Radiation Exposure & Hybrid Imaging: Panic Time or Not?

The issue of radiation dose is certainly an attention-grabber these days—not only within the medical community, but from the popular press as well. And the U.S. government also is enhancing its profile in this area, as evidenced by a Congressional hearing held on medical radiation in February, as well as an announcement by the FDA (accompanied by a two-day public hearing on the subject) that it will be working with manufacturers of advanced medical imaging devices to take steps to prevent patients from receiving excessive doses of radiation. So is there a hint of radiation exposure panic in the air in hybrid imaging?

It is clear that over the last year or so the subject of medical radiation exposure has become a touchy one. Just ask Kevin Donohoe, MD, of the Division of Nuclear Medicine at Beth Israel Deaconess Medical Center in Boston.

“As soon as the topic comes up with someone in the lay public, the immediate reaction is one of withdrawal,” Donohue says. “In some cases, it’s warranted. But other times, the concerns are overblown and they need to be tempered a bit. And that’s a difficult thing to do these days.”

The issue is certainly topical—enough so that as the “ad hoc” chair of SNM’s REIR (Radiobiological Effects of Ionizing Radiation) Donohoe has organized a session on “Putting Radiation Risks in Perspective for Health Care Providers” that will take place at the 2010 SNM Annual Meeting this month in Salt Lake City. He’ll be joined on the panel by Henry Royal, MD, professor of radiology and associate director, division of nuclear medicine, Washington University School of Medicine, and Pat B. Zanzonico, PhD, attending physicist at Memorial Sloan-Kettering Cancer Center in New York and manager of the center’s nuclear medicine research laboratory.

Donohoe believes that for all of the attention that’s been paid to the issue of radiation exposure, the public has remained somewhat level-headed when it comes to the issue of risks and benefits associated with radiation.

“I’ve been impressed that the public is not as concerned about this issue as much as government and people in the medical community are,” says Donohoe. “I believe it’s because the public knows that we’re talking about radiation exposure associated with medicine and when they’re not sick, it really doesn’t concern them. And when they are—when they have abdominal pain and they come into the emergency room—the last thing they are concerned with is radiation exposure.”

As the attending physicist at Memorial Sloan-Kettering, Zanzonico is often contacted by nuclear medicine physicians to give guidance on issues of radiation exposure. “And I’ve been very surprised since this issue came to the fore over the last year with the lack of reaction among the patients,” he says. Over that time he’s had just one case in which a physician asked for a consult about a patient who was expressing concern about the radiation associated with an FDG-PET scan.

“So, having this one patient over that length of time since the issue hit the popular press is surprising—and somewhat reassuring,” says Zanzonico.

Differentiation does have to be made among different patient groups as well. The potential harm to older cancer patients going through radiation therapy who have a follow up CT or PET/CT study, for example, is less of a concern because overall longevity is limited. Yet, radiation dose and its associated cancer risk is clearly a concern for a 40-year old patient undergoing a cardiac SPECT exam due to chest pain. Some 8 million cardiac SPECT scans are performed every year in the U.S.

Government intervention

While Donohoe and Zanzonico are somewhat reassured about the impact the radiation exposure issue is having on patient attitudes, they are less sanguine about government reaction.

Royal does believe that government has a role to play. For example, Royal refers to a study that appeared in the Archives of Internal Medicine in December that found that the effective radiation dose for a routine CT scan in four California hospitals varied by a factor of 13 depending on the hospital. 

“It’s hard to justify that kind of variation,” Royal says, and he points out that a diagnostic reference standard has been adopted in Europe that provides hospitals with accurate information on how much variability there is from hospital to hospital when imaging studies are ordered.

“They have a much better system [in Europe] than we have in the U.S., where we really have no system,” says Royal. “I’m not saying that the government should be determining what the radiation dose should be. What I am saying is that it’s the only central agency that can collect the data that let physicians know that when they do an imaging procedure they can see how they stand compared to their peers [when it comes to patient radiation exposure].”

Donohoe believes that, in general, the medical community “has the people who are best equipped to deal with the issue of radiation exposure, but I think we’re all worried that the government is going to come in with a heavy hand and use politics to make decisions, when we would rather see decisions made that are based on science.”

Zanzonico agrees that government policy is often driven by politics and emotion as opposed to science, and refers to a brewing controversy surrounding the rules governing the treatment of nuclear medicine patients to make his point.

The House Energy and Environment Subcommittee of the Energy and Commerce Committee in March released a report critical of the Nuclear Regulatory Commission’s rules surrounding the release of patients treated with radioactive iodine. Zanzonico explains those regulations require patients to be hospitalized if they have a certain amount of radioactivity in their bodies regardless of mitigating circumstances, while the rules currently in place in the U.S. are based on a projection of the radiation dose to individuals surrounding the patient. “And that’s really the soundest way of doing it,” says Zanzonico. “It’s the dose that represents the most potential harm, not the activity itself.”

Risk—unnecessary and otherwise

Donohoe says that since most physicists accept the linear no threshold model for radiation exposure that means that implies there is an acceptance that there is an inherent risk in any radiation exposure.

“But there’s a problem when you talk about something like releasing patients with radioactivity onboard into the public,” Donohue observes. “When the public or politicians hear that they begin thinking, ‘Oh that’s terrible, we have to hospitalize those people.’ And they don’t think about the other side—the risks associated with keeping someone in the hospital—the risk of infection, or the wisdom of using healthcare dollars [for treatment that only has] theoretical benefits. They don’t think about those things because they are so overwhelmingly biased against radiation exposure that it is very hard to consider things objectively.”

Royal says there is a tendency for some to use the linear no threshold model to suggest that radiation presents “some kind of special risk where there is no safe level of radiation.” But, he points out that there are kinds of human activities—using the simple act of crossing a street as an example—that have an element of risk.

“But we don’t think that crossing the street is unsafe just because there’s no threshold to the risk when you cross that street,” Royal says. “On the other hand, we wouldn’t recommend that our kids have a contest to determine how many times they can cross the street in an hour because that would be an unnecessary risk.” He suggests it’s roughly equivalent to treating someone with radiation—the benefits of treatment certainly outweigh the risks, but physicians should certainly avoid unnecessary tests.

While Royal says he does accept the linear no threshold model, he does have a problem with the current focus on cumulative radiation dose.

“The trouble with that, I believe, is that you should avoid the first unnecessary imaging exam as much as you should avoid the fiftieth,” Royal says. “In fact, I would argue it’s better to avoid the first one since it’s likely that the patient is going to be young and less likely to be ill, and therefore less likely to benefit from having the test done.”