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BCT Dosimetry: Orthodontic Considerationsharon L. Brooks

This article reviews the general principles of radiation biology and dose

measurement. Effective doses for typical imaging examinations used in

orthodontics include: panoramic, 5.5 to 22 microsieverts (�Sv); cephalomet-

ric, 2.4 to 6.2 �Sv; large field-of-view cone beam CT, 58.9 to 1025.4 �Sv. This

can be compared with average annual natural background radiation of 3000

�Sv/yr. Issues of radiation risk, particularly for children, as well as mechanisms

for dose reduction are discussed. (Semin Orthod 2009;15:14-18.) © 2009 Elsevier

Inc. All rights reserved.

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he discovery of x-rays more than 100 yearsago brought about an era of increased

iagnostic capability in the healing arts. Asechnology improved, clinicians moved fromimple bitewing and periapical intraoral radio-raphs to larger, more complex extraoraliews. It is hard to imagine an orthodonticffice today that does not use panoramic andephalometric radiographs to help make ahorough assessment of the orthodontic pa-ient before beginning treatment. Today manyrthodontic offices are substituting cone beamomputed tomography (CBCT) images for theraditional orthodontic views for all patients,hile others are adding this type of imagingnly in specific types of cases, such as thoseith impacted canines or requiring orthog-athic surgery. It is expected that the use ofBCT in orthodontics, as well as in other den-

al specialties, will continue to grow at a rapidate, bringing up the question about the radi-tion dose required for this type of imagingnd whether it is justified.

It did not take long after the discovery of-rays for the first reports of radiation injury to

From the University of Michigan School of Dentistry, Annrbor, MI.

Address correspondence to Sharon L. Brooks, DDS, MS, Univer-ity of Michigan School of Dentistry, Department of Periodontics andral Medicine, 1011 N. University Avenue, Ann Arbor, MI 48109-078. Phone: 734-764-1595; Fax: 734-764-2469; E-mail: slbrooks@mich.edu

© 2009 Elsevier Inc. All rights reserved.1073-8746/09/1501-0$30.00/0

cdoi:10.1053/j.sodo.2008.09.002

4 Seminars in Orthodontics, Vol 15, N

ppear. Of course, the dose levels in the earlyays were very high and the equipment not veryood, but there were numerous reports of radi-tion burns and the induction of cancer. One ofhe pioneers of dental radiology, Dr. C. Edmundells, developed cancer on his hands and arms,ue to prolonged exposure to the x-ray beam,hich led to more and more surgery and hisventual suicide.1

Because we no longer use the very high dosesf the past, it is unlikely that we will ever see theypes of radiation injuries also seen in the past.owever, does that mean that there is no risk of

iological damage from the doses currently inse? If the answer to that question is no, can weeasure the magnitude of the risk and thenake a judgment about whether this level of risk

s acceptable or not?It has been observed for many years that

roups of individuals who received radiation ex-osure for a variety of conditions showed anlevated risk of developing cancer.2 In the pastadiation was used to treat ankylosing spondyli-is, a type of arthritis, postchildbirth mastitis, andonsillitis, among other conditions, which in-reased incidence of leukemia, breast cancer,nd thyroid cancer, respectively, leading to theiscontinuation of this practice. These groups ofatients, along with the large number of people

iving in the Japanese cities of Hiroshima andagasaki at the time of the dropping of the

tomic bomb at the end of World War II, makep the population that has been studied to de-ermine the relationship between radiation and

ancer.

o 1 (March), 2009: pp 14-18

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15CBCT Dosimetry

Much of the information we have on radiationisks is based on a careful epidemiologic analysis ofhese populations, looking at radiation doses re-eived and radiation effects observed. Mathemati-al models were then developed to try to explainhe observations. Because the information is in-omplete for all dose levels, there is some uncer-ainty in the exact shape of the dose-responseurve. However, even though the exact risk isot known, there is little doubt that radiationas an effect on the human body even at lowoses.

It is well known that radiation is a carcinogen,long with various chemicals, viruses, and prob-bly other entities. X-rays striking the nucleus ofcell can disrupt cell mitosis and damage DNA,

eading to mutations that can be passed on touture generations of the same cells (somatic

utation) or to future offspring (genetic muta-ion). Some of the mutations may be lethal,eading to cell death, some may modify the func-ion of the cell to a lesser or greater degree,erhaps by affecting the production of enzymes,nd some may change the cell enough to induceancer, when coupled with other initiator orromoter entities.

