Tumefactive demyelinating disease mimicking malignant tumor in positron emission tomography with 11C‐methionine

Tumefactive demyelination is sometimes difficult differentiate from malignant tumors. Positron emission tomography with 11C‐methionine is useful for diagnosing cerebral malignant tumors, but there are previous reports for tumefactive demyelination. We experienced a 32‐year‐old man who suffered from subacute onset dysarthria and left hemiparesis with magnetic resonance image of a large lesion (5.3 × 4.2 cm) in the right frontal lobe. Positron emission tomography with 18F‐fluorodeoxyglucose showed a high ring‐shaped glucose uptake in the lesion and 11C‐methionine showed a remarkable methionine uptake in the whole lesion. Although a malignant tumor was suspected, the patient was finally diagnosed with tumefactive demyelinating disease based on brain biopsy results, and was treated completely by methylpredonisolone therapy. Although 11C‐methionine uptake with positron emission tomography commonly suggests a malignant tumor, there can still be a possibility of tumefactive demyelinating disease.