The effectiveness of the thyroid shield in dental radiology.

The effectiveness of the thyroid shield in reducing patient exposure during routine dental radiographic examinations was studied. A nonscreen film and two film-screen combinations were used as dosimeters and to show the spatial distribution of x-ray exposure. The error in the absolute accuracy of the film dosimeter and the error in the reproducibility of the film-screen combinations were less than +/- 10%. Measurements of thyroid exposure and exposure distribution, with and without the shield, were made on a Rando phantom and on patients. In the phantom study, the reduction by the shield of exposure to the thyroid from primary beam and from scatter was found to be highly dependent on the position of the primary beam relative to the thyroid gland but independent of kVp. Exposure reduction by the shield varied from 5% to 56% for a complete-mouth survey, 2% to 18% for a bitewing survey, and 10% to 79% for a panoramic survey. In the patient study, thyroid skin exposures measured on adults were 33% to 84% lower in offices in which the shield was used as compared to offices in which it was not used. Thyroid skin exposures measured on children were 63% to 92% lower. In accordance with the ALARA principle, these results support the routine use of the thyroid shield for all dental radiography.