Oncology Update: Histiocytic Disease

Emergency Veterinarian Toronto

Oncology Update
Central Toronto Veterinary Referral Clinic
Kevin Finora DVM, Diplomate ACVIM (Oncology and Small Animal Internal Medicine)
Histiocytic Disease

Histiocytic disease is a group of disorders for which our understanding is rapidly evolving. A lack of complete knowledge respecting etiology and pathogenesis has resulted in this group of diseases becoming a controversial area in veterinary oncology and pathology. Some degree of confusion surrounding this class of cancer is the result of the often changing and complex nomenclature used for each disease. Histiocytic disease can be divided into two groups based on cell of origin (dendritic cell and macrophage), as well as into reactive versus neoplastic. “Histiocytic” refers to cells of the monocyte/macrophage cell line.

There are 3 categories of histiocytic disease: Cutaneous Histiocytoma, Reactive Histiocytosis and Histiocytic Sarcoma.

Cutaneous Histiocytoma is a benign neoplasm commonly seen in young dogs, though it can appear in older dogs. Typically these lesions will spontaneously regress. They are tumours of epidermal origin involving the Langerhans cells.

Reactive Histiocytosis is divided into two groups: A) Cutaneous Histiocytosis and B) Systemic Histiocytosis. Both forms of Reactive Histiocytosis involve the non-neoplastic proliferation of activated interstitial dendritic cells. A) Cutaneous Histiocytosis is restricted to the skin only. This disease is benign and tends to be steroid responsive. In some situations, if steroid therapy fails, chemotherapy drugs are necessary to control the disease. Cutaneous Histiocytosis tends to wax and wane. The goals of treatment are to manage and control the disease and usually not to cure. Spontaneous regression has been reported, but is rare. B) Systemic Histiocytosis is similar to Cutaneous Histiocytosis but involves various regions, in addition to the skin. Mucous membranes and lymph nodes are common targets. Long-term treatment is needed and typically involves treatment with oral immunosuppressive drugs, such as cyclosporine. The treatment is prolonged, and the disease will wax and wane. In general this form of histiocytic disease is not thought to be fatal. However, due to difficulty in achieving adequate long-term control, many animals are eventually euthanized.

Histiocytic Sarcoma is the neoplastic form of histiocytic disease. Histiocytic Sarcoma exists in two forms, A) Localized Histiocytic Sarcoma and B) Disseminated Histiocytic Sarcoma. A) Localized Histiocytic Sarcoma originates from a single site (must be confirmed with appropriate staging). These tumours can usually be diagnosed by cytology. However histopathology is the gold standard. Immunohistochemistry can be useful in making the diagnosis as this tumour will be positive for CD18. The localized form may be amenable to surgery followed by chemotherapy. One study suggests a survival time of over 400 days is possible when Localized Histiocytic Sarcoma is treated with surgery and adjunctive chemotherapy. B) Disseminated Histiocytic Sarcoma (formerly known as malignant histiocytosis), is the most aggressive form of Histiocytic Sarcoma. This disease has multi-organ involvement (usually, spleen, liver and lungs) and is characterized by rapid progression. Diagnosis can be made by cytology. Histopathology and immunohistochemistry (also CD18 positive) remain the gold standard. This disease can be treated with chemotherapy with an expected survival time of 106 days. This disease is uniformly fatal.

Finally malignant fibrous histiocytoma is a group of tumours with histological features of fibroblasts and histiocytes. This disease is distinct from the general classification of histiocytic diseases mentioned above. They are localized to the subcutis and have a low metastatic rate. Despite their nomenclature, they are CD18 negative and are not believed to be of true histiocytic origin. They are best thought of as a type of mesenchymal sarcoma similar to fibrosarcoma. They can be treated with methods of local control. They are not thought to be very chemotherapy responsive.


Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.

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