A method of use of an isolated polypeptide conjugated with a radionuclide, wherein the isolated polypeptide binds specifically to HER2, or a variant thereof is described. The method is for monitoring the response to HSP90 inhibition and comprises the use of the isolated polypeptide conjugated with 18F. The application also describes the use of an isolated polypeptide conjugated with 18F, wherein the isolated polypeptide binds specifically to HER2 or variants thereof, as an imaging agent to monitor uptake thereof to measure HSP90 inhibition.