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Reference level for patient dose in dental skull lateral teleradiography

¹ Medical Physics Group, Radiology Department, Complutense University, 28040 Madrid, ² Private dental specialist, Ricardo de la Vega St., 3, 28901 Getafe, ³ Private dental specialist, Mostoles St., 56 bis, 28943 Fuenlabrada and ⁴ Radiodiagnostic Service, Dentistry School, Complutense University, 28040 Madrid, Spain

	Abstract

Top Abstract Introduction Materials and methods Results Discussion Conclusions Appendix 1. Quality criteria... References

 
The present work describes an experiment to obtain a local reference dose value in lateral skull teleradiography for dental applications. The reference value was based on patient measurements in seven dental X-ray units, using thermoluminescent chips, and measurements on a rubber globe filled with water at another 78 installations. Dosemeters were located initially in the head and neck of a human phantom to select the most suitable locations, and on the cephalostat of the X-ray unit at two appropriate locations, which did not interfere with the patient or with the usual imaging routine. The skin projection of the Porion point was considered the best position. 523 patients were monitored in the seven units; then patient doses and dose values from measurements on rubber balloons were compared to normalize and combine the data. The provisional reference value proposed is 400 µGy.

	Introduction

Top Abstract Introduction Materials and methods Results Discussion Conclusions Appendix 1. Quality criteria... References

 
The European Directive 97/43/Euratom on health protection of individuals against the dangers of ionizing radiation in relation to medical exposure [1] requires Member States to promote the establishment and the use of diagnostic reference levels, or reference values (RVs) for radiodiagnostic examinations. In medical X-ray diagnostic practices, such RVs are dose levels for typical examinations, for groups of standard-sized patients or standard phantoms for broadly defined types of equipment, which are expected not to be exceeded for standard procedures when good and normal practice regarding diagnostic and technical performance is applied. RVs have already been defined in the document EUR 16260 [2] from the European Commission (EC) for six frequent radiological examinations.

There have been no European RVs proposed for dental practice. The International Commission for Radiological Protection (ICRP) lists some RVs from national and international bodies [3]. The Spanish legislation has adopted the dose guidance value of 7 mGy as RV for intraoral radiography from the International Atomic Energy Agency [3]. The suitability of this value has been discussed previously [4] in the light of the reference dose for bite wing exposure in the UK [5] and the mean doses reported by the Finnish Radiation and Nuclear Safety Authority from molar X-ray dose measurements [6]. In the same literature, RVs were suggested for orthopantomography, based on dose–width product [5], dose–area product [6] or entrance dose [4]. A more recent survey [7], using the database from the National Radiological Protection Board (NRPB), completes information for the UK and proposes the adoption of reference doses. In the absence of formally recognized national RVs, the NRPB recommends adopting such values for local use [8].

Furthermore, Directive 97/43 establishes the need for optimization based on quality assurance (QA) including quality control (QC) and the assessment and evaluation of patient doses. These aspects were developed in Spain in a decree providing quality criteria to evaluate X-ray images, as part of a QA programme [9]. X-ray units must meet the acceptability criteria fixed in the decree, in agreement with the EC document on the topic [10].

The Spanish regulation adopts the EC recommended RVs for the six examinations covered by the EC quality criteria [2]. For other examinations, the specialist responsible can select a typical projection and adopt the average dose level based on 10 estimates as a provisional RV, until a representative RV derived from statistical data from within the country or proposed at a European level is available. Additionally, image quality should be evaluated by means of anatomical criteria developed for the purpose. A list of the proposed image quality criteria for skull lateral teleradiography (SLT), to be incorporated in dental QA programmes, has been included as Appendix 1 to this paper [11, 12].

SLT examination for dental purposes, frequently used by orthodontic and maxillofacial surgical specialists in the infant and teenage patient populations, does not yet have a proposed European RV. In the present work, an experiment is described, based on patient and phantom measurements, to obtain a local RV.

	Materials and methods

Top Abstract Introduction Materials and methods Results Discussion Conclusions Appendix 1. Quality criteria... References

 
Measurements were carried out at seven SLT units. Usual technical parameters used in every unit for clinical imaging were maintained throughout the experiment, aiming to reproduce patient exposure conditions. Table 1 shows the different types of equipment and related details. Prior to patient measurements, a head and neck phantom of polyurethane was used with the X-ray set from facility no. 3 to determine approximately the range of doses to be expected at different points of interest, as a way of selecting the most suitable locations. Lithium fluoride thermoluminescent dosemeter (TLD) chips (type TLD-100; Harshaw TLD/Bicron/NE-Technology; BICRON-NE, Solon, OH) were placed on the phantom at the anatomical points Nasion (Na) and Porion (Po), on their respective projections at the skin (Na' and Po') [13, 14], occipital bone, zygomatic bone, nose tip, vertex, point B, Pogonion, Gonion, thyroid and infraorbitary point. After analysis, all of the values, except those from Na' and Po', were discarded for patient monitoring, as the information they provided was found to be redundant to that obtained from Na' and Po'. Furthermore, chips at Na' and Po' are easy to place on the cephalostat.

