Signalment and clinical findings
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 2. A cutaneous histiocytoma in the medial canthus of a 10-year-old spayed female boxer. Surgical resection was curative.
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
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.)
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.