The body has some DNA repair mechanismsvailable, but these may not be sufficient to re-air all DNA breaks, particularly if they occur inoth strands or if large pieces of the DNA areemoved or rearranged. Thus, the nonrepairedamage can accumulate in the body.

There are many varieties of radiation-associ-ted effects that can occur in the body. In oneype, described as deterministic, there appearso be no damage until a certain threshold ofadiation is received. Once beyond the thresh-ld, the severity of the damage is proportional tohe radiation dose. Examples of this type of ef-ect include skin reddening, hair loss, and sali-ary gland dysfunction.

Other types of radiation effects, most notablyarcinogenesis and genetic mutations, are exam-les of stochastic effects, in which there is no

hreshold for radiation damage but where therobability of an effect is proportional to theadiation dose. In this situation, it is possible thatsingle “hit” of radiation can produce a nonre-aired mutation of the DNA that can lead toancer several years in the future. There is nouarantee that the subject will ever develop can-

er as a result of the radiation, but the more w

adiation that is received, the more chances thatomething will occur.

Not all cells in the body have the same degreef sensitivity to radiation. In general, cells thatre dividing rapidly over several generations, arerimitive or immature, and are nonspecializedre at higher risk for radiation effects due to thempact of radiation on DNA and cell division.hildren are considered to be more sensitive to

adiation than adults, an issue in orthodonticmaging due to the high proportion of childrenn an orthodontic practice.

eneral Principles of Radiationosimetry

o evaluate radiation risks from various imagingechniques, radiation doses must be measured.t is relatively easy for a health physicist to placen ionization chamber in the x-ray beam usedor intraoral and cephalometric radiographynd determine the amount of radiation thattrikes the patient, but unfortunately, this pro-edure does not make it easy—or even possi-le—to compare radiation doses for the various

maging examinations. For example, in pan-ramic radiographs and CBCT imaging, the x-ay beam moves around the patient’s head, par-ially or totally, depending on technique, andifferent parts of the anatomy receive differentmounts of radiation, depending on their loca-ion with respect to the center of rotation. Theurface exposure, while easy to measure, alsooes not take into account the size of the radi-tion beam or the radiosensitivity of the tissuesxposed.

To allow a meaningful comparison of radia-ion dose, and thus risk, radiation exposures arerequently converted to effective doses, mea-ured in Sieverts (Sv or milli- [mSv] or micro-�Sv]). In the calculation, the radiation dose topecific tissues is measured, adjusted for amountf that tissue in the field of view, and weightedased on radiation sensitivity of the tissue. Theeighted tissue/organ doses are then summed

o produce the effective dose. This could behought simplistically as a weighted average of theose over the entire body. When this is done, theose for various imaging techniques can be com-ared. Comparisons can also be done with other

hole body doses, such as background radiation.

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16 S.L. Brooks

The tissues/organs used to calculate the ef-ective dose are specified by the Internationalommission on Radiological Protection (ICRP).hile ICRP is an international advisory group,hich does not have force of law, its recommen-ations are accepted worldwide and form theasis for much of radiation protection.

The organs used to calculate effective doseor imaging of the head include the bone mar-ow, thyroid, esophagus, skin, bone surface, sal-vary glands, brain, and “remainder” tissues.3,4

he dose to the thyroid contributes the most tohe effective dose from dental imaging, but sal-vary glands, even though they have a lowereighting factor, are also a significant compo-ent of the effective dose due to their location in

he field of view.

ypical Orthodontic Radiation Doses

ffective radiation doses for various imaging ex-minations used in orthodontic practice haveeen calculated and published in the literature.n general, there is little difference in the doseor digital panoramic and cephalometric imagesompared with their film-based counterparts, inarge measure due to the use of intensifyingcreens with the film imaging that reduces theequired dose. The situation is not the same forntraoral imaging, in which the digital imagesre compared with direct exposure film thatequires a higher exposure.

