioli et al 2009
Loubele et al 2009
Roberts et al 2009
Pauwels et al, in press
|
Table 2.3b: The range of effective dose and the median
values in parentheses from dental CBCT in Sv for paediatric phantoms. Studies
are divided into “dento-alveolar” (small and medium FOV) and “craniofacial”
(large FOV).
Age
|
Dental CBCT unit type
|
Effective dose (μSv)
|
Reference
|
10 year-old phantom
|
Dento-alveolar
|
16-214 (43)
|
Theodorakou
et al (in press)
|
Craniofacial
|
114-282 (186)
|
||
Adolescent phantom
|
Dento-alveolar
|
18-70 (32)
|
|
Craniofacial
|
81-216 (135)
|
Table 2.3c: Effective dose from conventional dental imaging techniques in µSv. MSCT = multislice CT.
Effective
dose (μSv)
|
References
|
|
Intraoral radiograph
|
<1.5*
|
Ludlow et al 2008
|
Panoramic radiograph
|
2.7 – 24.3
|
Ludlow et al 2008
Okano et al 2009
Garcia Silva et al 2008b
Palomo et al 2008
Garcia Silva et al 2008a
|
Cephalometric radiograph
|
<6
|
Ludlow et al 2008
|
MSCT maxillo-mandibular
|
280 - 1410
|
Okano et al 2009
Garcia Silva et al 2008a
Loubele et al 2005
Faccioli et al 2009
Suomalainen et al 2009
|
In summary, the radiation doses (and
hence risks) from dental CBCT are generally higher than conventional dental
radiography (intraoral and panoramic) but lower than MSCT scans of the dental
area. Dose is dependent on equipment type and exposure settings, especially the
field of view selected. In particular, “low dose” protocols on modern MSCT
equipment may bring doses down significantly (Loubele et al 2005; Ballanti et
al 2008).
Effective
dose measurements of equipment reported here become dated very quickly, not
least by new equipment manufacturers appearing. Indeed, some of the studies
reviewed include dental CBCT equipment which has already been superseded by
newer models, although it is likely that existing equipment will remain in
clinical use for some years. As a method of overcoming this problem of
maintaining current and valid data on dental CBCT doses, computed dose
simulations offer considerable advantages. Work in the SEDENTEXCT project has
been performed using Monte Carlo modelling of computational phantoms for a
range of dental CBCT machines and imaging protocols. This facilitates
estimation of effective dose of dental CBCT without the need for repeated
dosimetry work on anthropomorphic phantoms.
Ballanti
F, Lione R, Fiaschetti V, Fanucci E, Cozza P.Low-dose CT protocol for
orthodontic diagnosis. Eur J Paediatr Dent. 2008; 9: 65-70.
European
Commission. Radiation Protection 125: Low dose ionizing radiation and cancer
risk. 2001. Office for Official Publications of the EC: Luxembourg. http://europa.eu.int/comm/environment/radprot/publications.
European
Commission. Radiation Protection 136. European Guidelines on Radiation
Protection in Dental Radiology. Luxembourg: Office for Official Publications of
the European Communities, 2004. Available from: http://ec.europa.eu/energy/nuclear/radioprotection/publication/doc/136_en.pdf
European Commission. Radiation
Protection 88. Recommendations for the implementation of Title VII of the
European Basic Safety Standards (BSS) Directive concerning significant increase
in exposure due to natural radiation sources. 1997. Office for Official
Publications of the EC: Luxembourg.
Faccioli N, M Barillari, S Guariglia, E
Zivelonghi, A Rizzotti, R Cerini, et al., Radiation dose saving through the use
of cone-beam CT in hearing-impaired patients. Radiologia Medica, 2009; 114:
1308-1318.
Garcia
Silva MA, Wolf U, Heinicke F, Bumann A, Visser H, Hirsch E. Cone-beam computed
tomography for routine orthodontic treatment planning: a radiation dose
evaluation. Am J Orthod Dentofacial Orthop 2008a; 133: 640.e1-5.
3: BASIC PRINCIPLES
3.1: Background
The
detailed methodology followed in the preparation of these guidelines is fully
described elsewhere (Horner et al 2009). Briefly, a Guideline Development Panel
was formed to develop a set of draft statements using existing EU Directives
and Guidelines on Radiation Protection. The draft statements covered
Justification, Optimisation and Training of dental CBCT users. These statements
were revised after an open debate of attendees at the 11th EADMFR Congress on
28th June 2008. A modified Delphi procedure was then used to present
the revised statements to the EADMFR membership, utilising an online survey in
October/November 2008. Consensus of EADMFR members, indicated by high level of
agreement for all statements, was achieved without a need for further rounds of
the Delphi process.
