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.


Signalment and clinical findings

Figure 2. A cutaneous histiocytoma in the medial canthus of a 10-year-old spayed female boxer. Surgical resection was curative.
Although histiocytomas can occur in dogs of any age, the incidence drops after 3 years of age.7 A breed predilection has been documented in boxers, dachshunds, cocker spaniels, Great Danes, Shetland sheepdogs, and bull terriers.12,13 No apparent sex predilection exists.14 Histiocytomas are usually < 2- to 3-cm, fast-growing, raised, buttonlike, hairless lesions (Figure 2). Solitary tumors most commonly arise on the extremities, head, ears, or neck. Despite their rapid growth, the tumors are benign. Some dogs have multiple lesions at diagnosis,14 and histiocytomas with regional lymph node involvement have been reported, most commonly in Shar-Peis.13


Figure 3. A fine-needle aspirate of a histiocytoma in a dog. Note the monomorphic population of round cells with round to oval nuclei, ropy chromatin, indistinct nucleoli, and scant light-blue-grey cytoplasm with poorly defined cell borders (Wright-Giemsa, 500X). (Photo courtesy of Dr. Elizabeth Little, DACVP.)
Histiocytomas are round cell tumors and are easily diagnosed. Cytologic examination of fine-needle aspirates reveals sheets of pleomorphic round cells with abundant pale-grey cytoplasm; centrally located, round to slightly indented nuclei; and inconspicuous nucleoli (Figure 3). Varying amounts of inflammatory cell infiltrate can be present and typically indicate lesion regression.15

Histologically, histiocytomas are characterized by sheets and cords of pleomorphic histiocytes infiltrating the dermis and subcutis. Interestingly, despite the tumor's benign behavior, the mitotic index is often high.13 The presence of CD8+ (cytotoxic) T cells is common in regressing lesions. Studies have demonstrated that histiocytomas express surface markers descriptive of their epidermal Langerhans cell origin (CD1a, CD1b, CD1c, major histocompatibility complex [MHC] II), CD11c, and E-cadherin).3,7,16 Their lack of CD4 and thymocyte differentiation antigen 1 (Thy1) expression distinguishes them from other histiocytic diseases, which consistently express these surface markers.3,6 Immunohistochemical staining by using a panel of surface markers is recommended to obtain an accurate diagnosis only when the cell of origin cannot be clearly determined by routine histology.

Treatment and prognosis

In most cases, histiocytomas spontaneously regress within three months.13,14 Metastases to the lymph nodes have rarely been reported.3 The more aggressive behavior in some cases resembles Langerhans cell histiocytosis in people, a disease characterized by extensive regional cutaneous histiocyte infiltration (resembling cells in a histiocytoma), although widespread systemic involvement occurs.17 In dogs with multiple histiocytomas, the clinical course can be protracted, with new nodules forming while others regress. Eventually, spontaneous resolution of all nodules is expected.18 In general, surgical excision is often curative and should be used in patients with nodules that are ulcerated, infected, or pruritic. Adjunct therapy (such as corticosteroids) is generally unnecessary. The prognosis for solitary regressing cutaneous histiocytoma is excellent.18 In dogs with multiple nodules, the regression may be more prolonged,14,18 but ultimately regression occurs and a good prognosis can be given.


Two forms of reactive histiocytosis occur in dogs—cutaneous and systemic.


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