Though the lens might contain substantial amounts of thorium (Th, with 232Th the most common isotope), most of the radiation is blocked by the glass itself (eventually turning it brown). Still, the surface emits some alpha particles, as @RossMillikan (any relative of the physicist?) stated. Keeping it in a container more than a few millimeters thick (e.g., a plastic box), will block all the 232Th alpha particles.

There are, however, decay products from Th, such as radium (Ra) and radon (Rn), and these emit more radiation and decay into other radioactive species. The half-life of 232Th is 14,000,000,000 years, about the age of the universe, which means that it is not intensely radioactive, and the majority is sealed in glass. You might not want to keep it in a living space because of Rn outgassing. Handle it only while wearing disposable rubber or vinyl gloves, because of those decay products.

This is not meant to be considered medical or radiological safety advice. See other sites, such as Movie Cultists and Lens Legend, too. And do not use a thoriated eyepiece (ocular) lens.