Localized anaplastic lymphoma kinase–positive histiocytosis in a cerebellar hemisphere with long-term treatment: illustrative case

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BACKGROUND Anaplastic lymphoma kinase (ALK)–positive histiocytosis (ALK-H) is an emerging entity in the category of histiocytic neoplasms that was first reported as a multisystemic disease in three infants in 2008. The clinicopathological spectrum of ALK-H has been expanded to include localized disorders in specific organs, but the features of this subtype are not well known. The authors report a case of ALK-H localized in the central nervous system that was difficult to treat and review the relevant literature. OBSERVATIONS The authors reviewed archival histiocytic tumors at their institute and found a pediatric case of ALK-H localized in a cerebellar hemisphere that had previously been reported as histiocytic sarcoma. Chemotherapy (approximately 1 year), additional surgery, and conventional chemotherapy (approximately 2.5 years) led to clinical remission, and maintenance chemotherapy was continued (approximately 1.5 years). Three years after completing treatment, a high-grade glioma was found in a cerebral hemisphere, and the patient died of the glioma 2 years later. LESSONS Although the prognosis of ALK-H is generally good according to prior cases, the authors’ case required long-term conventional chemotherapy, suggesting the tumor displayed aggressive characteristics. Early administration of ALK inhibitors may be necessary.