Histiocytic neoplasms are rare hematological disorders that have diverse clinical manifestations and can pose significant management challenges for clinicians. Previously considered inflammatory, several of these disorders are now included in the World Health Organization classification of hematopoietic and lymphoid tumors. This was due, in part, to the discovery of recurrent MAP-ERK pathway alterations in Erdheim-Chester disease (ECD) and Langerhans cell histiocytosis (LCH).2,3  Recently, such mutations were also found to be present in one third of patients with Rosai-Dorfman disease (RDD). This suggests that at least a subset of these disorders is neoplastic in nature. The prognosis of histiocytic neoplasms is variable, and the natural course may be relatively benign and self-limiting in some cases, whereas in others it may be much more aggressive and life-threatening.5-7 

Most adult patients with histiocytic neoplasms receive...

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