Plastic brachytherapy sources

103125131An implantable source of therapeutic radiation for brachytherapy is provided as a sealed, biocompatible capsule of plastic (e.g. polyethylene or PEEK) transparent to the radiation. The capsule contains a radiation source comprising particles of a radioactive isotope (e.g. Pd, I, Cs) in a fluid carrier that is resistant to radiation polymerization but solidifies at elevated temperature. It also has a marker, and desirably has a socket which accommodates attaching spacers and makes possible linear strands and planar arrays of the capsules. The spacers may be functional, e.g. heat-generating or medication-releasing.