Dose data from 78 X-ray dental machines used in SLT examinations, randomly chosen, were gathered. These were measured during routine yearly QC, as a part of the QA programme [9] by a QC company. At each facility, average exposure parameters to simulate real patient imaging conditions were used to irradiate a crystal TLD located at the level of the ear rod on a rubber balloon filled with water (about 2.5–3 l).

Routine QC tests were carried out on the unit used for measurements on the expanded polyurethane phantom, aiming to assure correct operation. With respect to the size of the irradiation field, an important extra cross-section was found. As a consequence, the thyroid gland area may lay inside the direct beam. Therefore, calibration was performed to reduce the radiation field to the volume of interest. After calibration, the unit was used to compare our dosimetry results on patients with those on the rubber balloon phantom, performing five irradiations to this end. A water-filled rubber balloon similar to that used by the QC company was used. In addition, chips from both dosimetry systems were irradiated together for comparison using an X-ray beam at typical SLT exposures. The dosimetry system used by the QC company is traceable to a national laboratory and our system is traceable to the NOFER Institute of Occupational Medicine (Poland), and the International Atomic Energy Agency (Vienna, Austria).

	Results

Top Abstract Introduction Materials and methods Results Discussion Conclusions Appendix 1. Quality criteria... References

 
Table 1 presents the entrance dose mean values measured at Na' and Po'. Mean values supplied from facility 5 were determined during routine practice, for a speed class of 100. Figure 1 shows the histogram of dose values from the sample. Values above 1100 µGy are omitted from the distribution for the sake of convenience. The mean value of the distribution is 370 µGy and the 75th percentile value is 380 µGy. TLD chips from both dosimetry systems irradiated together provided comparable readings (±15%).

View larger version (33K): [in this window] [in a new window]   Figure 1. Histogram of dose values from the complete sample of X-ray units studied in the present work. For convenience, values above 1100 µGy have been omitted.

	Discussion

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Patient doses from Table 1 show a large range. One of the evaluated facilities (facility 5) showed higher dose values than the rest. In fact, it is devoted to skull and oral radiology, not just to dental radiodiagnostics, and uses high-powered X-ray generators, unusual in dental units. Image systems consist of photostimulable phosphor plates and digital image processing is made by simulating a screen–film conventional radiography set with a sensitivity class of 100, lower than the minimum of 400 recommended in [2] for skull radiography. Further measurements performed simulating a sensitivity class of 200 yielded dose values significantly lower, although above the average for the dental facilities. A comparison between mean doses for speed classes of 100 and 200 at facility 5 shows that the latter are much lower than they should be, as technical imaging parameter settings at the X-ray units were not adapted accordingly. This fact reveals the need for dose optimization by setting a suitable operating technique, which seems to be independent (at least to some extent) of the equivalent sensitivity class simulated in the image processing software and does not convey apparent changes to the image quality. Note, however, that the doses are below the RV (3 mGy) proposed [2] for a lateral skull projection. However, for the purposes of this study, data from this facility were excluded when deciding a RV suitable for dental practice.

It is not surprising that doses increased when computed radiography was used, since there is no relationship between image quality and X-ray tube charge selected, at a given kilovoltage (except for exposures giving rise to quantum mottle or saturation). A similar conclusion was reported in recent studies [15, 16]. Therefore, one of the first actions in the QA programme should be to review initial dose levels as part of the necessary optimization.

From the available data, it is impossible to determine if the highest doses measured by the QC company came from similar situations to that described above. Therefore, they should not be considered to represent typical dental practice. Evaluation of the image quality has not been made, but it would be hard to justify the higher dose imparted on the basis of significant image improvement, particularly when increased detail is of little diagnostic importance in dental practice.

It should be noted that, in contrast to the lateral skull projection studied in [2], for which the beam cross-section is considered uniform in intensity, the beam used in SLT is non-homogeneous, as an anterior wedge filter is used to image the face soft tissue profile [17]. As the beam intensity is less at the anterior (face) side, one must position chips close to the external acoustic meatus (Po', representative of the unfiltered beam section). This point is usually close to the geometrical centre of the radiation field and allows the highest beam intensity to be recorded. In contrast, Na' provides information from the filtered beam side.

The choice of points Po' and Na' as dose measurement points has the advantage that they do not interfere with the normal practice of the examination, avoiding discomfort or disturbance to the patient. Furthermore, they are close to the anatomical areas where radiation sensitivity is important for this projection, as Po' is in the region of the parotid gland, and Na' is near the eye lenses. Both zones are those of the highest absorbed dose, due to their shorter distance from the X-ray tube focus [18, 19], when using points external to the patient to support the dosemeters. Moreover, it is worth mentioning that a point of interest such as the thyroid, monitored by some authors in previous work, receives very little radiation in comparison with the values measured at the points chosen here for suitably collimated radiation fields. Although a lead thyroid collar can be used to shield from the direct X-ray beam, adopting a suitable irradiated area should assure proper radiation protection. To achieve this goal, a light beam to check the position and size of the X-ray field on the patient is an essential tool.

