The invention provides a scanning suspension comprising a particle which is capable of at least in part disturbing a magnetic field, wherein said particle comprises a diameter of at least 1 mum, and use thereof for obtaining a scanning image. Preferably, said particle comprises holmium and a composition capable of essentially maintaining its structure during irradiation. A particle of the invention is suitable for preparing a kit of parts, comprising a diagnostic and a therapeutic composition which both comprise particles of the invention with essentially the same chemical structure, wherein said therapeutic composition is more radioactive than said diagnostic composition. Said kit of parts is especially suitable for treatment of a tumor. First, the distribution of a particle of the invention within an individual can be determined with a scanning image obtained with said scanning composition. Subsequently, a radioactive therapeutic composition can be administered, wherein a suitable dose of said therapeutic composition is derived from said scanning image.
