AHA: Cancer risk from cardiac CT overstated

NEW ORLEANS—The risk of radiation-induced cancer from cardiac CT is substantially lower than previously reported for general populations, according to a poster presentation at the 2008 American Heart Association (AHA) Scientific Sessions this week.

Previous studies on lifetime cancer risks from radiation at cardiac CT are based on extrapolation of data unrelated to medical imaging (victims of the atomic bombs dropped in Japan) and on presumptive scan protocols, but not on actual patient cohorts.

In previously published studies, different researchers concluded the risk of cancer from radiation exposure during CT for cardiovascular disease was approximately 1 in 114, but the new study suggests the risk is 1 in 1000.

Walter Huda, PhD, and colleagues at the Medical University of South Carolina (MUSC) in Charleston aimed to determine the realistic risk of radiation induced cancer based on a real-life clinical patient population undergoing cardiac CT.

Researchers studied 104 consecutive patients undergoing 64-slice cardiac CT (Somotom, Siemens) at their facility. They obtained organ dose estimates using the ImPACT dosimetry spreadsheet. ImPACT is the U.K.'s CT scanner evaluation center.
Huda and colleagues converted organ doses into patient lifetime risk of developing radiation-induced fatal and non-fatal cancer using the BEIR VII approach.

“BEIR” stands for Biological Effects of Ionizing Radiation and the BEIR series of reports are the most authoritative basis for radiation risk estimation and radiation protection regulations in the United States.

Patient cancer risks were adjusted taking into account patient sex, age and weight, the latter being an often neglected factor influencing radiation risk, according to co-author U. Joseph Schoepf, who presented the poster. These data were used to determine median values and the 10th and 90th percentiles.

Sixty-two percent of patients were male, with a median patient age of 59 years and a median weight of 202 lbs. All patients were scanned at 120 kV, and the median dose length product (DLP) was 1124 mGy-cm. (DLP is the measure for total radiation exposure for the whole series of images and is proportional to scan length.)

Researchers found that in normal sized patients, the organs receiving the highest radiation doses were the lung (76 mGy) and the female breasts (92 mGy).

The average patient lifetime risk for developing a radiation-induced cancer was 0.12 percent, of which 90 percent would be fatal. Approximately 85 percent of the radiation risk arises from the irradiation of the lung. Patients with an age and weight at the 10th percentile, who receive a DLP at the 90th percentile, would have a cancer risk double the average value.

In a typical clinical population undergoing cardiac CT, patient are predominantly male, heavier set and in their fifth to seventh decade of life. In such a patient population, the realistic risk of radiation-induced cancer from cardiac CT is 0.12 percent, with the risk to the most sensitive patients being about twice this value.

“Thus, in a real-life clinical patient cohort, the realistic risk of radiation-induced cancer from cardiac CT is substantially lower than previously reported for general populations,” Schoepf said. “Appropriate patient selection and indication remain the most important means for radiation protection.”

In a second poster presentation, MUSC researchers, this time led by Joseph A. Abro, MD, found that the average effective patient dose from coronary CT angiography (CCTA) was somewhat higher than that of invasive catheter angiography. The average skin doses for catheter angiography, however, significant higher than for CCTA.

Using data from 104 consecutive patients for CCTA measurements and 102 patients who underwent diagnostic catheter angiography (Axiom Artis, Siemens), researchers found an average patient effective dose of 21 mSv for CCTA and 15 mSv for catheter angiography.

For CCTA, researchers estimated the average skin organ dose to be around 150 mGy. For catheter angiography, the average reference point dose was 1,420 mGy.

“Average skin doses at catheter angiography are an order of magnitude higher than those associated with coronary CTA and may exceed the threshold dose (2,000 mGy) for inducing skin damage,” researchers concluded.