Wanted to is very different than doing. As far as I'm aware there has never been a dirty bomb attack. You could effectively create a dirty bomb with less radiation with more easily obtainable materials (still pretty expensive and labor intensive). The threat would be similar to a more radioactive substance because the biggest damage is the fear (that's what terrorism is about. They could do MUCH more _damage_ if they weren't as concerned about striking fear). The reason it hasn't been done is because it is impractical and difficult to do without killing yourself. There's better and far easier ways to strike fear into people's hearts. Killing a single person is more effective for them than increasing the chance of getting cancer in one's lifetime by 10% for a dozen people. The much bigger threat is a briefcase nuke but that is several more orders of magnitude difficult/expensive to obtain.