Thyroid shields and neck exposures in cephalometric radiography

Several radiosensitive organs such as the thyroid, the esophagus, and vertebral bone marrow are located in the neck tissues. The extent to which these organs are exposed to ionizing radiation under real-life conditions has not been reported and can vary due to person-to-person variability in location of thyroid position [1], due to the age-dependent caudal movement of the thyroid through puberty [2], due to clinician's request regarding what should be visualized on the radiograph [3], and due to differences in radiographic practices. There have been assertions that thyroid shields are commonly used in "modern" dentistry [4–6], but these assertions are inconsistent with survey information [7, 8].

Current guidelines from the National Council on Radiation Protection and Measurements (NCRP) on thyroid collar utilization during cephalometric radiography are unclear. The NCRP reported that: "thyroid shielding shall be provided for children, and should be provided for adults, when it will not interfere with the exam" [9], and that 'no anatomical structures inferior to the hyoid should be visualized'[9]. Since thyroid shields may 'interfere' with the soft tissue examination below the chin and the cervical vertebrae, it is unclear whether current guidelines suggests that thyroid shields should be used with cephalometric radiography. The goal of this study was to assess two issues on cephalometric radiographs: (1) to evaluate anatomical characteristics of neck exposures in the absence of shielding, and (2) to estimate the prevalence of thyroid shield utilization.

Findings indicate that in the absence of shielding, the amount of neck tissue exposed was larger in females than in males, and larger in children than in adults. Overall, usage of the thyroid shield occurred in less than 1 out of 5 cephalometric radiographs. The presence of a thyroid shield depended on calendar year, and during some periods both on the age and the gender of the patient. From 1973 until 1990, thyroid shields were used for less than 3% of the cephalometric X-rays. Thyroid shields were seen less often in children and females than in adults or males during this period. From 1991 until 2000, the prevalence of thyroid shield was 47%. No adult/child differences were observed during this period and females were more likely than males to wear thyroid shields. From 2001 until 2003, there was a drop in the thyroid shield utilization to 10% with a higher utilization in adults than children and no gender differences.

In the absence of a thyroid shield, the amount of neck structure radiated depends primarily on the orientation of the film, and less so on the age and the gender of the patient. The orientation, landscape or portrait, of the cephalometric film determines the amount of neck-tissue exposed to ionizing radiation (that is, if the beam is collimated to the film). If the long axis of the film is parallel to the spine (portrait orientation), thoracic vertebrae are typically exposed indicating a full exposure of the thyroid, and partial exposure of the esophagus. With a perpendicular orientation of the film to the spine (landscape orientation) only cervical vertebrae are typically exposed, and a fraction of the thyroid, as opposed to the whole thyroid, is exposed to ionizing radiation.

It has been reported that the soft tissues of the neck, including the thyroid, descend inferiorly with increasing age, but no data could be identified that describe this phenomenon [2]. The findings of this study suggest that children and females will have, regardless of film orientation, more neck structure exposed than adults or males. This finding may be because film sizes are constant, while the skull and neck of a children and females are smaller than those of adults and males. In addition to the amount of neck structure exposed in children, there is also evidence that the thyroid is positioned higher in the neck of children than of adults [2]. These findings suggest that neck structures are more likely to be exposed in those individuals who are most radiosensitive: children and females.

While the fraction of radiographs taken outside of the dental school could not be established, the observed secular changes in thyroid shield practices within this study may be partly explained by changes in radiological practices inside the dental school. Between 1973 and 1990 the cephalometric radiographs were taken either in the orthodontic clinic or outside. From 1991 and on, most cephalometric radiographs were taken in the oral radiology clinic. Most likely, the drastic increase in the utilization of thyroid shields coincided with the switch from taking radiographs in the orthodontic department to the oral radiology clinic. The drop in utilization from 2001 can partly be explained by the requirement to include at least the 3^rd cervical vertebra in the cephalometric radiographs to determine skeletal age [3, 11–17]. In order to achieve this goal, the thyroid shield was not utilized for many patients or placed much lower, or only a leaded apron was used as a shield. The use of a thyroid shield on small children was difficult because of the thyroid shield design, as was reported in another study [18] suggesting why aprons were the most common sole radiation protection device after 2000.

