The application provides polypeptides comprising or essentially consisting of at least one heavy chain variable domain of a heavy chain antibody (VHH) or a functional fragment thereof, wherein said VHH or a functional fragment thereof specifically binds to a target protein that is present on and/or specific for a solid tumor, e.g. HER2. The application further provides nucleic acids encoding such polypeptides; methods for preparing such polypeptides; host cells expressing or capable of expressing such polypeptides; compositions, and in particular to pharmaceutical compositions, that comprise such polypeptides, nucleic acids and/or host cells. The application further provides such polypeptides, nucleic acids, host cells and/or compositions, for use in methods for detection, imaging, prognosis and diagnosis of cancer as well as for predicting patient response(s) to therapeutics.