An overview of canine histiocytic disorders - Veterinary Medicine
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An overview of canine histiocytic disorders
The disorders that arise from histiocyte proliferation range from benign, self-resolving lesions to malignant, life-threatening sarcomas and include a newly identified splenic and bone marrow macrophage disorder. These clinicians give you the information you'll need to readily differentiate and manage these disorders.



Figure 5A. A fine-needle aspirate of a cutaneous histiocytic sarcoma in a dog. Note the marked anisocytosis, anisokaryosis, bizarre-shaped nuclei, and prominent nucleoli. One plasma cell is also present (arrow) (Wright-Giemsa, 500X). (Photo courtesy of Dr. Elizabeth Little, DACVP.), 5B. A fine-needle aspirate of a cutaneous histiocytic sarcoma in a dog. Note the pleomorphic cells, vacuolated cytoplasm, and large size compared with the neutrophil (arrow) (Wright-Giemsa, 500X). (Photo courtesy of Dr. Elizabeth Little, DACVP.)
Histiocytic sarcoma is diagnosed based on signalment along with clinical findings and is confirmed by cytologic and histologic examination. On cytologic examination, samples from organs or effusions of dogs with disseminated histiocytic sarcoma are typically highly cellular.28 Neoplastic cells (discrete mononuclear cells) are pleomorphic, demonstrating marked anisocytosis and anisokaryosis, abundant vacuolated granular cytoplasm, and bizarre mitotic figures (Figures 5A & 5B).28 In some cases, phagocytosis of red and white blood cells is a prominent feature.

Histologic evaluation reveals diffuse infiltration of tissues by atypical histiocytes with abundant granular cytoplasm and prominent nucleoli. Similar to cytologic samples, erythrophagocytosis, leukocytophagy, and numerous bizarre mitotic figures can be noted. The neoplastic cells are commonly arranged in sheets.28

Figure 6. A right lateral thoracic radiograph of a 4.5-year-old castrated male Bernese mountain dog with disseminated histiocytic sarcoma. Nodular and alveolar soft tissue pulmonary infiltrates and increased opacity in the regions of the cranial mediastinal and tracheobronchial lymph nodes are present, consistent with marked enlargement of these lymph nodes. (Radiographic interpretation courtesy of Dr. Justin Goggin, DACVR.)
Localized and disseminated histiocytic sarcomas are derived from dendritic cells,1 which can be confirmed by immunohistochemistry. Tumor cells uniformly express CD1b, CD1c, CD11c, CD18, CD45 (indicative of leukocyte origin), and MHC II. Low concentrations of E-cadherin and CD4-Thy1 expression differentiate localized and disseminated histiocytic sarcoma from histiocytomas and reactive histiocytosis, respectively.1 In addition, histiocytic sarcoma tumor samples are uniformly positive for lysozyme and negative for cytokeratin expression, distinguishing them from neoplasms of epithelial origin.28 Immunohistochemistr y to detect cyclooxygenase -2 expression (often overexpressed in a variety of human sarcomas)37 found strong staining in only one out of 20 tumor samples tested.38


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