The infrequent thyroid shield utilization may not be unique to the patients in this study. The previous manager of the X-ray control section of the Washington State Department of Health Radiation Protection has observed during surveys in the 1990s that thyroid shields are rarely used for intra-oral films (2%), and occasionally used for extra-oral x-rays (5%)[19]. A recent survey of routine cephalometric radiographs reported on the prevalence of thyroid and cricoid ossifications of laryngeal cartilages in children and adults[20] providing an indication of the potential general lack of use of thyroid shields in at least one other university setting. A review of 24 issues of the American Journal of Orthodontics (2003–2004) shows that less than 10% of the evaluable cephalometric X-rays had thyroid collars visible, a prevalence similar to that identified in this study. Current advertisements of cephalometric X-ray units [21, 22], or for professional dental meetings [23], provide additional examples of cephalometric radiographs that do not reveal the presence of a thyroid shield. The extent to which publications in a professional journal, X-ray unit advertisements, or professional meeting advertisements reflect or influence clinical practice is not known. Nonetheless, it portrays a picture of a generalized lack of use of thyroid shields in cephalometrics.

Several surveys have reported low thyroid shield utilizations in dental settings, suggesting that the findings reported in this study are not limited to cephalometric radiography. One mail survey of 7940 dental offices in 1985 in Virginia and Florida had a 28% response rate and the findings suggested that thyroid collars were not routinely used in 83% of the offices [7] and not available in 74% of the offices. A mail survey of 398 dentists in 1992 in Michigan had a 67% response rate and indicated that thyroid collars were not used in 51% of the practices[8]. In a survey of 132 staff members within the department of dentistry of a teaching hospital, 40% reported not using a thyroid cover [7]. In contrast to these low utilization rates, a mail survey of North American schools with a 100% response rate in 2000 indicated that thyroid shields were consistently used for extra- and intra-oral radiography in 74% and 85% of the schools respectively [24]. Such self-reported statistics are open to overt selection and reporting biases.

In general, the decision when to use a thyroid shield is complex. For panoramic radiography, the most common extra-oral radiation exposure, there is a consensus that thyroid shields should not be used [25]. For cephalometric radiography, a thyroid shield may be contra-indicated, if skeletal age is to be determined based on cervical vertebrae. For bitewings, the most common type of radiograph used in dentistry, a shield has been reported to be cost-ineffective with rectangular collimation since it does not significantly reduce radiation to the thyroid [26, 27]. These complexities combined with the lack of unequivocal guidelines in the use of thyroid shielding for both intra- and extra-oral radiography indicate that clinical decisions on thyroid shield utilization can be confusing.

Weaknesses of this study include the limited regional nature of the data, the lack of information on the diagnostic goals of the clinicians, and the measurement error in determining the presence or absence of shielding. The data only reflect the radiation protection practices seen in patients presenting in the dental school. At best, the practices are reflective for the Seattle urban area. Minimal information was available related to the diagnostic purposes of the clinician. In particular, we did not abstract information from the charts to assess whether indeed skeletal age was determined based on cervical vertebral morphology. The absence of a visible thyroid shield or an apron on a cephalometric radiograph does not necessarily imply the absence of shielding. Possibly, the shield could have been placed outside of the scope of the film. Also, we do not have information to what extent thyroid shields were not used because it may have been thought to interfere with the imaging. Such a practice may be hinted in the increased utilization of aprons after 2000.

In conclusion, young individuals, who are more radiosensitive than adults, have more neck structure exposed in cephalometric radiography. In this survey, thyroid shields were less likely to be employed in children than in adults. These findings indicate that dose determination studies in adult phantoms where the neck structures are carefully excluded from any direct radiation may be unrepresentative of real-life doses to the neck in children without thyroid shielding. Estimation of real-life doses is important in providing a better understanding of the risks associated with radiation.