Abstract: The present invention relates to small particles comprising a radionuclide and in particular to small particles comprising a radionuclide for implantation in organs or tissues or tumours of subjects. Embodiments of the invention have been particularly developed for embolisation into the arterial system using a technique known as radioembolisation or Selective Internal Radiation Therapy (SIRT) and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use. The small particles are preferably radioactive microspheres comprising a matrix and a radionuclide stably attached. These microspheres have a diameter ranging from 5 to 45 ?m and the radionuclide has a specific activity ranging from 100 to 2000 Bq per microsphere.