What is the lifetime risk of cancer from a single 64-slice CT coronary angiogram?

Bob: In the past decade, accessibility to CT scanning has increased and its use has skyrocketed. It is estimated that 62 million CT scans are performed each year in the United States. Of these, at least 4 million are performed in children.¹ The amount of ionizing radiation associated with CT scanning far exceeds that of conventional radiography. Because large doses of radiation are associated with subsequent cancer development, the authors of this study attempted to estimate the lifetime risk of cancer from a single 64-slice CT coronary angiogram.

For me, this article is one of the most important—but most frightening—articles I've read. The conclusion of this study should have an effect on the way every family physician practices medicine.

What does this article say?

Bob: This study is unlike your typical randomized, double-blind, controlled study. First, the authors used data from the BEIR (Biological Effects of Ionizing Radiation) VII Phase 2 report that estimates radiation-induced cancer risk from currently available epidemiologic models. These include data from atomic bomb survivors and medical and occupational radiation studies. The authors then estimated the amount of radiation to specific organs through standardized male and female anthropomorphic mathematical phantoms. From these data, the authors were able to estimate the lifetime risk of cancer for specific ages, sexes, and organs.

Here are the scary conclusions: for a 20-year-old woman who undergoes a single 64-slice CT coronary angiogram with aortic arch scanning, the lifetime risk of cancer is one in 114. If it is limited to a standard 64-slice CT coronary angiogram (instead of a CT of the whole chest), the lifetime risk of cancer is one in 143. For a 40-year-old woman who undergoes a 64-slice CT coronary angiogram, the lifetime risk is one in 284. The lifetime risks of cancer from a single 64-slice CT coronary angiogram are summarized in Table 1.

Andrea: As a woman and a mother, this information really hits home. First, I can understand that a woman's risk of breast cancer from ionizing radiation to the chest would be much higher than a man's. Additionally, data from Japanese atomic bomb survivors demonstrated that a young woman's breasts are more radiosensitive than an older woman's.² But even more concerning is that the BIER VII data also demonstrated that a 20-year-old woman has double the risk of lung cancer compared with an age-matched man at the same dose of radiation.

Second, as a mother of young children, it is clear that the younger the patient, the greater the risk of subsequently developing a radiation-induced malignancy.

Mark: To put this in some perspective, it is estimated that 1 to 2 percent of all cancers are the result of CT scanning.¹ Put another way—these are iatrogenic cancers.

In children, the risk from CT-associated radiation is not limited to its ability to induce malignancies, because it also has been shown to impair cognitive development. A Swedish study demonstrated that children younger than 18 months with cutaneous hemangiomas of the head and face who received radiation doses similar to those in a CT scan of the brain ultimately did worse on high school entrance exams at 15 to 16 years of age.³

Should we believe this study?

Bob: This study is unusual, simply because the results are based completely on mathematical models. Some may consider this the Achilles' heel of the study; however, think about it for a moment. How can you do this study with humans? You can't. No institutional review board would approve it, you wouldn't be able to recruit any volunteers (let's see who would sign up for a study that might give them cancer), and it would take decades to follow large numbers of patients. Because a randomized controlled trial is not feasible, we'll have to settle for the best models that can be generated.

Mark: And because this is all theoretical, there have been challenges to the notion that CT-associated radiation will result in increased rates of cancer. Some point to animal studies that have not detected increased cancer rates in animals exposed to doses of radiation similar to those in CT scans. Some note that induction of cancer has been noted only when cumulative radiation doses exceed 500 mSv.⁴ Others suggest that low doses of radiation can actually “up-regulate” physiologic mechanisms of protection against radiation-induced cellular damage.⁵ Therefore, it is understandable that even expert radiobiologists can't come to a consensus on the risk of radiation-induced cancer associated with CT scanning.⁶

What should the family physician do?

Bob: In Journal Club, we often talk about the adverse effects of certain medications or interventions. It is important to put these adverse effects into perspective. In this study, we are talking about the possibility of fatal cancers being induced by CT scanning. This is not something as benign as dyspepsia.

Because CT-associated radiation may pose a major health hazard, it would be prudent for the family physician to take a conservative approach to ordering CT scans. A cavalier attitude to ordering CT scans because of convenience, patient request, or “just to make sure” needs to be abandoned.

Andrea: I agree. We can no longer obtain a CT for every child because they have a bump on the head, or because the parent wants a test. It is our responsibility to inform patients and parents of the risks and benefits of each and every study we do. To help put this in perspective, I like to give the patients an idea of the magnitude of the amount of radiation associated with each CT scan by comparing it with the equivalent number of chest radiographs. After I have told a patient or parent that a head CT is the equivalent of 100 to 200 chest radiographs, you'd be surprised how many say they really don't need or want the test. Table 2 lists doses of radiation associated with CT compared with chest radiography.^7,8

Mark: And we haven't even touched on cost. Just imagine what 62 million CT scans per year costs the U.S. health care system. Now, add in the costs of cancer care for the CT-induced malignancies. Don't be surprised when your local health care insurers, under the disguise of patient safety, try to put the brakes on ordering CT scans in your community.