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British Journal of Radiology 74 (2001),153-156 © 2001 The British Institute of Radiology

Full paper

Reference doses in dental radiodiagnostic facilities

L González, PhD1, E Vañó, PhD1 and R Fernández, MD2

1 Medical Physics Group, Radiology Department, Complutense University, 28040 Madrid
2 3, Ricardo de la Vega St., 28901 Getafe, Spain

Correspondence: Prof. L González, Catedra de Fisica Medica, Facultad de Medicina, Universidad Complutense, 28040 Madrid, Spain


   Abstract
Top
 Abstract
Introduction
 Method
 Results
 Discussion
 References
 
The present work describes an experiment undertaken using thermoluminescent dosemeters to obtain a local reference dose level in orthopantomography, based on patient measurements in 11 dentistry installations. In addition, a critical audit of the dose recommended by the International Atomic Energy Agency for intraoral X-ray diagnostics is performed, based on data gathered from over 300 intraoral X-ray facilities. The provisional local reference levels proposed are 0.7 mGy entrance surface dose at the occipital region for orthopantomography and 3.5 mGy entrance surface dose for intraoral radiology.


   Introduction
Top
 Abstract
 Introduction
Method
 Results
 Discussion
 References
 
The Council of the European Union Medical Exposure Directive 97/43/EURATOM [1] and Directive 84/466/EURATOM [2] were implemented in Spanish legislation by Royal Decrees 1132/1990 [3], 1891/1991 [4] and 1976/1999 [5]. Previous work [6] has given the main details of the legislation, which lays down basic measures for protecting persons undergoing medical examinations or treatment with radiation sources.

In particular, the installation and use of X-ray equipment for medical diagnosis with regard to technical specifications in the design of a facility and the provision of radiation protection resources are governed by decrees 1891/1991 and 1976/1999. The latter provides quality criteria for the compulsory evaluation of X-ray images, on a yearly basis, as well as their adaptation to European quality standards, as the basis of a quality assurance programme that must have been implemented by June 2000. Compulsory initial acceptance tests on new equipment and reference performances for old equipment establish the baseline performance. These must satisfy the requirements of acceptability criteria in the decree, which agree with the European Commission document on criteria for acceptability of radiology (including radiotherapy) and nuclear medicine installations [7].

This regulation follows the recommended image quality and reference dose criteria of document EUR 16260 [8] for chest, skull, lumbar spine, pelvis and urinary tract examinations. For examinations using other projections, image quality can be evaluated by means of anatomical criteria developed for this purpose by the specialist responsible. This may set the initial average dose level as the provisional reference value for the examination, until such time as representative reference levels are determined.

Dose evaluations must be based on a minimum of 10 estimates for simple examinations (i.e. less than four projections, without fluoroscopy), which is the case in dentistry. The image reject rate should also be evaluated and recorded. Exceeding the reference doses requires investigation of the causes, may lead to a correction of the protocols and/or suitable repair or reconditioning of the equipment, followed by a subsequent check on the efficacy of the changes made.

As in the case of all examinations that include projections other than the ones described in document EUR 16260 [8], there are no anatomical criteria developed to assess dental radiological images, although a reference dose value of 7 mGy is adopted for intraoral dental radiology, as proposed by the International Atomic Energy Agency (IAEA) "Basic safety standards" [9]. In previous work, a set of specific quality criteria has been proposed for intraoral [10], orthopantomography and cranial teleradiography images [11] so that image quality can be compared.

There is no dose value for orthopantomography. Previous literature provides doses measured in phantoms [12–16] or following hybrid procedures using patients and phantoms [17, 18]. Owing to different measurement points, parameters used, equipment and perhaps also different sensitivity of the films available when measurements were performed, results from the various authors are not comparable, and it is possible to find average values from below 100 µGy to almost 3 mGy. In particular, measurements performed on patients are scarce. Moreover, entrance surface dose measurement is not the only solution to determine reference levels. For example, in the UK, rather than measuring the actual patient dose at given skin locations, it is accepted practice to measure the dose–width product at the film for the whole panoramic exposure and average adult settings, using a dosemeter and film. The reference value is 75 mGy mm [19]. As the width of the beam should be less than 10 mm, the dose measured at the cassette should be less than 7.5 mGy for the whole exposure. This method provides information that is independent of geometry, but it is difficult to correlate this with skin doses owing to beam movement. In Finland during 1995–1999, regulatory control of panoramic units has been performed by measuring the dose–area product [20], with values in the range 34–254 mGy cm2 and a mean of 94 mGy cm2.

