Answer to Question #2766 Submitted to "Ask the Experts"

Q

I am very concerned since reading a David Brenner study on pediatric CT scans. My six-year-old received a head CT three years ago. At the time it seemed that everything I read was emphasizing the minimal amount of radiation given by CT scans. I read that they delivered far-less radiation that traditional x-ray films and in one pregnancy book I even read that these were the only type of x ray that is "safe" during pregnancy. After the Brenner article came out, shortly after my son's CT, I felt very angry that I was so misinformed about the radiation risks. Both my son's doctor as well as the radiology technician dismissed my concerns with vague comments suggesting that the risk was nil. However Brenner states that head CTs are within the range of exposure of Hiroshima survivors and that 1,500 children out of 600,000 may develop cancer in their lifetimes. I am now very worried about the risk to my son. I asked the technician to give him the lowest dose possible but she commented that she needed "clear pictures." When she came out she said she had taken 10 slices one mm apart. How does this compare with the average head CT that Brenner used of 12.5 slices? I know all machines are different so it is probably next to impossible to ascertain a dose, but I am especially concerned about the risk of thyroid and brain cancer. I don't understand how so many books/sites compared CT scans to dental x rays and minimal doses, and now I find out that the doses are hundreds or thousand of times higher and are really comparable to a nuclear bomb. This can't have been new information, so why is it that three years later everyone is talking about massive doses of CT? I just feel so betrayed by the "experts" I consulted back then, angry at myself for not being more assertive and persistent with the doctor and the technician, and most of all, extremely concerned about my son.

A

The best information that I have regarding doses from CT scans of the head are from the U.S. Food and Drug Administration's Center for Devices and Radiological Health's "Nationwide Evaluation of X-Ray Trends (NEXT)" studies. While this most recent study does not present data for pediatric head studies, the mean dose to the head for adults is indicated as 40–50 mGy (4–5 rad). For an axial scan, they estimate the effective dose to be 2 mGy (200 mrad). It is anticipated that the dose to a pediatric head would be lower that the dose for an adult. The "effective dose" is a calculated dose that determines an average dose to the entire body that would represent the same risk as a high dose to only a portion of the body. Therefore, a dose of 5 rad to the head would be equivalent, in risk, to a dose to the whole body of approximately 2 mGy (200 mrad). A dose of 2 mGy is approximately equal to one year's worth of natural background radiation exposure, i.e., radiation that we are continuously exposed to from cosmic rays, radioactive materials present in the earth and building materials, and radioactive materials normally present within our own bodies. Epidemiological studies, such as the studies you mentioned of the atom bomb survivors, have shown that there is an increased risk of developing cancer when the radiation dose is high. There have not been any studies that have demonstrated a proportionate risk for low doses. The Health Physics Society's Position Statement regarding radiation risk states: "Epidemiological studies have not demonstrated adverse health effects in individuals exposed to low doses, i.e., less than 10 rem, delivered in a period of years." This statement refers to whole-body doses of 10 rem. For x rays, a 10 rem whole-body dose is the same as a 100 mGy (10 rad) whole-body dose. Your son's whole-body equivalent dose was probably less than 2 mGy (200 mrad). The risk involved, if any, is extremely small. Kenneth L. Miller, CHP, CMHP