RADIOLOGY: HEAD: Case# 33668: BRAIN METASTASES. This 60 year old man presents with a history of two previously irradiated brain metastases and a clinical suspicion of recurrence. The patient was injected with 10 mCi of 18-FDG and after allowing 30 minutes for localization, emission and transmission data were acquired. One day later, the patient was injected with 10 mCi of 11-C-methioine intravenously followed by emission and transmission PET imaging. 18-FDG images demonstrate hypometabolic areas in the right frontal and occipital lobes with smaller foci of increased metabolism near the cortex that have the same intensity as grey matter. Carefully registered 11-C-methioine data demonstrate increased protein synthesis in these two foci and clearly suggest recurrent tumor. Hypometabolic areas in the right frontal and occipital lobes correspond to sites of irradiated metastases. Smaller foci of increased metabolism may represent residual normal cortex or tumor that have the same intensity as cortical grey matter. Supplemental imaging using 11-C-methioine reflected increased protein metabolism in these foci. In combination, the studies suggested recurrent tumor in both foci. 18-FDG brain studies may be difficult to interpret when tumor foci are of the same intensity as grey matter and in a peripheral location as normal cortex. 11-C-methioine brain studies provide minimal anatomical information but can offer higher contrast for identifying tumors near the cortex. Careful registration is crucial for proper interpretation.