Dental Patient Doses Information

E. Russell Ritenour and S. Julian Gibbs

In dental radiography, the part of the head that receives the greatest dose is the skin in the area where the x rays enter. In the table below are some "maximum" skin doses received each time an exam is performed. Of course, these doses vary somewhat from different machines, but the figures listed below are probably within 10-20% of the actual amounts received. The dose to the thyroid gland from a single, intraoral exposure such as a bitewing is approximately 0.3 millirem (mrem) (Bengtsson, et al.).
 

In 1991, a research team at the Academic Center for Dentistry in Amsterdam made an elaborate series of measurements of dose to all areas of the head and neck during bitewing radiography using a plastic "head phantom." They found an effective dose (ED, defined in the next paragraph) for the bitewing of 0.4 mrem. To put this in perspective, background radiation from naturally occurring radionuclides in our environment and from cosmic rays delivers almost a thousand times as much radiation every year (approximately 360 mrem). Furthermore, differences in background levels in different parts of the country are about a thousand times larger than the ED from a bitewing. For example, moving from a lower-background region such as Minneapolis, Minnesota, to a higher-background region, such as Denver, Colorado, for a year would result in an increase in ED for that year that is about the same as a several bitewing exams (the increase is approximately 15 mrem per year)

Epidemiological studies comparing cancer rates in high- and low-background-radiation regions have repeatedly failed to show any association with background levels in this or in other countries. It also appears that radiation doses at levels of as much as several times natural background do not play a significant role in causing cancer. To predict the probability of radiation causing harm, we calculate a quantity called ED in units of millisieverts (mSv) or mrem. This quantity takes into account the type of radiation, which is x rays in this case, and the body parts or organs involved, for example, salivary glands, sinus areas, mandible, thyroid, etc. Organs and body parts have been assigned "tissue weighting factors" by the International Council on Radiation Protection and Measurements (ICRP). These factors are derived from review of the epidemiological data that exist for humans exposed to large amounts of radiation, primarily the survivors of the atomic weapon detonations in Hiroshima and Nagasaki. The factors indicate the relative likelihood of harm (that is, cancer, birth defects, or increased risk of genetic disorders in future generations) per unit dose. Since the dose to reproductive tissue is much less than 0.1 mrem for all of the dental exposures here, the only health issue considered is cancer induction.

It is important to point out that in epidemiological studies of humans, no actual increase in cancer incidence has ever been found in groups of humans who have received EDs below 100 mSv, or 10,000 millirem. The EDs associated with dental exposures are much, much smaller than this. But, in order to come up with some estimate of harm for purposes such as setting standards for reasonable levels of exposures in medicine, it is assumed that the probability of harm seen at high doses decreases proportionally with dose and never becomes zero. In 1995, a joint study on the role of medical radiation in thyroid cancer was conducted in Sweden by the US and Swedish National Cancer Institutes. Sweden was a better country for this type of study than the United States because its entire healthcare system, including its medical records, is more centralized and standardized. The study showed that patients with thyroid cancer had received the same number of diagnostic x-ray studies, including dental x rays, as the general population. If it had been found that people with thyroid cancer had had more exposure, it could have indicated some connection between the radiation exposure and thyroid cancer. Of course, it's tough to prove that something is totally unrelated to something else, but this was pretty good evidence that there isn't much of an association between medical x rays and thyroid cancer (Inskip, et al.).

There is also a 1988 study funded by the National Cancer Institute and conducted in Los Angeles by a team at the University of California, Los Angeles that found a positive correlation between cancer of the parotid gland and previous dental x-ray exposure. It didn't seem to be as definitive a study as the Swedish study. The Los Angeles study information was obtained strictly from interviews with parotid cancer patients, whereas the Swedish study used actual medical records. The Los Angeles study population included only about 400 cancer patients compared with over 4,000 for the Swedish study. Also, US citizens tend to move around the country during their lifetimes, which causes a bigger difference in their lifetime EDs than is caused by variation in dental radiography practices. So, there is probably some controversy in this area (Preston-Martin, et al.).

There are reports of epidemiologic studies showing associations between dental x ray and certain head and neck cancers. However, most of these were published years ago and are the results of dental exposures before World War II, when equipment was much cruder and doses much greater than they are today. These epidemiologic studies show only associations and do not establish cause and effect. There are no such reports from recent exposures. Risks from dental x rays are very small when compared with other sources of radiation.

— Written by E. Russell Ritenour and S. Julian Gibbs

References

• Ballinger PW. Merrill's atlas of radiographic positions and radiologic procedures. 5th ed. Vol. 1. St. Louis: CV Mosby; 1982
   
• Bengtsson G, Blomgren PG, Bergman K, Aberg L. Patient exposures and radiation risks in Swedish diagnostic radiology. Acta Radiologica, Oncology, Radiation Physics and Biology 17:81-105; 1978.
   
• Inskip, et. al. Medical diagnostic x rays and thyroid cancer, Journal of the National Cancer Institute 87:1,613-1,621; 1995.
   
• Preston-Martin, et. al. Prior exposure to medical and dental x-rays related to tumors of the parotid gland. Journal of the National Cancer Institute 80:943-949; 1988.