I found this on the internet, take it for what it's worth.
Radiation patients set off border scanners
July 28, 2005
Canadians who have undergone certain diagnostic tests or treatment involving radiation are setting off detectors at crossings along the border with the United States.
The scanners are used to detect radiation in the hunt for nuclear weapons, said Chief Ron Smith of U.S. Customs Border Protection at the Detroit-Windsor border.
"They are sensitive," said Smith. "They are going to pick up on medical isotopes that are used on people."
Radioistopes are used in a variety of medical procedures, from heart tests to cancer treatment. A small amount of radioactive material is injected into the body, where it can remain for hours or a few weeks.
"We certainly tell all our patients that if they're going to be travelling in the near future, and that really does mean two to three months, that they should be carrying some form of information with them that they've had a treatment," said McEwan.
Many doctors now issue letters for patients to show at the border with the U.S.
Border agents said the letters will help avoid problems, but they still need to confirm a patient isn't trying to transport nuclear material.
The extra security measures meant waiting 30 minutes, said Annamarie Aartsen, a radiation therapy patient in London, Ont.. As she waited, agents verified her treatment and searched the car.
Border agents said the delays are to be expected if patients set off a radiation scanner.