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Issue Description
A histiocytoma in the dog is a benign tumor. It is an abnormal growth in the skin of histiocytes (histiocytosis), a cell that is part of the immune system.

Histiocytic Disorders
A histiocyte is a differentiated tissue cell that has its origin in the bone marrow. The source for histiocytes is the monocyte/macrophage line. Monocytes (found in the blood) and macrophages (found in tissue) are responsible for phagocytosis (ingestion) of foreign material in the body. Langerhans cells are dendritic cells found in the skin and function by internalizing antigens (foreign particles) and presenting them to T cells. They arise from monocytes. Histiocytic disorders refer to diseases that are caused by abnormal behavior of these cells.

They include the following:

Reactive Diseases of Langerhans Cells
  • Histiocytomas
  • Cutaneous histiocytosis
  • Systemic histiocytosis

  • Reactive Diseases of Macrophages
  • Hemophagocytic syndrome - a condition where macrophages phagocytose myeloid and erythroid precursors (similar to hemophagocytic lymphohistiocytosis in humans)

  • Malignant Diseases of Langerhans Cells
  • Malignant histiocytosis - a condition found in Bernese Mountain Dogs
  • Diffuse histiocytic sarcoma
  • Localized histiocytic sarcoma

  • Malignant Diseases of Macrophages
  • Histiocytic lymphoma

  • Common Breeds At Risk
  • Bulldogs
  • Scottish Terriers
  • Greyhounds
  • Boxers
  • Boston Terriers

  • Symptoms
    Most commonly histiocytomas are found in young dogs and appear as a small, solitary, hairless lump,[6] although Shar Peis may be predisposed to multiple histiocytomas. They are most commonly found on the head, neck, ears, and limbs, and are usually less than 2.5 cm in diameter. Ulceration of the mass is common.

    Definitive diagnosis is established via cytology or biopsy. Your veterinarian may want to wait and watch the lump, based on the age of your dog and the location and presentation of the lesion. Histiocytomas will spontaneously regress in 2- 3 months. If in doubt, biopsy will provide an answer as to the nature of the lesion.

    Most histiocytomas will regress within two or three months. Surgical removal may be necessary if the tumor does not regress or if it is growing rapidly to a large size. Histiocytomas can also be treated with an intralesional injection of a corticosteroid, but this is not always successful.

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