+ 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