Thyroid shielding in cone beam computed tomography: recommendations towards appropriate use

The objective of this paper is to provide recommendations towards the appropriate use of thyroid shielding in dental cone beam CT (CBCT). Based on current evidence of thyroid radiosensitivity, dosimetric data in the presence and absence of shielding, and a depiction of potential adverse effects of thyroid shielding, a concise set of recommendations was prepared. According to current risk models, thyroid sensitivity is particularly high at a young age, and much higher for females. In the literature, involving adult male, female and paediatric reference phantoms, the use of a tightly fitted thyroid collar with a lead-equivalent thickness of at least 0.25 mm has consistently shown a significant reduction (average: 45.9%) of the equivalent dose to the thyroid. It can therefore be recommended that thyroid shielding should be routinely used for children undergoing CBCT scanning and is recommended for adults up to the age of 50. The increase of the X-ray tube current from automatic exposure control systems due to thyroid shielding can be avoided by placing the shielding collar after acquiring the scout images. Should real-time tube current modulation be implemented in dental CBCT imaging in the future, perspectives regarding the appropriate use of shielding may change according to current trends in CT. In view of the manifestation of metal artefacts, shielding is best avoided if radiological evaluation of tissues below the lower border of the mandible is needed.