Primary diffuse large B‐cell lymphoma of the central nervous system with rapidly progressing lesions after dimethyl fumarate treatment, showing relapsing and remitting symptoms: A case report

Background We present a case of B‐cell type primary central nervous system lymphoma that rapidly progressed after dimethyl fumarate (DMF) administration. Case presentation An asymptomatic white matter lesion of the left frontal lobe was observed in a 56‐year‐old Japanese man on magnetic resonance imaging during a medical checkup. For the subsequent 5 months, the sporadic white matter lesion showed no change and no contrast effect. He suddenly presented with right upper limb paralysis on day 74. After improvement, he had a recurrence of right upper limb paralysis and diminished vision loss. Based on the 2017 revised McDonald criteria, two attacks, objective clinical evidence of one lesion and cerebrospinal fluid oligoclonal band assay positivity, he was diagnosed with relapsing–remitting multiple sclerosis and administered DMF. Three months after DMF administration, he developed new brain lesions that progressed rapidly; additional immunotherapy was ineffective. He was pathologically diagnosed with B‐cell type primary central nervous system lymphoma using brain biopsy on day 301. Conclusion Patients with rapidly progressing white matter lesions after DMF administration should be suspected for B‐cell type primary central nervous system lymphoma and pathologically diagnosed using brain biopsy. Pathological findings for the right basal ganglia lesion, based on immunostaining with (A) hematoxylin‐eosin, (B) anti‐CD3, (C) anti‐CD20, and (D) anti‐CD5.