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. A recent study was performed at the Department of Diagnostic Sciences at the University of North Carolina School of Dentistry in Chapel Hill, North Carolina, using a realistic head phantom and state-of-the-art imaging systems (Ludlow et al. 2008). In the table below are some typical skin and thyroid doses received for the exams indicated. The effective dose is explained below. Of course, these doses vary somewhat from different machines, but the figures listed below are probably within 10 to 20 percent of the actual amounts received by the patient.
 

To put these values in perspective, background radiation from naturally occurring radionuclides in our environment and from cosmic rays is approximately 3,100 µSv (NCRP 2006) every year. Furthermore, differences in background levels between different parts of the country are larger than the effective dose for 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 effective dose for that year that is about the same as 30 bitewing exams, or approximately 150 µSv.  

Epidemiological studies comparing cancer rates in high- and low-background radiation regions have repeatedly failed to show any association with background levels in the United States 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 increasing cancer above the natural incidence rate. The fact that routine dental exams listed above are significantly lower than background radiation exposures leads to the idea that there is no increased risk from such exams.

To predict the probability of radiation causing harm, we calculate a quantity called the effective dose in units of the millisievert (mSv) or microsievert (µSv), where 10 mSv equals 1 rem in the older radiation dose units. The effective dose takes into account the type of radiation, which is x rays in this case, and the body parts or organs involved, for example, the skin, salivary glands, bone marrow, mandible, thyroid, etc. The absorbed doses to the individual organs are, unfortunately, also expressed in mSv or µSv. The old unit for organ does was the rad, where 100 millirad (mrad) equals 1 mSv. Doses to individual organs, however, do not represent the risk or harm to the organ, as various cellular repair mechanisms attenuate the radiation effects. Rather, each organ or body part is assigned a tissue weighting factor (w_T) determined by the International Council on Radiation Protection and Measurements (ICRP 2007). For example, the w_T values for the thyroid and skin are 0.04 and 0.01, respectively, and do not have any measurement unit associated with them. The sum of the individual organ w_T values equals 1.0. Organs that do not receive radiation do not contribute to the effective dose.

The w_T values 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, such as 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 1 µSv 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 effective doses below 100 mSv. The effective doses associated with dental exposures are much, much smaller than this. Nevertheless, 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 United States and Swedish National Cancer Institute. Sweden was a better country for this type of study than the United States because its entire health care 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. 1995).

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, U.S. citizens tend to move around the country during their lifetimes, which causes a bigger difference in their lifetime effective doses than is caused by variation in dental radiography practices. So there is probably some controversy in this area (Preston-Martin et al. 1998).

there are reports of epidemiologic studies showing associations between dental x ray and certain head and neck cancers. Most of these reports, however, were published years ago and are based on the results of dental exposures before World War II, when equipment was much cruder and doses much greater than they are today. In addition, these epidemiological studies show only associations and do not establish a cause-and-effect relation between exposures and cancers. No such reports are associated with recent dental exposures. Risks from dental x rays are very small when compared with other medical exams involving radiation exposures.

— Written by E. Russell Ritenour and S. Julian Gibb
Revised and updated by John P. Jacobus in July 2010

References  

• International Commission on Radiological Protection. The 2007 recommendations of the International Commission on Radiological Protection. New York: Pergamon Press; ICRP Publication 103, Ann ICRP 37(2-4); 2007.
• Inskip PD, Ekbom A, Galanti MR, Grimelius L, Boice JD Jr. Medical diagnostic x rays and thyroid cancer. Journal of the National Cancer Institute 87:1613-1621; 1995.
  
  
• Ludlow JB, Davies-Ludlow LE, White SC. Patient risk related to common dental radiographic examinations: The impact of 2007 International Commission on Radiological Protection recommendations regarding dose calculation. Journal of the American Dental Assoc 139:1237-1243; 2008.
  
  
• National Council on Radiation Protection and Measurements. Ionizing radiation exposure of the population of the United States. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 160; 2006.
• Preston-Martin S, Thomas DC, White SC, Cohen D. 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.