The invention can be summarized as follows. According to the present invention there is provided an attenuated, tumour-targeting bacterium comprising a heterologous nucleotide sequence encoding a protein permitting the accumulation of a radionuclide within the bacterium. The heterologous nucleotide sequence may encode a sodium iodide symporter protein. The invention also relates to the use of such bacteria and methods of treating diseases using the bacteria of the present invention.