According to European Union article 4(2) of the Medical Exposure Directive [1], member states shall promote the establishment and use of diagnostic reference levels (DRLs). DRLs are defined as dose levels in groups of standard sized patients or standard phantoms, for typical examinations and for broadly defined types of equipment. These levels should not be exceeded for standard procedures when good and normal practice is applied with regard to diagnostic and technical performance. According to Radiation Protection Document 109 from the European Commission [21], DRLs play an important role in clinical auditing to guarantee the performance of diagnostic services and as a support to improve procedures. Point 14 of the document states the applicability of DRLs for standard procedures in all areas of diagnostic radiology, although it identifies areas of particular utility where a relatively large reduction in risk can be achieved, as in the case of examinations with more radiosensitive patients such as children.

Since orthopantomography is often performed in paediatric cases, this study describes an experiment undertaken to obtain a local reference dose value based on patient measurements. Also, a critical evaluation of the adopted reference dose in intraoral X-ray diagnostics is undertaken using data gathered from over 300 intraoral X-ray facilities.


   Method
Top
 Abstract
 Introduction
 Method
Results
 Discussion
 References
 
The European Commission Radiation Protection Document 109 [21] considers the frequency curve of a number of examinations and their doses, and proposes adoption of the 75th percentile as an appropriate DRL value. We have used a similar approach in this study.

Doses in orthopantomography were monitored with lithium fluoride thermoluminescent dosemeters (TLDs) of type TLD-100 (Harshaw TLD/Bicron/NE-Technology, Solon, OH). These TLDs were calibrated individually. A sample of at least 10 patients per X-ray unit was used. TLD chips inside sachets were used in pairs to reduce uncertainties in measurements. The sachets were located using an elastic ribbon in the occipital region at the level of the posterior cerebral fossa at the occlusal plane in each patient and, in a number of cases, on the temporal region, to compare results.

Exposures were classed according to the tube potential (kV) and tube current (mA) settings usually employed in each facility (up to three different parameter sets were found to be used in given facilities, although operation with a constant technique is not uncommon), using a sachet in each typical setting for between 5 and 21 patients. Data from three University facilities, a large hospital, a private centre devoted exclusively to dental radiology and five private dental clinics, were collected. 12 X-ray units were studied. A total of 190 measurements on adult and child patients were performed. Scanning times in all machines ranged from 12–15 s.

An overall statistical uncertainty of 7% was estimated in the TLD read-out process. Reliability in doses owing to uncertainty in the placement of sachets was tested in a phantom, simulating possible shifts due to changes in focus-to-chip distances that could occur as a result of the presence of more or less hair, and was estimated to be better than 10%. Uncertainties due to patient size have been simulated by measuring anteroposterior distances in a sample of patients, concluding shifts of about 20% related to the mean value.

For intraoral exposures, dose estimates have been gathered based on quality control tests performed on 307 X-ray odontology installations. Data on X-ray tubes together with usual maximum and minimum exposure times used in the different projections were obtained. A mean exposure time was used (not weighted by the number of exposures). A mean entrance surface dose was calculated at the appropriate focus-to-skin distance, using a backscatter factor of 1.1 and a mass attenuation coefficient ratio from human tissue to air of 1.06.


   Results
Top
 Abstract
 Introduction
 Method
 Results
Discussion
 References
 
For orthopantomography, a mean surface dose value of 530 µGy was obtained at the occipital region, with individual values ranging between 250 µGy and 870 µGy. Median dose was 550 µGy and the third quartile dose was 660 µGy. Doses measured on the temporal region (in the cases tested) varied between 40% and 60% of the values at the occipital location. Mean values for each X-ray machine are plotted in Figure 1Go.



