Histiocytoma

General Considerations

•histiocytomas are common in dogs and account for 3%-14% of canine skin tumors

•cause unknown

•intracytoplasmic reticular aggregates are suggestive of a viral cause but a causative agent has not been identified and experimental horizontal transmission has been unsuccessful

•histiocytomas arise from epidermotrophic Langerhans cells in the skin and are different from the macrophage immunophenotype

•histiocyte proliferation expresses major histocompatability complex class II and several leukocyte antigens characteristic of dendritic cell differentiation

Clinical Features

•age: 50% < 2 years

•breed predisposition: Boxer, Dachshund, Cocker Spaniel, Great Dane, Shetland Sheepdog, and Bull Terrier

•sites: head (especially pinna), pelvic limb, feet, and trunk

•benign tumor despite rapid growth and high mitotic index

•nucleus variable in size and shape, cytoplasm pale blue, and lymphocytic, plasmocytic, or neutrophilic infiltrates

•spontaneous regression possible with concurrent lymphoid infiltration which is mediated by CD8+ T cells

•treatment: surgery or cryosurgery

•prognosis: excellent

Cutaneous Histiocytosis

•cutaneous histiocytosis: benign proliferation of multiple erythematous dermal or subcutaneous plaques or nodules

•spontaneous asynchronous regression over 8-12 weeks

•treatment: corticosteroids ± azathioprine and polyethylene glycosylated L-asparaginase

•prognosis: good

Localized Histiocytic Sarcoma

•synonym: malignant fibrous histcioytoma, giant cell fascial sarcoma, epitheliod sarcoma, malignant histiocytoma, reticulum cell sarcoma, and giant cell tumor

•malignant fibrous histiocytoma is a diagnosis that is being phased out in both veterinary and human pathology

•primitive, pleomorphic sarcomas arising from undifferentiated mesenchymal cells

•storiform-pleomorphic subtype is most common in dogs and giant cell subtype is most common in cats

•histiogenesis of malignant fibrous histiocytoma is controversial and may represent final common pathway of tumor progression (for both STS and other tumor types) or misdiagnosis as histologic re-evaluation of malignant fibrous histiocytoma in humans has demonstrated many different histologic types

•mean age 8-9 years (but reported in 4-month-old puppy)

•breed predisposition: Flat-Coated Retrievers

•biologic behaviour: invasive with high recurrence rate and moderate metastatic potential dependent on tumor grade

•malignant fibrous histiocytoma has also been reported in the cat with the same biologic behaviour

•gross appearance: firm and invasive arising from subcutis

•histologic DDx: FSA, peripheral nerve sheath tumor, and extraskeletal OSA

•prognosis is guarded as majority are grade III tumors with no response to either chemotherapy or radiation therapy

•giant cell variant of malignant fibrous histiocytoma has a 70% metastatic rate at diagnosis and overall metastatic rate of 90% with an overall MST 61 days and MST 161 days for treated dogs

•metastatic predictors in humans: histologic grade

Systemic Histiocytosis

•breed predisposition: familial disease in middle-aged Bernese Mountain Dog

•mean age 4 years

•prolonged course of disease with a mean of 15 months

•histiocytic infiltrates do not demonstrate clinical signs of malignancy

•predilection for skin, eyes, and lymph nodes

•skin lesions on flanks, muzzle, nasal planum, eyelids, and scrotum

•poorly responsive to anti-inflammatory and immunosuppressive therapy

Disseminated Histiocytic Sarcoma (or Malignant Histiocytosis)

•synonym: systemic histiocytosis

•neoplastic transformation of tissue macrophages leading to excessive phagocytosis of erythrocytes

•breed predisposition: Bernese Mountain Dog, Golden Retriever, and Rottweiler

•familial disease in older Bernese Mountain Dog with probable polygenic mode of inheritance

•sex predilection: male

•mean age: 7 years (range, 4-10 years)

•rapidly fatal condition with relatively non-specific clinical signs such as weight loss, splenomegaly, hepatomegaly, dyspnea, neurologic signs, and severe regenerative anemia

•necropsy findings: multiple, solid, pale tumors in a variety of organs including spleen, liver, lymph node, and lungs

•malignant histiocytosis resembles anaplastic tumors and lysosome immunoreactivity can be used to differentiate systemic and malignant histiocytosis from other histiocytic disorders