Described embodiments include an apparatus (20, 21), which includes a support (22), including an outer surface (24) and configured for insertion into a body of a subject. The apparatus further includes multiple atoms (26) of a radionuclide, which radioactively decays to produce a daughter radionuclide, coupled to the outer surface, and a layer (28, 33) of a polymer, which is permeable to the daughter radionuclide, that covers the atoms. Other embodiments are also described.
