What are the limitations of a pet scan?

A few: Lesions below a certain size may be too small to show up on pet (typically around 8 mm). It may show many "abnormal" lesions that may be unrelated to cancer. It cannot show lesions in the brain as well. Finally, in diabetics with very high glucose levels it may be inaccurate.

Tricky: Not all cancers are hypermetabolic on pet scans and not all hypermetabolic activity in a pet scan is cancer. Inflammation/infection can also show hypermetabolic activity. Pet scans are performed in conjugation with ct (pet/ct) and though pet is great for detecting metabolism (high sensitivity), its specificity is not as great and also has poor anatomic resolution. Reading a pet requires care/skil.

FDG PET: Fdg pet is used often for oncology. Fdg uptake does not necessarily mean cancer. Infection, granulomatous disease, and tumor all can take up radioactive glucose. Very small lesions may also not light up on pet. It is always important to take into account the clinical history, distribution of tracer and lesions, and to examine ct and other anatomic imaging available.