Efficacy of thyroid collar in CBCT in children. A laboratory study.

Despite thyroid shielding has proved to be effective in reduction of exposure at thyroid area, there is still no evidence in child phantoms which shows a need for its use in this population during dental CBCT. The aim of this study was to evaluate the efficacy of different types of thyroid collars in reducing dose to the thyroid gland during exposure to dental CBCT in children.A neck head and shoulders part of an anthropomorphic phantom, which simulates a 10 years old child, was used (ATOM Model 706-C). Ten thermoluminescent dosimeters (TLD-100H) were placed in the position of the thyroid gland to determine the average dose and the dose at three different heights of the gland. The TLD-100H were read-out using an automatic hot gas reader (Harshaw 5500). A 3D Accuitomo 170 (J. Morita) CBCT device was used with a small FOV (4x4cm) and large FOV (17x12 cm) at 90 kV. The region of interest was the upper right canine in the case of the small FOV. With the large FOV, the region of interest was the facial bones. Five different thyroid collars of different thickness, material and designs were tested and compared with the results from the exposure of the dosimetry phantom without thyroid shielding.Up to 28.9% in average dose reduction was seen when using thyroid shielding during CBCT examinations with large FOV. Inconclusive results were obtained when using thyroid shielding with the small FOV. Thyroid collar seems to be effective in dose reduction for the thyroid shield mainly when the large FOV is used. Different types of thyroid collar perform differently in terms of dose reduction for the thyroid gland, being the thickness of the collar an important factor to be considered. In the small FOV, the dose reduction would be due to the reduction of the FOV size more than to the use of a thyroid collar.