Study assuages cardiac CT radiation worries

March 8, 2008

Patients’ risk of developing a radiation-induced cancer is actually far lower than reports suggest, ECR delegates concerned about the level of radiation associated with cardiac CT were told in Vienna.

Patients' risk of developing a radiation-induced cancer is actually far lower than reports suggest, ECR delegates concerned about the level of radiation associated with cardiac CT were told in Vienna.

The good news came courtesy of Dr. Joseph Schoepf, an associate professor of radiology at the Medical University of South Carolina in Charleston. It follows a study of 104 consecutive patients (64 men, 40 women) scheduled for cardiac CT on a 64-slice scanner.

Organ doses were calculated using the ImPACT dosimetry spreadsheet and then corrected for patient weight. The risk of patients developing radiation-induced cancers was then determined using the BEIR VII approach, a method that draws on data from Japanese atomic bomb survivors.

"Weight is an important factor in radiation risk. The heavier the patient, the lower the risk from radiation," Schoepf said.

Researchers found that the risk of an average patient in their group developing cancer was 0.12%. The risk of mortality was put at 0.1%, with lung cancer forecast to be the biggest killer (85% of radiation-induced cancers).

These figures differ markedly from reports claiming a one in 114 risk of contracting cancer from cardiac CT, Schoepf said. This alarming statistic, taken from a study published in the Journal of the American Medical Association, was calculated using presumed scan protocols for a 20-year-old female patient.

In contrast, the calculations made by Schoepf and colleagues are based on actual scan parameters for a real-life patient population. Their cohort was predominantly male with a median age of 59 years and a median weight of 91 kg.

"It is always the most sensationalist numbers that get repeated over and over again," he said. "The vast majority of patients who undergo cardiac CT are in their fifth to seventh decade of life. That goes along with a significantly reduced risk of seeing a radiation-induced cancer in their lifetime. Also, our patient population was pretty heavy, again a factor that significantly reduces the risk of radiation-induced cancer."

The cancer risk doubled for the most sensitive patients in the study group. These patients were younger than average, lighter than average, and scanned with a particularly high dose of radiation. The one in 500 risk of contracting cancer faced by these patients is still far lower than the headline-grabbing one in 114 figure, however.

"If appropriately indicated, the gain in diagnostic information obtained noninvasively almost always outweighs radiation risk," Schoepf said. "Appropriate patient selection and indication are the most powerful tools for radiation protection."

The theme of radiation reduction was continued by Dr. Vicky Goh, a radiologist at the Paul Strickland Scanner Centre, Mount Vernon Hospital, in London. The evolution of CT technology means that coronary artery anatomy, myocardial perfusion, and left ventricular function can theoretically be performed in a single combined assessment. The added dose required to quantify myocardial perfusion, however, would take the patient's radiation burden well above acceptable limits.

The solution may be to use the test bolus performed during routine coronary CT angiography to carry out the desired quantification. A small-scale trial has confirmed that this strategy is feasible.

The prospective study involved 14 patients (mean age 66.5 years, eight male, six female) with suspected coronary artery disease. All underwent a retrospectively ECG-gated dynamic test bolus acquisition at the midventricular level prior to combined coronary CTA and rubidium-82 perfusion PET. The effective dose for the modified test bolus acquisition was 1.4 mSv.

"Compare this with the 12 to 15 mSv for a standard perfusion study," Goh said.

The test bolus acquisition revealed normal resting perfusion levels in 13 of the 14 patients and evidence of an infarct in one patient. Interobserver agreement was good. The CT perfusion data tallied well with that from Ru-82 PET, also from previously published perfusion studies using a range of modalities.

"We have been able to show that the quantification of perfusion is actually possible as part of a gated CTA test bolus study. It is certainly useful in a general setting, and it bodes well for combined studies in the future in coronary artery anatomy and myocardial perfusion," Goh said. "Particularly with the onset of prospective gated techniques and greater cardiac acquisition, we now should see greater integration of these techniques together."