Answer to Question #14969 Submitted to "Ask the Experts"

I know from previous answered questions that the radiation dose of a single dental x ray is quite small, smaller than the total radiation dose received in a typical day, and thus poses nearly zero risk What about 60-plus dental x rays over the course of three months, all directed at the same tooth, and, unlike daily total exposures, all concentrated at the same location? How would this affect risk in general, compared to if the x rays had been spread out over many years instead of a few months? Specifically, would the skin at this site be more at risk of developing cancer?

Since you have asked the question with two parts, I will divide the answer in two ways making an assumption that the visits were at intervals of every two weeks, such as that for a relatively complicated set of treatments involving endodontic diagnosis, attempted conservative therapy leading to surgical endodontics with follow-up care or some such dental therapy. This assumed dental care would then result in approximately six dental visits with 10 periapical radiographs exposed of the same region at each visit. Most dentists (95%) are using a digital radiographic system with round collimation, which yields an effective radiation dose of ~9.5 microsieverts (µSv), for a per visit dose of ~95 µ, or ~570 µ over the entire three months.

It is also useful to evaluate the actual skin entrance dose for each radiographic exposure, since you asked how the skin may be affected. The average periapical radiograph in this scenario has a skin entrance dose of 1.0 milliggray (mGy) of absorbed radiation dose. The exposure time for modern dental exposures is measured in tenths of seconds, meaning that over the course of a 60-minute procedure with 10 radiographs, there may be a radiograph every six minutes, followed by a two-week period of no radiographs then a repeat of the 60-minute procedure with 10 radiographs. This concept is the "dose rate" which, combined with the total radiation dose is exceedingly important when considering the overall skin effect. Yes, as far as considering the total number of dental radiographs to one site, this is a considerable number; however, the total dose remains quite small.

Researchers such as Hall and Giaccia in Radiobiology for the Radiologist document quite nicely the relationship of increased radiation and the dose rate effect on biologic tissues; but, they were using much higher levels of radiation than used in diagnostic radiation—those used in radiation therapy for example. However, their studies cannot be extrapolated to the very low doses (µ levels) that are used in dentistry.

Having said that, there is an effect that is well-known in radiation research known as the "stochastic" effect, which basically says that the cancer effects of radiation do have a type of random pattern that is dependent on an "odds" pattern—the more often a radiation event occurs, then the more often you may develop a cancer. For example, the more chest x rays a patient has, the more likely a patient may develop a cancer. Did the patient develop cancer due to the chest x rays, or did the patient have an underlying disease process that resulted in them having more chest x rays, but the underlying disease process was the actual cause of the cancer? This is a conundrum no one really knows the answer to, but it is a known phenomenon.

Final summary of whether the skin at the site is at higher risk for developing cancer. Modern science cannot yet answer the question definitively. Some researchers would say probably not, others would say possibly, but if so, the chance would be exceedingly small, but only the "stochastic" risk. The good news is that Congress recently has again funded research in low dose radiation research so answers may be on the horizon.

Jeffery B. Price, DDS, MS, MAGD, FICD