Concern About Risks From CT Scans

Posted on January 1, 2010

Written by Ira Gordon, DVM DACVR-RO (Radiation Oncology)

In December 2009, several studies were widely covered by the national media that attempted to assess the risk of radiation induced cancer from having a diagnostic CT scan. While this data focused on the risks of human patients, the concern about this issue will undoubtedly also be seen from pet owners. What will you say when you recommend a CT scan and a pet owner asks you about the risk of that scan causing cancer in their pet?

The recent study that received the most media attention was titled “Projected Cancer Risks From Computed Tomographic Scans Performed in the United States in 2007” and was published in the Archives of Internal Medicine (Arch Intern Med. 2009;169(22):2071-2077). In this study, the researchers used survey and insurance claims information to assess the types of scans that were performed in 2007. Then, they used a cancer incidence model derived from the latest surveys of cancer risks in Japanese atomic bomb survivors and the estimated doses of radiation they received. Keep in mind that these calculations are complex and are based on the dose and organs receiving radiation as well as the age of the patient at the time of the procedure.

Overall, the study projected 29,000 cancers including 14,500 cancer deaths from CT scans performed in 2007 from 56.9 million CT scans performed. This works out to a 0.05% risk (1 in 2000) and depends on the type of scan since different scans result in different absorbed doses.

A related study in the same journal reports on the average doses received from CT scans per patient and are outlined in the table below. For reference, we have also added other common radiographic procedures. The milliSievert (mSv) is the accepted unit for absorbed dose in radiation protection.

The American College of Radiology responded to this study by pointing out that the study relies on data which equates effects experienced by atomic bomb survivors in Japan to present day patients who receive CT scans. Most CT studies are performed in controlled settings and result in limited radiation exposure to a small portion of the body whereas atomic bomb survivors experienced instantaneous exposure to the whole body. CT exams expose patients solely to X-rays. Atomic blast survivors were exposed to X-rays, particulate radiations, neutrons, and other radioactive materials. The known biological effects are very different for these two scenarios. The statement also critiques the lack of consideration of the medical status of the patient population receiving CT scans. The full statement can be found at: http://www.acr.org/HomePageCategories/News/ACRNewsCenter/StatementonRecentStudiesRegardingCTScans.aspx.

Ideally, to answer a question about how much a CT scan increases the risk of cancer, one would need to perform a randomized study comparing patients that could receive either a CT scan or some other diagnostic test and then follow them for the rest of their lives to determine if there is a difference in cancer risk. Because such a study is unlikely to occur, the real answer to that question may not be known for a long time. However, it is always important to consider all of the risks of a diagnostic or therapeutic procedure in addition to the potential benefits when selecting the ideal course of action for your patient. This is how I try to weigh my recommendations and is what I explain to clients who ask about the risks of a diagnostic or therapeutic procedure.