Veterinary Society of
Surgical Oncology


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


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