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Extranodal NK/T‐cell lymphoma, nasal type, manifesting as rapidly progressive dementia without any mass or enhancing brain lesion

Among the many potential etiologies for rapidly progressive dementia (RPD), primary central nervous system extranodal NK/T‐cell lymphoma, nasal‐type (ENKL) is a rare entity. We present the first reported case of autopsy‐proven RPD due to ENKL without any mass or enhancing lesion of the brain. A 54‐year‐old immunocompetent man presented with RPD, myoclonus and ataxia. The mini‐mental state examination (MMSE) score was 22/30. His brain MRI revealed progressive brain atrophy without gadolinium enhancement or mass lesion. Five months after the initial evaluation, cognitive impairment further worsened with an MMSE score of 3/30. At the advanced stage, lumbar MRI showed swollen cauda equina with gadolinium enhancement. The number of Epstein‐Barr virus (EBV) DNA in cerebrospinal fluid had gradually increased. Twelve months after onset, the patient died of respiratory failure. Pathological findings revealed that lymphoma cells had diffusely invaded the meninges, parenchyma of the brain, spinal cord and cauda equina. Cells were positive for CD3, CD56 and EBV‐encoded small RNAs and negative for CD20. No evidence of malignancy was identified in the visceral organs. This report indicates that ENKL should be recognized as one of the rare causes of RPD. Early testing for EBV‐DNA in cerebrospinal fluid and imaging of cauda equina would be useful diagnostic tools.