Giant Cell Tumors in Dogs

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Malignant Fibrous Histiocytoma

A malignant fibrous histiocytoma refers to an invasive tumor that contains an excessive number of histiocytes, the white blood cells that reside within the normal connective tissue of the body. Referred to as tissue macrophages, histiocytes play a defensive role in the body’s immune response, engulfing cellular debris and infectious agents, as well as initiating defense mechanisms in the system.

Generally, histiocytomas are benign growths, but there are documented cases of malignant fibrous histiocytomas, where the tumor is composed of both histiocytes and fibroblasts. Fibroblasts are the most common cells found in the body’s connective tissue, playing a major role in wound healing. This condition involves the cells of both, with the addition of giant multinucleated cells, which occur as the result of the immune system cells attacking infectious agent cells and fusing together.

Giant cell tumors do not commonly afflict dogs; this is a rare canine disease. 

Symptoms and Types

The most common symptoms include:

  • A firm and invasive tumor in the fat layer of the skin
  • Lack of appetite
  • Weight loss, often rapid
  • Lethargy


The causes for malignant fibrous histiocytoma are currently unknown.


Upon examination, your veterinarian will need to rule out a variety of other medical issues prior to issuing a diagnosis and treatment plan. Some other medical conditions which can cause giant cell tumors to form include:

  • Fibrosarcoma - a malignant tumor located in the fibrous tissues
  • Chondrosarcoma - a tumor that can be found in the cartilage of the animal
  • Liposarcoma - a tumor which develops in the fat cells of the animal
  • Periopheral nerve sheath tumors


Laboratory work, as well as X-ray imaging, will be used in diagnosing the condition. Your veterinarian may perform a histologic examination, analyzing the tissue under a microscope, during the course of taking a biopsy of the tissue, so that the exact composition of the mass can be defined and an appropriate treatment plan put into place.

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