A
set of 20 “Basic Principles” on the use of dental CBCT were thus established.
These act as core standards for EADMFR and are central to this Guideline
publication.
1
|
CBCT
examinations must not be carried out unless a history and clinical
examination have been performed
|
2
|
CBCT
examinations must be justified for each patient to demonstrate that the
benefits outweigh the risks
|
3
|
CBCT
examinations should potentially add new information to aid the patient‟s
management
|
4
|
CBCT
should not be repeated „routinely‟ on a patient without a new risk/benefit
assessment having been performed
|
5
|
When
accepting referrals from other dentists for CBCT examinations, the referring
dentist must supply sufficient clinical information (results of a history and
examination) to allow the CBCT Practitioner to perform the Justification
process
|
6
|
CBCT
should only be used when the question for which imaging is required cannot be
answered adequately by lower dose conventional (traditional) radiography
|
7
|
CBCT
images must undergo a thorough clinical evaluation („radiological report‟) of
the entire image dataset
|
8
|
Where
it is likely that evaluation of soft tissues will be required as part of the
patient‟s radiological assessment, the appropriate imaging should be
conventional medical CT or MR, rather than CBCT
|
9
|
CBCT
equipment should offer a choice of volume sizes and examinations must use the
smallest that is compatible with the clinical situation if this provides less
radiation dose to the patient
|
10
|
Where
CBCT equipment offers a choice of resolution, the resolution compatible with
adequate diagnosis and the lowest achievable dose should be used
|
11
|
A
quality assurance programme must be established and implemented for each CBCT
facility, including equipment, techniques and quality control procedures
|
12
|
Aids
to accurate positioning (light beam markers) must always be used
|
13
|
All
new installations of CBCT equipment should undergo a critical examination and
detailed acceptance tests before use to ensure that radiation protection for
staff, members of the public and patient are optimal
|
14
|
CBCT
equipment should undergo regular routine tests to ensure that radiation
protection, for both practice/facility users and patients, has not
significantly deteriorated
|
15
|
For
staff protection from CBCT equipment, the guidelines detailed in Section 6 of
the European Commission document „Radiation Protection 136. European
Guidelines on Radiation Protection in Dental Radiology‟ should be
followed
|
16
|
All
those involved with CBCT must have received adequate theoretical and
practical training for the purpose of radiological practices and relevant
competence in radiation protection
|
17
|
Continuing
education and training after qualification are required, particularly when
new CBCT equipment or techniques are adopted
|
18
|
Dentists
responsible for CBCT facilities who have not previously received „adequate
theoretical and practical training‟ should undergo a period of additional
theoretical and practical training that has been validated by an academic
institution (University or equivalent). Where national specialist
qualifications in DMFR exist, the design and delivery of CBCT training
programmes should involve a DMF Radiologist
|
19
|
For
dento-alveolar CBCT images of the teeth, their supporting structures, the
mandible and the maxilla up to the floor of the nose (eg 8cm x 8cm or smaller
fields of view), clinical evaluation („radiological report‟) should be
made by a specially trained DMF Radiologist or, where this is impracticable,
an adequately trained general dental practitioner
|
20
|
For
non-dento-alveolar small fields of view (e.g. temporal bone) and all
craniofacial CBCT images (fields of view extending beyond the teeth, their
supporting structures, the mandible, including the TMJ, and the maxilla up to
the floor of the nose), clinical evaluation („radiological report‟)
should be made by a specially trained DMF Radiologist or by a Clinical
Radiologist (Medical Radiologist)
|
3.4: References
Council of the European Union. Council
Directive 96/29/Euratom of 13 May 1996 laying down basic safety standards for
the protection of the health of workers and the general public against the
dangers arising from ionizing radiation. Official Journal of the European
Communities N° L 159, 1996. Available from: http://ec.europa.eu/energy/nuclear/radioprotection/doc/legislation/9629_en.pdf
Council
of the European Union. Council Directive 97/43/Euratom of 30 June 1997 on
health protection of individuals against the dangers of ionizing radiation in
relation to medical exposure, and repealing Directive 84/466/Euratom. Available
from: http://ec.europa.eu/energy/nuclear/radioprotection/doc/legislation/9743_en.pdf
European Commission. Radiation
Protection 136. European Guidelines on Radiation Protection in Dental
Radiology. Luxembourg: Office for Official Publications of the European
Communities, 2004. Available from: http://ec.europa.eu/energy/nuclear/radioprotection/publication/doc/136_en.pdf
Horner
K, Islam M, Flygare L, Tsiklakis T, Whaites E. Basic Principles for Use of
Dental Cone Beam CT: Consensus Guidelines of the European Academy of Dental and
Maxillofacial Radiology. Dentomaxillofac Radiol. 2009; 38: 187-195.
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