ALK-positive histiocytosis with isolated central nervous system involvement: Report of two cases and review of a newly described entity.

Anaplastic lymphoma kinase (ALK)-positive histiocytosis is a novel and rare entity commonly described as a systemic disease affecting infants, while isolated systemic involvement including the central nervous system (CNS) have been reported in older children and young adults. We report 2 cases of CNS ALK-positive histiocytosis, with detailed histopathological and radiological information, and provide a review of literature. Two patients, a child and a young adult, presented with extra-axial mass lesion. The radiological differentials considered were meningioma and schwannoma. The histopathological examination of both cases showed sheets of cells resembling histiocytes admixed with scattered Touton-type and foreign body giant cells. These tumors do not have any distinct diagnostic radiological, features and hence histopathological examination is crucial in the diagnosis of these tumors. The cells were immunopositive for CD68, CD163, and ALK. Understanding the histopathological spectrum of ALK-positive histiocytosis is important as targeted therapy (ALK inhibitor therapy) exists and the prognosis is better.