Radiolabelled antibody fragments for use in treating cancer

Provided herein are methods, kits, and compositions for stratifying and treating subjects, e.g., subjects having cancer. In some examples, the methods involve use of a radiolabelled heavy chain variable domain derived from a heavy chain antibody (VHH), or a functional fragment thereof, as both a screening agent and a treatment agent. In some examples, the VHH, or a functional fragment thereof, that is radiolabelled with a radioisotope that is both a γ-emitter and β-emitter.