Table 2 shows the doses quoted in the literature. Values tabulated refer exclusively to locations close to those used in the present work, as determined from measurements in patients or in phantoms. Our results for Po' would compare with the data from other authors for the parotid gland and nearest skin, temporomandibular joint and the point 10 mm posterior and below the tragus. In addition, our dose at Na' compares with those of the eye lenses and to the external edge of the left eye. In both cases, results are similar. For example, Tsiklakis [19] reports between 70 µGy and 280 µGy for the eye external edge, and Visser [18] reported from 26 µGy to 45 µGy in Na, or 34 µGy and 81 µGy for digital and conventional equipment in the lens, which are compatible with our results.

The adoption of a RV requires an estimate of uncertainties in the experiment. Uncertainty sources originate from dosemeter misplacement, statistical uncertainty in the TLD read-out process and patient size. The corresponding values were investigated earlier in orthopantomography exposures [4], with values of 10%, 7% and 20%, respectively, which could reasonably be applied to these measurements. The combined uncertainty of 23% allows a more convenient rounded RV of 400 µGy, instead of the 380 µGy found at the 75th percentile, to be provisionally proposed when monitoring is performed at Po'.

This provisional RV is fairly conservative, as the mean value from patient measurements in this work, when the data from facility 5 are excluded, is 300 µGy±10% (standard deviation), combining the statistical deviation in TLD read-out and weighting each mean value by its number of measurements in this uncertainty. Thus, if these mean values were supposed to fit to a normal distribution, the 75th percentile value should be approximately 340 µGy. Moreover, the RV proposed by the American Association of Physicists in Medicine [3] is 250 µGy for the kerma in air at the entrance. Finally, the distribution of Figure 1 provides a range of values with a lower threshold of 60 µGy, although the diagnostic information of the corresponding images may be poorer than in images obtained with higher doses. In any case, the proposed RV allows for further reductions consistent with suitable image quality.

	Conclusions

Top Abstract Introduction Materials and methods Results Discussion Conclusions Appendix 1. Quality criteria... References

 
An RV for SLT of 400 µGy at the point Po' is proposed. This value is provisional, until further experience relating dose and image quality is available, and is only applicable where conventional dedicated dental imaging systems are employed. Otherwise, the need to observe other image details might necessitate larger doses, outside of the standard.

Special attention should be given to digital equipment to determine optimum settings with respect to image quality and patient doses. Excluding special cases in which lower sensitivities may be appropriate, optimized operation should be set by selecting a speed class similar to 400 or higher as used in conventional screen–film systems, then following the image criteria in the appendix can help to define the most suitable setting.

During the study, the importance of checking field size during QC checks was demonstrated. Confining the field size to the anatomy under examination also ensures very low doses to the thyroid gland as it is then outside the direct beam. The fitting of light beams to SLT equipment, together with adjustable diaphragm collimation used on an individual basis, is to be recommended.

	Appendix 1. Quality criteria in lateral skull teleradiography

Top Abstract Introduction Materials and methods Results Discussion Conclusions Appendix 1. Quality criteria... References

 
1. Diagnostic requirements

 1.1 Image criteria

Visualization of*

  1.1.1 Soft tissue profile

  1.1.2 Sella turcica

  1.1.3 Orbit profile

  1.1.4 Porion

Reproduction of*

  1.1.5 Nasion

  1.1.6 Incisor superior and inferior profile

Visually sharp reproduction of*

  1.1.7 Mandibular edge

  1.1.8 Maxillar anterior profile

2. Criteria for radiation dose to the patient

Entrance surface dose for a standard-sized patient: 400 µGy at Porion Po' point.

3. Example of good radiographic technique

 3.1 Radiographic device: cephalostat

 3.2 Nominal focal spot value: 0.5 x 0.5 mm²

 3.3 Total filtration: 2.5 mm aluminium equivalent

 3.4 Focus to film distance: 1.5 m

 3.5 X-ray tube voltage: ge;70 kV

 3.6 Screen–film system: green series, speed class 400 or higher

 3.7 Protective shielding: protective thyroid collar may provide protection in equipment in which the beam exceeds the imaging field (not properly collimated). Lead apron may be desirable in special cases.

* The degrees of visibility used along with this proposal are those of the EUR-16260 document. They are defined as:

   Visualization: characteristic features are detectable, but details are not fully reproduced; features just visible.

   Reproduction: details of anatomical structures are visible, but not necessarily clearly defined; details emerging.

   Visually sharp reproduction: details are clearly defined; details clear.

	Acknowledgments

 
The authors gratefully acknowledge INFOCITEC for the data supplied and Mr A Ortiz for the statistical treatment, Dr M Baos and P Paniagua, for their help in the experimental work.

Received for publication February 6, 2003. Revision received February 26, 2004. Accepted for publication March 30, 2004.

	References

Top Abstract Introduction Materials and methods Results Discussion Conclusions Appendix 1. Quality criteria... References

 

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