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  		Figure 1. Mean values of skin doses in orthopantomography for each X-ray machine tested in the present study.

 
For intraoral equipment, the estimated value was 2.89±2.12 mGy, with a median of 2.43 mGy, and the 75th percentile was 3.37 mGy. Figure 2Go shows a frequency plot, which excludes five cases with mean dose values above 10 mGy.



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  		Figure 2. Frequency plot of doses for intraoral radiography, excluding five cases with mean doses above 10 m.

 

   Discussion
Top
 Abstract
 Introduction
 Method
 Results
 Discussion
References
 
The mean dose measured in the occipital region is one of the highest entrance surface doses to be measured along the X-ray beam trajectory. It may actually be the highest [22], since orthopantomographs either increase the selected kilovoltage when scanning this area or reduce the scanning speed to avoid a shadow produced by the cervical spine. Since the statistical uncertainties in TLD processing (or in whatever instrument is used) decrease when measuring higher values, the occipital region has been considered the most suitable for establishing a measurement procedure to derive a reference value. Doses at this location are consistent; experimental values measured at the temporal region have shown poorer consistency, since the location of the TLD chip with reference to the X-ray beam appears to be subject to higher discrepancy because the chip is seldom centered in the beam spot. In orthopantomography, the X-ray scanning movement generates a complex dose distribution [12], thus entrance surface dose does not correlate with radiation risk. However, one must use this reference dose as a tool only to improve practices [21] and, according to this approach, the dose value obtained at the central plane by locating the TLD chip near the posterior cerebral fossa can make a suitable value for optimization purposes.

The third quartile value, 670 µGy, is not owing to old equipment, as units with scanning times over 15 s (the oldest ones) have not been surveyed. Accordingly, a provisional local DRL value could be set at 0.7 mGy, with the aim of starting the optimization process.

Regarding dose estimates in intraoral radiology, the mean value differs from the value proposed by IAEA and adopted as the Spanish reference dose. The standard deviation suggests that very few X-ray units irradiate the patient above 7 mGy (less than 5% based on this work). Thus this value is not indicative of good practice and 3.5 mGy (approximately corresponding to the third quartile value) is a more appropriate DRL. This would also agree with the guidance value of 3 mGy considered by IAEA in initial draft documents.

With regard to the suitability of this value as a reference dose, it is worth noting that the average exposure rate produces a maximum dose value, using the average value of maximum exposure time, of 3.5 mGy; thus, the DRL proposed is not over restrictive, although the exposure time used in its estimation may be judged not to represent the most usual exposure condition. The recent appearance of an F-type film, which ensures E-type behaviour in manual processing, should lead to a further reduction in this value. Digital dental radiology would need to use specific DRLs and patient dose measurements, thus such units have not been included in the present study.

In the UK, the reference dose for bite wing exposure is 2.5 mGy at 70 kVp using E-speed film, and 5.0 mGy at 50 kVp. These values are higher when using D-speed film [19]. As the film currently used in most centres is the E-type, the reference doses proposed here agree with the UK approach. In addition, the mean dose value reported by the Finnish Radiation and Nuclear Safety Authority from molar X-ray dose measurements made on 909 dental units in 1999 is 3.5 mGy [20], equal to the value assessed in this work with the average maximum exposure times, and within the range of 0.8–16.4 mGy, close to the values presented here.


   Acknowledgments
 
The authors gratefully acknowledge INFOCITEC for the data supplied, and A Ortiz for his statistical treatment of the data.

Received for publication May 30, 2000. Revision received July 28, 2000. Accepted for publication September 25, 2000.


