A radiotracer composed of AE105, an urokinase-type plasminogen activator receptor (uPAR) peptide antagonist, conjugated with the bifunctional, macrocyclic chelating agent 1,4,7-triazacyclononane-N,N',N''-triacetic acid (NOTA) and labeled with the radionuclide gallium Ga 68, with potential imaging activity using positron emission tomography (PET). Upon administration, the AE105 moiety of gallium Ga 68-NOTA-AE105 targets and binds, with high affinity, to uPAR expressed on tumor cells. Upon PET imaging, uPAR-expressing tumor cells can be visualized and the degree of tumor aggressiveness can be assessed. uPAR expression is correlated with increased tumor invasiveness and aggressiveness, as well as a poor prognosis.