Study finds measurable cancer risk from CT angiography scans

January 16, 2007

As the number of cardiovascular CT angiography scans continues to grow, researchers are attempting to gauge the potential effects of radiation exposure during the procedure. Although extensive epidemiologic studies are not yet available, new research suggests a measurable risk.

As the number of cardiovascular CT angiography scans continues to grow, researchers are attempting to gauge the potential effects of radiation exposure during the procedure. Although extensive epidemiologic studies are not yet available, new research suggests a measurable risk.

A study presented at the American Heart Association meeting by Andrew J. Einstein of the Mount Sinai School of Medicine in New York City found that a single cardiovascular CTA study produced a non-negligible lifetime attributable risk (LAR) of cancer in patients. The risk was greatly influenced by factors such as the patient's age and gender, as well at the scan protocols used.

Using data from the National Research Council's Biological Effects of Ionizing Radiation VII (BEIR VII) report released in 2006, Einstein and colleagues determined that the LAR of cancer from cardiovascular CTA was highest among young women. Risk falls as a function of age for both sexes, dropping off most noticeably after the age 60.

Attributable risk to a 20-year-old woman from a standard scan without tube current modulation was about 7000 cases per million, or one in 143, Einstein said. For a 40-year-old woman, risk dropped to 3500 cases per million, or one in 284. For a 60-year-old woman, risk was one in 465, and for an 80-year-old woman, about one in 1300.

Risks were considerably lower in men. For a 20-year-old man, LAR of cancer incidence from a single study was one in 686, equivalent to the risk in a 70-year-old woman, Einstein said. Risks to men also decreased as a function of age, to one in 1000 for a 40-year-old, one in 1200 for a 60-year-old, and one in 3200 for an 80-year-old.

This difference may be caused by a few factors, Einstein said. Women face a higher risk of breast cancer than men, and the breast tends to lie within the field of radiation. Women also tend to be more sensitive to radiation as a whole, according to BEIR VII data. A 100-mSv dose of radiation to the lung will cause lung cancer in 3460 cases per million in a 20-year-old woman, for example, while it will cause lung cancer in only 1490 cases per million in a 20-year-old man.

The scan protocols used and the areas scanned can also have a dramatic effect on LAR of cancer. Tube current modulation protocols can reduce LAR of cancer by up to a third, Einstein said. According to the BEIR VII data, using these protocols to reduce the tube current by 35% would result in a 35% reduction in cancer risk. In a 20-year-old woman, for example, the risk dropped from one in 143 without tube current modulation to one in 219 with tube current modulation. The researchers noted similar findings across both age and sex.

This finding plays up the importance of heart rate control in patients. Tube current modulation can be used to reduce dosage by as much as 50% if the patient's heart rate is sufficiently low.

"Beta blocking is not just for image quality, it's also for cancer protection," Einstein said.

By the same token, scan protocols that expand the field-of-view can increase LAR of cancer. Including the aorta in the triple rule-out study dramatically increased cancer risk, he said. Extending the baseline scan by 10 cm to cover the aortic arch increased the risk of cancer incidence by about 45% in men and about 25% in women. Those results did not vary strongly by age.

Einstein attributed the lower increase in women to the fact that the breast dose did not significantly increase with the extended scan.