The activity could be bone breaking down, or bone repairing itself. There are several possible reasons for bone activity – cancer is one of them, but other reasons include arthritis, fractures and bone infections, so the test results need to be carefully interpreted.

For the test, a mildly radioactive substance is injected into a vein in the arm. Then a large machine called a gamma camera takes a scan of the body. The radioactive substance helps to show up any active areas of bone – these are called hot spots.