Answer to Question #10695 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:

Q

I would like some help interpreting the article published last year in The Lancet regarding cancer risks from pediatric CT (computerized tomography) scans. At the beginning of the article they state that the risk is that one child will develop cancer from every 10,000 CT scans performed. Then, at the end, they go on to state that the risk of cancer from a single CT is 1 in 500 for an abdominal scan.

If I remember correctly they enrolled around 170,000 children in the study and followed them for 10 years. During this period there were cancers found in several participants across all the age groups, in the range of 10 to 50 additional cancers. I don't understand how these number correspond to the stated occurrence of brain cancer to be between one and three cases per 100,000 children when there were less than double that number in the study. I also don't know what they mean by "people years." They state that two to three CT scans double the risk of cancer in children, but how then, did they find so many cases in every age group?

It was very discouraging to me to read this article because my son had a CT of the head at age seven. It was something that I was very concerned about at the time, but I was told that the risk was extremely minimal. However 1 in 500 does not sound all that minimal. I don't think that comparing this to the risk of 1 in 25 or so for developing cancer for anyone is very helpful since the risk for brain tumor is no where near that, and that is what showed up, along with leukemia in their study. Thank you very much for your help.

A

You did not cite The Lancet article, so I cannot specifically comment on it. Nevertheless, I can reply to some of the information to help you better understand what they are trying to report using the information you supplied.  
 
My first thought is that they were looking at the increase of cancer in a population of children after one or more CT scans, which I think is not a well-defined analysis. If you lump all head, chest, and abdomen CT scans together, the increased risk may be 1 in 10,000, but it is 1 in 500 for abdominal CT scans. This is due in part to more "sensitive" organs, including lymphatic tissue, in the abdomen. Sensitive in this case means that the natural cancer incidence rate is higher for tissues in the abdomen not from radiation exposure per se.  
 
My guess is that the study identified and followed 170,000 children who had at least one CT scan at some age, say from 5 to 15 years of age. They then followed this population for 10 years. By the end of the study, the age of the population was from 15 to 25 years of age. The number of identified cancers was then normalized to a reference value of 100,000 individuals rather than 170,000. If the natural number of brain cancers in the population from 5 to 25 years of age per 100,000 is subtracted out of the brain cancer identified, the result is one to three cases per 100,000 children. This normalization to 100,000 would explain the "less than double" of cancer in the original number of cancers. "Person-years" usually refers to the exposure received in a population over how many years the members of the population were studied and is related to the absolute risk in the population, not to individuals. See http://www.rerf.jp/glossary_e/absorisk.htm.
 
Regarding your concerns about your son, you should consider that the study involves populations and not individual risks. If your son is in a population of 10,000 children who each receive a single CT scan, one excess cancer, not necessarily brain cancer, may be identified. Of course, 9,999 children will not be at an increased risk. You comment about 1 in 500 is for abdominal CTs not brain CTs.   
 
I am not sure where your comment of 1 in 25 comes from so I cannot comment on it. The risk of men developing a solid cancer in their lifetimes is about 4,550 per 10,000, or about one in two. That is a very high risk, and I would say that 1 in 10,000 is really an insignificant increase in risk for one CT scan. I would also note that the estimated doubling of risk was found in those children receiving two to three CT scans. I would think that your son is at no increase of risk from his single CT brain scan.
 
Personally, I would not hesitate allowing one of my children to have a CT scan if his/her physician told me it was necessary for a proper diagnosis. Even if the scan result was negative, the information would be useful to the physician. 

I hope this information is helpful.

John Jacobus, MS
Certified Health Physicist

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