   References
Top
 Abstract
 Introduction
 Method
 Results
 Discussion
 References
 

  1. European Union. Council Directive 97/43 Euratom, on health protection of individuals against the dangers of ionizing radiation in relation to medical exposure, and repealing Directive 84/466 Euratom. Official Journal of the European Communities No L 180, 9th July 1997:22–7.
  2. European Union. Council Directive 84/466 Euratom, laying down the basic measures for the radiation protection of persons undergoing medical examination or treatment. Official Journal of the European Communities No L 265, 5th October 1984:1–3.
  3. BOE. Royal Decree 1132/1990, from the Health and Consumer Affairs Department, establishing fundamental measures for radiation protection of the persons undergoing medical examination or treatment. In State Official Bulletin, September 18th, 1990:27261. (In Spanish.)
  4. BOE. Royal Decree 1891/1991, from the Industry and Energy Department, on installation and utilization x-ray devices for medical diagnostic purposes. In State Official Bulletin, January 3rd 1992:138–48. (In Spanish.)
  5. BOE. Royal Decree 1976/1999, from the Health and Consumer Affairs Department, establishing quality criteria in radiodiagnostics. In State Official Bulletin, January 29th 1999:45891–900. (In Spanish.)
  6. Gonzalez L, Vano E, Oliete S, Manrique J, Hernaez JM, Lahuerta J, et al. Report of an image quality and dose audit according to Directive 97/43/Euratom at Spanish private radiodiagnostics facilities. Br J Radiol 1999;72:186–92.[Abstract]
  7. European Commission. Criteria for acceptability of radiological (including radiotherapy) and nuclear medicine installations. Radiation Protection 91, 1997.
  8. Carmichael JHE, Maccia C, Moores BM, Oestmann JW, Schibilla H, Teunen D, et al, editors. European guidelines on quality criteria for diagnostic radiographic images. EUR 16260. European Commission, 1996.
  9. International Atomic Energy Agency. International basic safety standards for protection against ionizing radiation and for the safety of radiation sources, Safety Series No. 115. Vienna: IAEA, 1996.
  10. Ferna'ndez R, Gonzalez L, Vaño' E, Villa A, Martinez JM, Ortega R, et al. Image quality criteria in dental radiodiagnostics. Arch Odont Est 1996;12:501–7. (In Spanish.)
  11. Ferna'ndez R, Gonzalez L, Image quality criteria in dental radiology. Review of the Spanish General Council of Colleges of Odontologists and Stomatologists 2000 (submitted for publication). (In Spanish.)
  12. Goldstein A. Exposure and dose in panoramic radiology. Med Phys 1998;25:1033–40.[Medline]
  13. Wall BF, Fisher ES, Poynter R, Hudson A, Bird PD. Doses to patients from pantomographic and conventional dental radiography. Br J Radiol 1979;52:727–34.[Abstract]
  14. Tyndall DA, Washburn DB. The effect of rare earth filtration on patient exposure, dose reduction and image quality in oral panoramic radiology. Health Phys 1987;52:17–26.[Medline]
  15. Underhill TE, Chilvarquer I, Kimura K, Langlais RP, McDavid W, Preece JW, et al. Radiobiologic risk estimation from dental radiology. Part 1: absorbed doses to critical organs. Oral Surg Oral Med Oral Pathol 1988;66:111–20.[Medline]
  16. Freeman JP. Radiation doses of commmonly used dental radiographic surveys. Oral Surg Oral Med Oral Pathol 1994;77:285–9.[Medline]
  17. Bankvall G, Hakansson HAR. Radiation absorbed doses and energy imparted from panoramic tomography, cephalometric radiography and occlusal film radiography in children. Oral Surg Oral Med Oral Pathol 1982;53:532–40.[Medline]
  18. Nuñez de Villavicencio C, Gomez P, Ferna'ndez M, Galva'n M, Prieto N, Ortiz E, et al. Dose measurements in patients in orthopantomography examinations: validity of the used methods. Radioproteccio'n 1998;(May):107–9. (In Spanish.)
  19. The Institute of Physics and Engineering in Medicine. Recommended standards for the routine performance testing of diagnostic X-ray imaging systems, IPEM Report 77. York: IPEM, 1997.
  20. Radiation and Nuclear Safety Authority. Radiation practices, Annual Report 1999. STUK-B-STO 42. Rantanen E, editor. Helsinki: Radiation and Nuclear Safety Authority, 2000.
  21. European Commission. Guidance on diagnostic reference levels for medical exposures. Radiation Protection 109,1999.
  22. Goldstein A. Panoramic radiology quality assessment. Med Phys 1998;25:1028–32.[Medline]



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