Affects: Cats, Dogs
A mastocytoma in dogs (or mast cell tumor in dogs) is a neoplasm (neoplasia) originating from mast cells in the domestic dog, which occurs mainly in the skin and subcutis. Mastocytoma are not only extremely common in dogs, but also tend to be much more malignant in them than in other animal species. The average survival time for malignant tumors is only four months, whereas for benign tumors it is over two years.
Mast cells are cells of the immune system that play a role in the innate immune response. They produce a number of biologically active substances, including primarily histamine. Mastocytoma account for about one-fifth of all skin tumors in dogs. They present as nodules or raised patches, and about one-fifth of affected animals have ulcers and bleeding in the stomach and duodenum. Metastasis in malignant mastocytoma occur primarily in lymph nodes, liver, spleen, and bone marrow. Any lump in the skin or subcutaneous tissue can be a mastocytoma. Detection is only possible by taking tissue with a fine needle (fine needle biopsy) followed by staining and microscopic examination (cytopathology).
Although the classifications according to the clinical appearances and cell appearance in cytodiagnostics give indications of the biological behavior (benign or malignant) and thus the prospect of cure, this tumor disease is unpredictable and should be treated at an early stage. The treatment of choice is complete surgical removal, possibly combined with radiotherapy or chemotherapy. Tumors for which surgical removal is not possible or only incompletely possible can also be treated with tyrosine kinase inhibitors.
Diagnostics: A visual or palpatory diagnosis is not possible, since neither appearance nor consistency allow differentiation from other skin tumors.
The diagnostic tool of choice is fine needle biopsy, since sufficient cells can be obtained from mastocytomas. In the cytological preparation, mast cells can be distinguished relatively easily from other cell types on the basis of their granules, although certain rapid staining solutions stain mast cell granules only unreliably, and cells of poorly differentiated mastocytomas may contain very few granules.
Changes are rarely observed in the blood count; occasionally, an increase in a subtype of white blood cells (eosinophilia) may occur. In systemic mastocytosis, a decrease in white blood cells (leukopenia) often occurs. Circulating mast cells in the blood are usually not observed.
Treatment: Although the classifications based on clinical appearances and cellular appearance in cytodiagnostics provide clues to the biological behavior, a mastocytoma remains unpredictable and is potentially to be considered malignant. The first-line treatment method is surgical removal of the tumor at the earliest possible stage. Concomitant chemotherapy and radiation may be necessary, especially if complete removal is not possible or unsafe for anatomic reasons. For inoperable tumors, treatment with tyrosine kinase inhibitors may be attempted. In general, the prospect of cure is best in well-differentiated mastocytomas (low-grade or grade 1) and in animals without general signs (substages a). Young dogs (<1 year of age) also have a better prognosis than older dogs.