11c-labelled peptide for detecting an antibody

There is described the use of a peptide (1) for the preparation of an agent for detecting an antibody (4), where the peptide (1) binds to a paratope (5) of the antibody (4) and includes an 11 C carbon atom. Also provided is a radiopharmaceutical for locating a tumour (18) comprising a peptide (1) which includes an 11 C carbon atom, where the peptide (1) binds to a paratope (5) of an antibody (4) which is directed against the tumour (18).