Histiocytic Neoplasms

Histiocytes are bone marrow-derived cells of macrophage or dendritic cell lineage. There are several dendritic cell lineages, the most studied being the antigen presenting cells located in the epidermis, namely Langerhans cells, and in the dermis, namely dermal interstitial dendritic cells. Immunohistochemical cell markers have been developed for use in identifying these cells in dogs, the domestic species most affected by histiocytic diseases. Cutaneous histiocytoma is a common benign tumor of epidermal Langerhans cells in dogs. Histiocytic sarcoma is a malignant tumor of interstitial dendritic cells and has been reported in various tissues including, but not restricted to, lymph nodes, spleen, liver, skin, skeletal muscle, and bone marrow. Histiocytic sarcoma is frequently disseminated at the time of diagnosis and hematologic abnormalities, such as cytopenia(s) are often present (see Chapter 5: Cytology, Case 7, Pierre). Hemophagocytic histiocytic sarcoma arises from tissue macrophages rather than dendritic cells and generally involves the spleen, although other tissues may also be infiltrated. The neoplastic macrophages aggressively phagocytose RBCs and often platelets resulting in regenerative anemia and thrombocytopenia. The prognosis for disseminated histiocytic sarcoma and hemophagoctic histiocytic sarcoma is extremely poor. Leukemia is rare with these neoplasms and these are not included in the classification scheme for acute myeloid neoplasms.