There are two main types of PET imaging probe: a radiotracer and a radioligand. Radiotracers are analogues of enzyme substrates, and can report on the rate of a biological process. An example is the glucose analogue 18F-FDG. Radioligands and analogues of receptor ligands (molecules that bind to receptors) and can report on the expression of that particular receptor. An example of a radioligand is 68Ga-PSMA.
PET imaging has several roles in oncology. As well as helping with the detection and diagnosis of cancer, PET can provide information on the stage (how much there is) and has an increasing role in the early detection of therapy response and disease recurrence.