Radiation risks to health staff from nuclear medical imaging

What to do when the patient is hot http://www.theheart.org/article/1535927.do 7 May 13, MAY 7, 2013 Shelley Wood Boston, MA – Patients who’ve had a nuclear imaging study with radioactive tracers become, themselves, radiation emitters—something that hospital staff should keep in mind, say researchers in a new analysis trying to quantify that risk. Their research letter is published today in the Journal of the American College of Cardiology [1].

While there are plenty of studies addressing radiation exposure to patients from medical imaging tests, there’s scant information on how much radiation is emitted from patients who have undergone myocardial perfusion imaging (MPI), Dr Connie Tsao (Beth Israel Deaconess Medical Center, Boston, MA) and colleagues write.

“The tracer that we use, technetium-99m, has a half-life of six hours, so the radiation goes away relatively quickly,” senior author Dr Thomas Hauser told heartwire. “But if you are routinely very close to a patient who has had one of these studies, for a period of time shortly after the tracer has been injected, you could potentially get a radiation exposure that is not insignificant. . . . This came up because our echocardiography techs have sometimes been asked to do echocardiograms right after a [nuclear] stress test, and they were concerned about the radiation exposure.”

Patient-level radiation

Tsao, Hauser, and colleagues measured radiation levels in 46 of 56 prospective MPI patients using an ionization chamber and Geiger-Muller survey meter at one, two, and four hours after arrival. Film badge dosimetry was obtained in an additional 10 patients.

At one and a half hours after radionuclide administration, the dose equivalent measured right at the anterior chest wall was 0.37 mSv while the right chest wall dose equivalent was 0.58 mSv.

Investigators then calculated the exposure potentially faced by sonographers coming into contact with patients, as well as any exposure to orderlies and other transport staff. Sonographers, they estimate, might have a potential radiation dose equivalent as high as 0.16 mSv (lower if the sonographer was left-handed and scanning on the left) during a 24-minute echo. A transport worker might face a dose equivalent of 0.02 mSv during a 10-minute patient transfer.

This level of exposure is very low, Hauser agreed. “If you are more than three feet away, the radiation is nothing to worry about; you’d have to be sitting right next to someone for half an hour or so, and even then, if you had just that single exposure, that’s really very small. It’s only when you have these repeated exposures, over time, that there might be an issue.”

For example, at Hauser et al’s institution, roughly one patient per day is transported directly from nuclear testing to the echo lab.

“Even if you are only seeing one or two of these patients a week, over the course of a year, that’s 50 or 100 studies,” he said. And there are increasing pressures on hospitals to schedule patients for multiple tests on the same day, either to expedite hospital discharge or consolidate outpatient testing for the sake of patient convenience, the letter notes.

Safe practices

It’s impossible to predict the long-term health effects of recurrent radiation exposure, Tsao and colleagues acknowledge. “While it is unlikely that repeated exposure to post-MPI patients will exceed [recommended] limits in adults, our data suggest that close and repeated contact should be avoided in populations that are more radiosensitive, such as pregnant women and children,” they write.

Some simple steps can be taken, such as scheduling echocardiograms before rather than after nuclear tests or instituting four-hour minimum delays between tests, Hauser told heartwire. “And if there is someone where you have to do [the echocardiogram] right after the nuclear imaging study, one thing you can do is wear a lead vest.”