The invention relates to a conjugate PT comprising (a) a peptide P and (b) a targeting moiety T; wherein the peptide P contains from 4 to 20 amino acid residues with optionally a single thiol moiety at the C- or N-terminus replacing one of these amino acid residues, and including a complexing amino acid sequence comprising (iv) a tridentate or tetradentate planar chelator containing a ligand backbone defined by a perimeter containing a single sulfur donor atom, two or three non-sulfur donor atoms, and intervening atoms between the donor atoms; and (v) a stabilizing sidechain comprising a further donor atom, the further donor atom being covalently linked to the ligand backbone of the planar chelator through two or three intervening atoms, provided that said further donor atom is neither the nitrogen nor the oxgen atom of a peptide bond and if said further donor atom is part of a heterocyclic ring, no more than one of the intervening atoms can be part of that ring; and a radioactive metal complexed to the donor atoms of the planar chelator.