Answer to Question #14048 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
My teenage son just received extraoral bitewing x rays from the dentist, using a panoramic machine. The dentist stated the x-ray beam is very concentrated (collimated) and only target the specific area, so a lead apron is not necessary. The dentist said the radiation dose from the extraoral bitewing is much lower than standard intraoral digital bitewings. Should I be concerned about a lead apron not being used?!
The bottom lines: (1) Dental x-ray beam collimation has improved substantially over the years, resulting in a substantially narrower beam. (2) A thyroid-protective collar should be used routinely for dental x-ray procedures, whereas a full lead apron need not be used. (3) The difference in radiation dose between extraoral bitewing and standard intraoral bitewing procedures is not yet well described in the scientific literature.
Multiple methods are employed together to limit the radiation dose received by a patient during an x-ray examination. One method is the restriction of the size of the x-ray beam; this restriction is also called "collimation." Collimation protects the patient by limiting the beam's area and shape to fit the body structure(s) to be examined. Collimation of conventional dental x-ray machine beams is evident from the rectangular or circular collimator that protrudes from the machine. Collimation of panoramic machine beams is also employed but is not obvious, as the beam restriction device is hidden inside the machine housing.
Another method to limit radiation dose is to add shielding to the outside of the patient, using radiation-attenuating devices. Examples include (1) the lead apron, which covers the chest, abdomen, and pelvic region, and (2) the thyroid gland-protective collar, which surrounds the patient's neck. Decades ago, dental x-ray beams were less well collimated and less well filtered, and lead aprons were then recommended. More recent improvements in technology and procedures, including collimation, have made the lead apron unnecessary in today's dental imaging. The thyroid collar, however, is recommended for all dental x-ray exposures, including intraoral and panoramic examinations, except for those procedures in which the collar would reduce the quality of the image. This recommendation is based on the relatively higher sensitivity of the thyroid gland to radiation-induced development of cancer. Some, but not all, dental x-ray procedures do involve thyroid exposure that can be reduced by use of the thyroid collar.
In extraoral bitewing examinations, a dental panoramic x-ray machine is configured to produce a more collimated beam to image the regions of the teeth susceptible to decay. Extraoral bitewing radiography is a new technology, and there is currently little published research on the dose and risk differences between extraoral bitewing procedures and standard intraoral bitewings. X-ray dose from an extraoral bitewing series does seem to be lower than the dose from a standard full-view panoramic.
Again, although a lead apron is unnecessary for properly performed dental x-ray imaging, a thyroid-protective collar is recommended. An extraoral bitewing x-ray series is likely to be lower in dose than a full view panoramic. Dose comparisons between extraoral bitewings and standard intraoral bitewings are so far minimally reported in the research literature.
S. Thomas Deahl DMD, PhD
Comment from the editor: I would like to add that the thyroid dose is very low, and the majority of the dose (80–90%) is due to scattering of the x rays inside the patient. Thyroid shields don't provide any protection from the internal scatter In one study the absorbed dose to the thyroid, based on measurements in a phantom (a plastic model) without a thyroid shield was estimated to be between 0.027 mGy and 0.068 mGy. Another study showed that the risk of thyroid cancer increases after 50 to 100 mGy (Iglesias et al. 2017). Given that the minimum dose that is likely to slightly increase the risk of thyroid cancer is more than 700 times higher than the maximum dose expected from a full panoramic dental x ray, and the fact that extraoral dental x rays may use less radiation than the full panoramic dental x rays, you shouldn't worry.
Deirdre H. Elder MS, CHP, CMLSO
Iglesias ML, Schmidt A, Ghuzlan AA, Lacroix L, Vathaire F, Chevillard S, Schlumberger M. Radiation exposure and thyroid cancer: a review. Arch Endocrinol Metab. 2017 Mar-Apr;61(2):180–187. doi: 10.1590/2359-3997000000257. Epub 2017 Feb 16. PMID: 28225863.