Answer to Question #4168 Submitted to "Ask the Experts"

We were informed a few days ago that our son will likely lose his permanent lateral teeth. The second opinion I have consulted has advised that we get a NewTom CT Scan of his mouth for further information. I have two concerns and two questions:

1. Our son has had numerous x-ray exams, (a) x-ray films every six months as a standard procedure from ages three to 10, (b) seven panoramic films for orthodontic work, (c) approximately 10 other x-rays exams for various orthodontic reasons, and (d) one cephalometric film for orthodontic work. How much radiation in terms of days/months/years in the normal environment is this equivalent to? Should I be concerned about this additional CT exam?

2. Our son has had anaphylaxis to milk, eggs, tree nuts, peanuts, and sesame since he was six-months old. I was told his immune system is so busy fighting his allergies that it is not as "effective" as the normal person. Would there be any reason to believe that the same radiation applied to a normal person versus a highly allergic person might affect the highly allergic person more at lower doses?

To answer Question 1, I will have to be somewhat general since we don't have information on specific film type, beam size, exposure time, etc., for the individual examinations, but I have used "worst-case scenarios" for the calculations.

Let's assume that your child  received two bite wing x rays every six months from age three to age 10, which would be 14 sets of images. There are seven panoramic radiographs, about 10 periapical films, and one cephalometric view for orthodontics. Based on information cited in a widely respected textbook (White et al. 2004), the equivalent number of days of background radiation exposure for adults is:

Bitewings* — 14 sets (2-film sets) x 2 background days/set = 28 background days
Panoramic — 7 sets x 3 background days/set = 21 background days
Periapical* — 2/3 set x 14 background days/set = about 10 background days
Cephalometric — 1 set x 27 background days**/set = 27 background days

Total equivalent days of background radiation = 86 days

*(For the intraoral radiographs, for example, bitewings and periapicals, if the dentist used a faster film, the dose would be lower.)

**(Based on a "skull view" Typically cephalometric radiographs for orthodontics are collimated to a smaller dimension, and have an aluminum filter over part of the anatomy to reduce the dose.)

The number of days of background radiation equivalent have not been calculated for children. While children are more at risk for radiation effects due to the growing tissues, the radiation doses required to produce a diagnostic image are lower due to their smaller size. I am making the assumption that these cancel each other out and thus used the published information for adults.

You may find other equivalent days of background estimates than those listed above, which may somewhat lower in value. There is some disagreement in calculating the effective dose and, thus, the equivalent background days, as to whether the dose to the salivary glands should be included as a separate organ in the calculation. For medical imaging, the salivary gland is not included separately since it receives such a small amount because it is far from the area being examined. For dental imaging, however, the salivary glands are in the path of the beam and thus do pick up some radiation exposure. If they are counted separately, the dose estimate is higher than if they are not counted. The dose estimates in the table above do include the salivary glands and thus are higher than some other published doses.

You should also be aware that background radiation will vary considerably around the country, with persons living at higher altitudes receiving more cosmic radiation. There are also variations in the amount of radon that people are exposed to in their homes. The calculations of background equivalent are based on the average dose for the United States.

Regarding the addition of a NewTom CT scan, I assume from your opening paragraph that the recommendation for this type of scan is to evaluate the amount of bone available for dental implants in case your son should lose the lateral incisors. The NewTom is one of a new generation of cone-beam CT scanners that allow us to obtain 3-D information, including cross-sectional views of proposed implant sites, at a relatively low radiation dose. Some orthodontists are using this type of imaging to replace the usual panoramic and cephalometric views.

There is some published information on radiation doses from the NewTom (Ludlow et al. 2003). The radiation effective dose for a full-head NewTom image is the equivalent of about four to six panoramic radiographs, depending on whether the salivary glands are counted separately or not. To put this into perspective, the cone-beam CT requires 30-90 times less radiation than a conventional CT scanner for dental implant imaging.

First of all, the decision to make a radiographic examination should be based on the type of information needed to make an accurate diagnosis and treatment plan and to monitor changes in the condition, if needed. In addition, the imaging technique that requires the lowest amount of radiation to produce that information should be selected if there is more than one technique available.

Regarding Question 2 and your son's allergies and a potential increased risk of radiation effect, I was not able to find any literature on this issue. Because the dental radiographs are to a very limited part of the body and are relatively low in dose, it is unlikely that the allergies will cause any significant additional risk to him.

Sharon L. Brooks, DDS, MS
Diplomate, American Board of Oral and Maxillofacial Radiology

References
White SC, Pharoah MJ, Oral Radiology Principles and Interpretation, 5^th edition, St Louis: Mosby; Chapter 3, p 54; 2004.

Ludlow JB, Davies-Ludlow LE, Brooks SL. Dosimetry of two extraoral direct digital imaging devices: NewTom cone beam CT and Orthophos Plus DS panoramic unit. Dentomaxillofac Radiol 32: 229-234; 2003.