A radioconjugate consisting of the benzamide ligand ISO-1 labelled with the radioisotope fluorine F18 with positron-emitting radioisotope activity. Upon administration, fluorine (F18)ISO-1 binds to sigma-2 receptors, located on tumour cells, allowing visualisation of sigma-2 receptor-expressing tumour cells with positron emission tomography (PET). Sigma-2 receptors, expressed in a variety of normal healthy tissues such as liver, kidneys, endocrine glands, and in the central nervous system, may be overexpressed in tumour cells.