Metastatic brain lesions may demonstrate photopenia on FDG-PET.

Metastatic brain lesions generally demonstrate hypermetabolic foci on fluorine-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) images. A large area of photopenia of the brain on FDG-PET is most likely consistent with a benign etiology. The author reports 2 cases of cerebral metastases from lung cancers, which display complete photopenia on FDG-PET. The brain is a special organ with high background uptake on PET and significant cytotoxic edema surrounding the metastatic deposits. In some cases, it is a dilemma to identify malignant intracranial foci on PET. Anatomic imaging, for example, MRI or CT correlation is needed for proper interpretation of brain PET on evaluation of metastases.