PET imaging for differentiating recurrent brain tumor from radiation necrosis.

The exact incidence of true radiation necrosis is largely unknown. It is probably much less frequent than indicated by MR or CT findings. Differentiating radiation necrosis from recurrent tumor is a diagnostic challenge, however, and has important implications for the patient's management. Even though the first results were published 20 years ago, the total number of case studies using FDG-PET in this indication remains limited. Several reports are also hampered by methodologic limitations. The technique has been largely criticized, notably in articles that themselves were not completely free of methodological flaws. Overall however, FDG-PET seems to be a valuable clinical tool. As a general rule, suspicious lesions on MR imaging that show increased FDG uptake (ie, uptake equal to or great than that in normal cortex) are likely to represent tumor recurrence. Sensitivity is an issue, especially but not exclusively with low-grade gliomas. Although false-positive results may occur, specificity is usually high in routine clinical practice. Coregistration with MR imaging surely improves the diagnostic performances of FDG-PET because it helps delineate the suspicious area. Another important aspect is the prognostic value of FDG uptake, which is now well established. It seems clear that only the combination of FDG with a radiolabeled amino acid analogue (MET or a more recent fluorinated compound) can provide a comprehensive characterization of suspected brain tumor recurrence.