LettersFebruary 2021
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    Background: Histiocytoses are rare disorders characterized by tissue infiltration by macrophages, dendritic cells, or monocyte-derived cells (1). Different subtypes have been described, which collectively encompass a wide range of clinical manifestations from mild to disseminated or life-threatening forms. These diseases are classified into 5 groups based on histology, clinical phenotype, and molecular features: Langerhans-related, cutaneous or mucocutaneous, and malignant histiocytoses; Rosai–Dorfman disease; and hemophagocytic lymphohistiocytosis (1).

    Langerhans-related histiocytoses comprise Langerhans cell histiocytosis (LCH) and Erdheim–Chester disease (ECD), both inflammatory myeloid neoplasms characterized by clonal activating mutations along the mitogen-activated protein kinase or related pathways, most commonly BRAF V600E (1, 2).