Effective doses in the literature are expressedsing one of two ICRP criteria: the 1990 report3

r the 2007 report4 (available earlier in draftorm), with the major difference being in the

able 1. Effective Doses of Imaging Examinations U

Examination E �Sv (without sal g

Panoramic (digital) 2.4-6.2Cephalometric (digital) 1.6-1.7CBCT (full FOV)

NewTom 9000 36.3NewTom 3G 44.5MercuRay 846.9i-CAT (9�) 68.7i-CAT (12�) 134.8

Conventional CT 42 to 657Background radiation 3 mSv/yr, �8 �Sv/

ll doses are from the literature and are expressed as effectivlands” (sal gl), which refers to how the dose to the salivary

with salivary glands” is probably a better representation of the reaxillofacial imaging. FOV � field of view.

andling of dose to the salivary glands. In thearlier report the salivary glands were consid-red as part of the “remainder” organs and asuch received a low weighting. In the 2007 re-ort the salivary glands have been removed fromhe “remainder” and have their own weighting.ecause the salivary glands receive a relatively

arge dose in dental imaging, due to their loca-ion, considering them separately leads to aigher calculated effective dose. In reading the

iterature on effective dose, it is important toetermine which ICRP report was used to calcu-

ate the dose since this will affect any compari-ons made.

Published effective doses for digital pan-ramic radiographs range from 5.5 to 22.0 �Sv,hen the salivary glands are considered 2.4-6.2Sv without,5,6 while digital cephalometric ra-iographs have effective doses of 2.2 to 3.4 �Svith salivary glands, 1.6 to 1.7 �Sv without.7,8

herefore, a typical panoramic � cephalometricrthodontic examination will expose the patiento 7.5 to 25.4 �Sv effective dose (with salivarylands). This compares with an average annualatural (not including medical imaging or con-umer products) background radiation dose inhe United States of 3.0 mSv (3000 �Sv).9 Seeable 1.

BCT Doses

n the last few years, the number of CBCT sys-ems available has increased dramatically. Unfor-unately, published dosimetry data are not avail-ble for most of these machines, although works under way to test them. Data have been pub-

n Orthodontics

E �Sv (with sal gl) Reference

5.5-22.0 52.2-3.4 7

77.9 658.9 10

1025.4104.5193.4

Reported in 11Reported in 9

e (E) in �Sv. They may be reported as “without/with salivaryis treated in the calculation of the effective dose. The dose

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17CBCT Dosimetry

ished on four of the large field-of-view systems:he NewTom 9000 (QR, Verona, Italy), NewTomG, CB MercuRay (Hitachi Medical Systems, To-yo, Japan), and the i-CAT (Imaging Sciencesnternational, Hatfield, PA).10,11 See Table 1.

There are a number of factors that will affecthe radiation dose produced by a CBCT system:he imaging parameters used (kVp, mAs);ulsed beam versus continuous beam; amount,

ype, and shape of beam filter; full 360° rotationersus lesser rotation; and limited versus fulleld of view. Some of these factors, such as typef beam and filtration, are unique to a specificachine, while other factors, such as field of

iew, are under the control of the operator.In general, the smaller the field of view, the

ower the radiation dose.10 Since the effectiveose is computed from a weighted summation ofoses to various organs, removing some organs

rom the path of the x-ray beam will reduce theffective dose. For example, since the radiationeceived by the thyroid gland contributes a largemount to the effective dose, limiting the beamo the maxilla instead of the whole head pro-uces a lower effective dose.

In orthodontics, if CBCT is being used toeplace the standard panoramic and cephalo-etric radiographs, it may not be possible to

educe the height of the beam and still obtainhe desired information. On the other hand, ifhe CBCT is being used to supplement othermaging in specific cases, such as for the assess-

ent of impacted canines, beam height can beeduced to cover only the area of interest with-ut exposing the entire head.

Operators of some CBCT machines have thepportunity to affect radiation dose by the im-ging parameters they select. Some machinesllow the operator to select tube potential andurrent (kVp, mA), others use a fixed setting,nd still others use a “smart beam” that bases thearameters on patient size. Using higher settings

ncreases the signal-to-noise ratio (SNR) but alsoncreases the dose. A higher SNR may look “pret-ier,” but it has not been shown to increase theiagnostic quality of the image.12 It does notffect the density and contrast of the image:hese are controlled by image processing of theata and can be adjusted on the monitor by the

perator. f

adiation in Children—As Low aseasonably Achievable (ALARA)

oncerns have been raised in medical imagingbout the increased use of CT scans in childrenecause of the higher susceptibility of childreno the effects of radiation, with increased num-ers of cancers predicted for the future of thesexposed children.13 In general, radiation dosesrom medical CT scanners are higher than fromBCT, but if CBCT becomes widespread in orth-dontics, the collective dose to children coulde quite high. Even with the lowest dose CBCTcanner on the market, the radiation dose isigher with a full head scan than it is with pan-ramic and cephalometric radiographs.

