The Collaborative International Dictionary of English v.0.48:

radiothorium \ra`di*o*tho"ri*um\
   (r[=a]`d[i^]*[o^]*th[^o]r"[=e]*[u^]m), n. (Chem.)
   an earlier name for the thorium isotope thorium-228, given
   by its discoverer Otto Hahn. It is a radioactive substance
   formed as one of series of products in the chain of
   radioactive decay of thorium. Its immediate predecessor in
   the chain is Actinium-228, and it decays by alpha emission to
   radium-224 with a half-life of 1.91 years. The name
   radiothorium was given prior to the full understanding of the
   nature of isotopes of elements.
   [PJC]

WordNet (r) 3.0 (2006):

thorium-228
    n 1: radioactive isotope of thorium with mass number 228 [syn:
         thorium-228, radiothorium]