A radiotracer consisting of sarcosine, the N-methyl derivative of the amino acid glycine, labeled with the positron-emitting isotope carbon C 11, that can be used for tumor imaging upon positron emission tomography/computed tomography (PET/CT). Upon administration, C-11 sarcosine is taken up by and accumulates in tumor cells, thereby allowing tumor imaging with PET/CT. Sarcosine, a non-proteinogenic amino acid and oncometabolite, is elevated in certain tumor cell types. Its expression seems to correlate with increased tumor invasiveness.