The risk of cancer development as a result ofBCT is not specifically known, particularly forhildren. The effective dose calculations haveeen used to try to estimate risk, usually ex-ressed in terms of X number of excess cancerser Y persons. Almost all of the risk estimatesave been based on adults because most of thepidemiologic information available is ondults. Using the ICRP probability coefficient of.0 � 10–2 Sv–1 and the effective doses includingalivary glands,3,5-7,10 the risk estimates of cancernduction or other stochastic effect are 0.3 to 1.3

10–6 for a dental panoramic radiograph, 0.1o 0.2 � 10–6 for a cephalometric, and 3.5 to1.5 � 10–6 for a full field-of-view CBCT.

At this point it is unknown whether thenformation provided by a CBCT is sufficientlyreater than for traditional orthodontic imag-ng to justify its routine use in children. Onhe other hand, it is also not clear whether thestimated risk is significant enough to worrybout at all.

However, since stochastic effects of radiationncrease with exposure and the effects are noteen for many years after exposure, the prudentourse is to apply the ALARA (As Low As Rea-onably Achievable) principle to orthodontic im-ging, as it is to other areas of radiation protec-ion.14,15 That means that research should beone to determine the true value of the extra

nformation in the diagnosis and managementf orthodontic cases, including the elucidationf selection criteria that will identify those situ-tions where the 3D information makes a sub-tantial contribution to case management. Ef-

orts should also be made by the manufacturers

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18 S.L. Brooks

f CBCT equipment to reduce the radiationose produced by their machines and by theperators to use the equipment in ways that willeep the dose as low as possible while still ob-aining the needed information. This may in-lude reducing the height of the beam when aull head view is not required and reducing themaging parameters to the lowest that would stillrovide the desired information. In this way theenefits of CBCT imaging can by realized whilehe risks are minimized.

eferences1. Langland OE, Langlais RP: Early pioneers of oral and

maxillofacial radiology. Oral Surg Oral Med Oral PatholOral Radiol Endod 80:496-511, 1995

2. National Research Council: Health risks from expo-sure to low levels of ionizing radiation. BEIR VII Phase2. Washington, DC, National Academies Press, 2006

3. International Commission on Radiological Protection:1990 Recommendations. ICRP Publication 60. AnnICRP 21:1-201, 1991

4. International Commission on Radiological Protection:2007 Recommendations. ICRP Publication 103. AnnICRP 37:1-332, 2008

5. Gijbels F, Jacobs R, Debaveye D, et al: Dosimetry ofdigital panoramic imaging. Part I: Patient exposure.

Dentomaxillofac Radiol 34:145-149, 2005

6. Ludlow JB, Davies-Ludlow LE, Brooks SL: Dosimetry oftwo extraoral direct digital imaging devices: NewTomcone beam CT and Orthophos Plus DS panoramic unit.Dentomaxillofac Radiol 32:229-234, 2003

7. Gijbels F, Sanderink G, Wyatt J, et al: Radiation doses ofindirect and direct digital cephalometric radiography.Br Dent J 197:149-152, 2004

8. Visser H, Rodig T, Hermann KP: Dose reduction bydirect-digital cephalometric radiography. Angle Orth-odont 71:159-163, 2001

9. Frederiksen NL: Health physics. In: White SC, PharoahMJ, eds: Oral radiology, principles and interpretation.5th ed. St Louis, Mosby, 2004

0. Ludlow JB, Davies-Ludlow LE, Brooks SL, et al: Dosim-etry of 3 CBCT units for oral and maxillofacial radiology:CB Mercuray, NewTom 3G and i-CAT. DentomaxillofacRadiol 35:219-226, 2006

1. Mah JK, Danforth RA, Bumann A, et al: Radiation ab-sorbed in maxillofacial imaging with a new dental com-puted tomography device. Oral Surg Oral Med OralPathol Oral Radiol Endod 96:508-513, 2003

2. Swan KA: Image quality and radiation dose in conebeam computed tomography for orthodontics. Master’sthesis, University of Michigan, 2007

3. MacNeil JS: Children highly vulnerable to imaging radi-ation. Pediatric News 2006 40:47. Available at: http://www.pediatricnews.com (Accessed 26 June 2008)

4. Brand JW, Gibbs SJ, Edwards M, et al: Radiation protec-tion in dentistry. NCRP Report No. 145, 2003

5. Farman AG: ALARA still applies [editorial]. Oral SurgOral Med Oral Pathol Oral Radiol Endod 100:395-397,

2005