Isolated Rosai–Dorfman disease of the cerebellum: A rare presentation of a histiocytic disorder mimicking a posterior fossa neoplasm

BACKGROUND: Rosai-Dorfman disease (RDD) is a benign histiocytic proliferation, typically presenting with massive cervical lymphadenopathy. Isolated intracranial RDD is rare, and primary cerebellar involvement is exceedingly uncommon [1, 2]. CASE PRESENTATION: We report a case of a 42-year old gentleman presenting with a 2-month history of early morning vomiting, balance disturbance, vertigo, and occipital headache. MRI revealed a 35 mm solid mass in the right cerebellar hemisphere with avid peripheral enhancement with cystic change/necrosis centrally, radiologically suggestive of high-grade primary tumor or a solitary metastasis. Surgical excision was performed. Histology showed cerebellar tissue diffusely infiltrated by a densely cellular lymphohistiocytic inflammatory infiltrate composed of abundant small lymphocytes and sheets of large histiocytic cells with round to oval nuclei, small nucleoli, and voluminous pale eosinophilic cytoplasm. Several histiocytic cells showed evidence of emperipolesis, whereby histiocytic cells are seen to engulf lymphocytes, occasional plasma cells, and red blood cells. The histiocytes were S100 positive, CD68 positive, and CD1a negative, consistent with RDD. CONCLUSION: Isolated cerebellar RDD is an exceptionally rare entity that can closely mimic neoplastic lesions radiologically. Awareness of this diagnosis and recognition of its characteristic immunophenotype are essential to avoid misdiagnosis and unnecessary adjuvant therapy [2, 3].