+ 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