Thorium-232 is a silvery, radioactive metal that is particularly good at absorbing X-rays. In the early days of X-ray imaging, doctors routinely injected patients with thorium dioxide because it produced high contrast images. Between the 1930s to the 1950s, some 10 million people received these doses.
The advantage of thorium dioxide, or Thorotrast as it was called, is that it had almost no immediate side effects on the patients, unlike other contrast agents, which were often dangerous. And the half-life of thorium is about 14 billion years, so it is relatively stable.
What doctors didn't appreciate at the time were the long term effects on the body. Once injected, Thorotrast settles in various organs where it tends to stay. The biological half life of the stuff is 22 years.
When thorium eventually decays it sets in train a sequence of five further decays producing alpha particles. These all happen relatively quickly; four of them in a matter of hours or fractions of a second.
For that reason, Thorotrast turned out to be highly carcinogenic but often on a timescale measured in decades. It was eventually withdrawn as a contrast agent in the 1950s.
The problem for physicists is to calculate the effects of elements like thorium on the body. They've long known that the high energy particles released during a decay damage the body by smashing into and damaging molecules like DNA.