TCT: Nuc imaging scrutinized for radiation exposure

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WASHINGTON, D.C.—Cardiac imaging accounts for 15 percent of the radiation exposure in the U.S. While particular attention has been paid to reducing dose for coronary CT angiography (CCTA), nuclear studies still emit high levels of radiation, according to Andrew J. Einstein, MD, during a presentation Wednesday at the 2010 annual Transcatheter Cardiovascular Therapeutics (TCT) meeting.

"We don't want to overlook the fact that the biggest clinical significance of radiation exposure are the benefits. Ultimately, this exposure to ionizing radiation can improve patient and societal outcomes." said Einstein, who is from Columbia University Medical Center in New York City.

He noted that two forms of radiation effects—deterministic or stochastic—are crucial to evaluate. Deterministic effects, such as skin burns and radiation sickness, are caused when radiation kills cells, while stochastic effects are caused when radiation mutates cells.

"Deterministic effects occur only above the threshold dose, which typically is going to be high, and only after a large proportion of cells have been killed by radiation—which varies from individual to individual," he said.

Regarding stochastic effects, the common assumption is that risk is proportional to dose at low doses. However, stochastic effects do not necessarily depend on dose. Additionally, these cancers occur only after a latency period of about five to 10 years.

Einstein asked, "What are the typical effective doses of cardiac studies using traditional protocols and how do we know that patients are not at risk of developing cancer downstream?"

There also can be wide variation. A diagnostic cardiac cath has a typical effective dose of almost 7 mSv, but the dose may vary depending on equipment, operator experience and technique. Just a few years ago, the typical dose from a CCTA exam averaged 15 mSv to 20 mSv. Today, it is possible—with certain equipment and protocols—to conduct the same exam with less than 5 mSv exposure. CT calcium scoring on average emits about 2 mSv of radiation.

While research into CT imaging has resulted in dramatically reduced exposure, nuclear and fluoroscopy imaging have some catching up to do.

SPECT imaging with a single-day, low-dose rest, high-dose stress protocol can emit 10 mSv, while a dual-isotope protocol has a typical effective dose of almost 25 mSv. "How do we know these nuclear cardiology patients may develop malignancies?" Einstein asked.

Previous epidemiological studies on which radiation dose exposure is based are from Japanese survivors of atomic bomb blasts, as well as from workers exposed to radiation. No data exist from large medically exposed populations, he noted.

Atomic bomb survivors were exposed to a median radiation dose of 29 mSv, leading to a 2 percent excess relative risk of cancer. Comparatively, fetuses exposed to x-ray exposures experience a typical dose of 10 mSv, which was related to a 40 percent excess relative risk of cancer. Lastly, radiation workers in 15 countries exposed to an average of 10 mSv saw a statistically significant relative risk of cancer.

"Even though we need better epidemiological data, we think these low radiation doses [in cardiac imaging] can be related to developing an increased risk of malignancy," Einstein said.

"If these procedures were only used infrequently, there wouldn't be a potential public health problem," he added.