Debate Sparked Over Risks of CT-based CAC Screening

A recent study suggesting that CT-based coronary artery calcification (CAC) screening puts patients at an increased risk of developing cancer over time has raised questions about the need for screening protocols and whether equipment and screening recommendations referenced in the study are in fact commonly utilized.

Andrew J. Einstein, M.D., Ph.D. Columbia University Medical Center

Geoffrey Rubin, M.D. Stanford University

Based on computer modeling of radiation risk, the study suggests that widespread screening for the buildup of calcium in the arteries using CT would lead to an estimated 42 additional radiation-induced cancer cases per 100,000 men and 62 cases per 100,000 women. The study appeared in the July 13, 2009 issue of the Archives of Internal Medicine (AIM).

Due to differences in scanner models and techniques, the radiation dose from a CT scan to detect CAC varied more than tenfold, researchers discovered.

“For the vast majority of patients undergoing CT and all radiology procedures utilizing ionizing radiation, the benefits outweigh the risks,” said study author Andrew J. Einstein, M.D., Ph.D., director of cardiac CT research at Columbia University Medical Center in New York. “However there is a small subset of patients for whom potential risks outweigh the benefits.”

An editorial appearing in the same issue of AIM points out the controversy surrounding the value of CAC screening and stresses the need for a standardized protocol for CAC quantification. Some cardiac imagers are also taking issue with the study, asserting that the authors’ findings are not adequately supported by the research.

“I don’t think it’s a robust finding for CT practice in 2009,” said Geoffrey Rubin, M.D., a professor of radiology at Stanford University in California. “While the results highlight the importance of responsible imaging to avoid unnecessary radiation exposure, the combination of older CT protocols and highly aggressive CAC screening recommendations should substantially overestimate the risk to the population under current practice standards.”

Variation in Radiation Dose Surprises Researchers

To reach their conclusions, authors found that an effective dose range measured from 0.8-10.5 mSv, and extrapolated data from that measurement and utilized SHAPE (Screening for Heart Attack Prevention and Education) recommendations of screening every five years for men ages 45 to 75 and women ages 55 to 75. A median dose of 2.3 mSv was used in the study calculation.

The wide variation in radiation dose detected by the team was unexpected, said Dr. Einstein. “It really surprised me,” he said. I didn’t think we were going to find such outliers with high doses for calcium scoring. At the same time, we thought it was worth investigating.

“I was most surprised that one of the CARDIA sites had an effective dose of calcium scoring over 10 mSv,” continued Dr. Einstein. “I suspect that scanner, which is older technology, is no longer being used, but it taught us a lesson that we must be careful about the technology being used.”

Variation in radiation doses from such screening is one of the key take-home points from this research. CT protocols can be optimized to minimize the dose and, therefore, risks, he said.

“We observed a more than tenfold variation in radiation doses from CAC screening with multidetector CT depending on the protocol and, therefore, a wide variation in the estimated radiation-induced cancer risk,” said the study’s lead author, Kwang Pyo Kim, Ph.D., then of the National Cancer Institute in Bethesda, Md., and now an assistant professor in the Department of Nuclear Engineering at Kyung Hee University in Gyeonngi-do in South Korea. “This means a patient may receive radiation doses 10 times higher or lower depending on the hospital he visits.”

Screening Guidelines Fuel Debate

Dr. Rubin takes issue with researchers basing their data on SHAPE recommendation of screening every five years.

“The goals of the SHAPE task force are laudable, however their recommendations for atherosclerosis screening are among the most aggressive paradigms proposed to date.” he said. “None of the major medical societies advocate this approach and I am not aware of any populations for whom this screening is being applied. At this time, this approach is purely theoretical.”

In the AIM editorial, authors Raymond J. Gibbons, M.D., and Thomas C. Gerber, M.D., Ph.D., both of the Mayo Clinic in Rochester, Minn., point out that, “The U.S. Preventive Services Task Force recommends against routine CAC screening with CT. The American Hospital Association has indicated that CAC scanning may be reasonable in selected, intermediate-risk patients but does not recommend it in low or high-risk patients.”

Dr. Kim rebutted, saying, “The point is that it depends on comparison between benefits and risks. If benefits from the screening apparently exceed the risks, the guideline is not too aggressive. If not, we need to think about that more carefully. Therefore, it is necessary to perform such a study to compare the benefits and risks.”

Dr. Rubin said he does not believe the study represented state-of-the-art methodologies.

“Of the 20 CT acquisition protocols considered, 15 came from published studies where the protocols were 7-9 years old and used equipment that was among the earliest generation of multidetector-row CT,” he said. “The other protocols were collected from hospitals that apparently were selected without justification as to why they would be representative of current practice. Protocols derived from a formalized survey of current practice that would legitimately represent the spectrum of CT scan protocols used for CAC screening in 2009 would have been preferable.”
 
CAC Screening Standard Urged

Ultimately professional organizations are responsible for advocating for standardization of CAC imaging with CT, stressed Drs. Gibbons and Gerber in their editorial. “In the meantime, every effort should be made to reduce patient dose while balancing image noise and quality sufficient for confident interpretation,” they write.