Answer to Question #10489 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:

Q
What proportion of dose to the thyroid from intraoral dental exposures is from internal scatter versus external scatter that exits the jaw and then impacts the thyroid through the skin? Similarly, how effective is the thyroid shield at reducing dose (absorbed dose with shield divided by absorbed dose without shield)?.

I have been unable to find a journal article or other legitimate reference material that covers the topic specifically for dental. All I can find are ALARA (as low as reasonably achievable) arguments, but not quantifiable numbers that clearly show the thyroid shield effectiveness in dental settings. We had a physicist on staff that estimated its effectiveness at 1 percent, but I cannot find a legitimate source that backs that up.
A

It is hard to find a journal article that quantifies the effectiveness of leaded collar shields on the reduction of dose to the thyroid from dental radiographic procedures. Fortunately, I found a journal article from 1984 that specifically addresses your question. 

However, if you take a look at the recommendations contained within the National Council on Radiation Protection and Measurements, NCRP Report 145, Radiation Protection in Dentistry, you will find the following guidance for lead aprons and collars:  

3.1.8 Leaded Aprons

Leaded aprons for patients were first recommended in dentistry many years ago when dental x-ray equipment was much less sophisticated and films much slower than current standards. They provided a quick fix for the poorly collimated and unfiltered dental x-ray beams of the era. Gonadal (or whole-body) doses from these early full-mouth examinations, reported as large as 50 mGy (Budowsky etal., 1956), could be reduced substantially by leaded aprons. Gonadal doses from current panoramic or full-mouth intraoral examinations using state-of-the-art technology and procedures do not exceed 5 µGy (5 x 10-3mGy) (White, 1992). A significant portion of this gonadal dose results from scattered radiation arising within the patient’s body. Technological and procedural improvements have eliminated the requirement for the leaded apron, provided all other recommendations of this Report are rigorously followed (NRPB, 2001). However, some patients have come to expect the apron and may request that it be used. Its use remains a prudent but not essential practice.
The use of leaded aprons on patients shall not be required if all other recommendations in this Report are rigorously followed. However, if under exceptional circumstances any of these recommendations are not implemented in a specific case, then the leaded apron should be used.

3.1.9 Thyroid Collars

The thyroid gland, especially in children, is among the most sensitive organs to radiation-induced tumors, both benign and malignant (Appendix B). Even with optimum techniques, the primary dental x-ray beam may still pass near and occasionally through the gland. If the x-ray beam is properly collimated to the size of the image receptor or area of clinical interest, and exposure of the gland is still unavoidable, any attempt to shield the gland would interfere with the production of a clinically-useful image. However, in those occasional uncooperative patients for whom rectangular collimation and positive beam-receptor alignment cannot be achieved for intraoral radiographs, then thyroid shielding may reduce dose to the gland without interfering with image production (NRPB, 2001).

Thyroid shielding shall be provided for children, and should be provided for adults, when it will not interfere with the examination”
In the paper by P.A. Sikorski and K.W. Taylor (1984), the authors demonstrated that for some single-film projections where they maximized the thyroid irradiation by positioning the x-ray cone as inferiorly and as steeply as possible, the thyroid shield was able to intercept the primary beam and reduce the thyroid dose by as much as 42 percent. However, for normal positioning, the dose reduction was less than 10 percent.

Additionally, for some nonstandard (extraoral) dental exams like a lateral cephalogram (whole skull) and submentovertex (base of skull) projections, the thyroid is in the radiographic field and in these cases, thyroid shielding may be of some use, especially in children, if it does not compromise the exam goals. It should be noted that these types of exams are usually performed in a regular radiology or orthodontic practice and not in a dental office.

Consequently, while the dose reduction in normal practice is small, I find the NCRP guidance to be prudent on an As Low As Reasonably Achievable (ALARA) and patient expectation basis.

Mike Bohan
Radiation Safety Officer

References

  1. Sikorski PA, Taylor KW. The effectiveness of the thyroid shield in dental radiology. Oral Surgery 58:225–236; 1984.

  2. Gibbs SJ. Effective dose equivalent and effective dose: .Comparison for common projections in oral and maxillofacial radiology. Oral Surg Oral Med Oral Pathol Oral Radiol & Endod 90:538-45; 2000.
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