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LONDON, July 21-The threat of terrorism has made it tougher for patients who have undergone radioisotope procedures to slip quietly past radiation detectors deployed in airports and many other public places.

A 55-year-old commercial pilot learned that the hard way, when he was detained for questioning at Moscow's airport two days after having a thallium-201 myocardial perfusion scintigram.

After "extensive interrogation," the pilot was released, only to trip radiation detectors again four days later while leaving Moscow, according to a case report in the July 23 issue of The Lancet.

The case and others like it suggest "it is important to warn patients having had a thallium scan that they may trigger radiation detectors for up to 30 days," wrote Richard Underwood, M.D., and colleagues of London's Royal Brompton Hospital.

In fact, the authors suggested, it should be standard practice to give patients an information card after they've had diagnostic or therapeutic procedures involving radioisotopes.

The card would give:

• The date and place of the procedure.
• The radioisotope used and its half-life.
• The potential duration of radioactive emissions.
• Who to call for verification.

More than 18 million such procedures are carried out every year and more and more radiation detectors are in place in what the U.S. Nuclear Regulatory Commission calls "critical infrastructure," including airports, banks, railway stations, and highway tunnels.

In 2003, the commission urged that nuclear medicine specialists make a point of emphasizing that patients should follow the written instructions they get after a procedure, which usually suggest not using public transport within two days.

The recommendation came after a New York State police pulled over a bus traveling from New York to Atlantic City after it tripped a radiation detector in a highway tunnel leading out of the city.

It turned out that one of the passengers had received a dose of iodine-131 earlier the same day, but had ignored her doctor's written suggestion to avoid public transit for at least two days.

To "avoid unnecessary concern by law enforcement authorities," the commission suggested nuclear medicine specialists should "consider" giving patients their business card and written information about the procedure.

A spokesman for the commission said there's no legal requirement to report such events, so the commission doesn't know how common they are.

But reports date back to at least 1986, when a letter to the New England Journal of Medicine noted two incidents in which Secret Service agents seized patients who set off radiation detectors at the White House after thallium stress tests.

The half-life of thallium-201 is 73 hours; the usual intravenous dose is 80 mega Becquerel. That means, wrote Dr. Underwood, that patients may trigger radiation detectors for at least two weeks.

In fact, highly sensitive modern detectors -- like those issued to police and other security personnel -- may detect the traces of thallium-210 for as much as a month, according to a report at the 2004 meeting of the Radiological Society of North America.

The least durable radioisotope was fluorine-18, which lasted about a day; the most durable was iodine-131, which in some cases could still trigger detectors 95 days after it was administered.

The pilot -- who had never complained of chest pain or shortness of breath -- was shown to have a dilated and hypertrophied left ventricle with no evidence of inducible ischemia, the authors noted.

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