Mass‐forming primary angiitis of central nervous system with Rosai‐Dorfmann disease‐like massive histiocytosis with emperipolesis

Primary angiitis of the central nervous system (PACNS) is a vasculitis restricted to the CNS without systemic involvement. We report a case of PACNS that was radiologically tumor‐mimicking, and pathologically similar to the Rosai–Dorfmann disease. A 20‐year‐old woman presented with a focal facial motor seizure. Magnetic resonance image revealed heterogeneously enhanced well‐demarcated solitary cerebral mass in the posterior frontal lobe. Histopathologically, the lesion showed lymphoplasmacytic vasculitis with massive parenchymal infiltration of large histiocytes with emperipolesis. Diffuse ischemic change, necrosis, hemorrhage of the brain parenchyma with neuronophagia, and extensive reactive gliosis by gemistocytic astrocytes were accompanying microscopic features. The patient was doing well for 3 years after complete resection of the lesion, except for occasional occurrence of alcohol‐ or sleep deprivation‐associated seizure. We describe this unique case to provide evidence that mass formation can be developed in PACNS by accompanying parenchymal lymphohistiocytic infiltration, necrosis, and marked reactive gliosis.