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Care Guide

About Care Guides[x] These care guides are written to help your clients understand common conditions, tests, and procedures, as well as to provide basic information about pet care. They are based on the most up-to-date, documented information, recommendations, and guidelines available in the United States at the time of writing. Pharmaceutical product licensing, availability, and usage recommendations are based on US product information. Use the Download Handout button to generate a PDF for printing or e-mailing to your clients.

Mast Cell Tumors in Dogs

    • Mast cell tumors are malignant (cancerous) masses that can occur anywhere in the body, but they are usually associated with the skin in dogs.
    • These tumors cannot be diagnosed from appearance alone, because they can look like many other benign (non-cancerous) and malignant (cancerous) masses.
    • Diagnosis may require analysis of a fine-needle aspirate (cell sample), a biopsy (tissue sample), or the mass after surgical removal.
    • Additional diagnostic tests that may be recommended to determine if the mass has metastasized (spread) include a complete blood count (CBC), a chemistry panel, and urinalysis; radiographs (x-rays); samples of other tissues, such as lymph nodes, liver, spleen, or bone marrow; and advanced imaging such as ultrasound, CT (computed tomography), or MRI (magnetic resonance imaging) scans.
    • Surgical removal of the mass is the cornerstone of treatment, but radiation therapy, chemotherapy, and other medications may also be recommended.
    • Because early diagnosis and treatment can improve the prognosis, it is important to check your dog’s skin regularly and ask your veterinarian to examine any new lumps and bumps or changes in existing masses.

    What Are Mast Cell Tumors?

    Mast cells are normally found throughout the body and are often involved in functions such as inflammatory and allergic reactions. Mast cell tumors are masses of these cells that have collectively become malignant (cancerous). No one knows exactly what causes normal mast cells to develop into malignant tumors.

    Although mast cell tumors can develop anywhere in the body, they are usually found within or just under the skin and account for approximately 20% of skin tumors in dogs. These tumors generally occur in older dogs, and breeds that are commonly affected include boxers, Boston terriers, bulldogs, basset hounds, golden retrievers, and Labrador retrievers.

    What Are the Clinical Signs of Mast Cell Tumors?

    It is impossible to diagnose a mast cell tumor simply by its appearance, because it can look just like many other benign or malignant lumps. A mast cell tumor can appear as a firm or soft lump attached to the skin or just under the skin. The mass may fluctuate in size and show signs of redness, swelling, bruising, hair loss, and ulceration (development of a sore), or the skin may appear perfectly normal. Dogs may have a single tumor or several masses in different locations. Most mast cell tumors in dogs are found on the trunk or limbs and, less commonly, on the face and neck.

    If the tumor has metastasized (spread) to other locations in the body, such as the spleen, liver, bone marrow, or digestive tract, there may be additional clinical signs, such as lethargy (loss of energy), anorexia (loss of appetite), nausea, vomiting, diarrhea, abdominal pain, and blood in the stools.

    How Are Mast Cell Tumors Diagnosed?

    Your veterinarian may recommend a number of tests to help determine if the lump is a mast cell tumor:

    • Fine-needle aspiration: A needle is inserted into the mass, and cells are extracted for examination under a microscope. If suspicious mast cells are found, your veterinarian will usually recommend surgical removal of the mass.
    • Complete blood count (CBC), chemistry panel, and urinalysis: These tests will help your veterinarian assess your pet’s general health before anesthesia and, in some cases, provide clues about possible tumor spread to other organs in the body.
    • Radiographs (x-rays): Abdominal and/or chest radiographs may help identify masses and indicate if the tumor has spread within the body.
    • Ultrasound examination: This can provide a more detailed view of the mass and sometimes evidence of tumor metastasis.
    • Biopsy: If a larger sample is needed than what is possible with fine-needle aspiration, your veterinarian may recommend a biopsy (tissue sample).

    What Are Tumor Grades and Staging?

    Some mast cell tumors can be more aggressive than others. Veterinarians often submit tissue samples—from a biopsy or from surgical excision (removal) of a mass—to a laboratory for analysis. Mast cell tumors are assigned a grade from I to III, with grade I tumors being least likely to metastasize and grade III tumors being more aggressive.

    Staging is the process of determining the extent of metastasis (tumor spread) throughout the body. Staging is usually done for dogs with more aggressive tumors and may require:

    • Fine-needle aspirates or biopsies: Cell or tissue samples from the lymph nodes, spleen, liver, or bone marrow may help assess organ involvement.
    • Radiographs (x-rays): Abdominal and/or chest radiographs may help indicate if the tumor has spread within the body.
    • Advanced imaging: Ultrasound examination, CT (computed tomography), or MRI (magnetic resonance imaging) may be recommended to explore the extent of the mass to prepare for surgery or to check for signs of tumor metastasis.
    • Fecal test for blood: Blood in the feces may indicate digestive tract involvement.

    Treatment and Prognosis

    Depending on whether the mass(es) are limited to the skin or other organs are involved, treatment may include:

    • Surgery: Surgical removal of mast cell tumors, taking wide skin and tissue margins around the masses, is the cornerstone of treatment.  If laboratory analysis of the excised tissue shows that the mass was completely removed and there is no evidence of metastasis, no more treatment may be required. If the mass was not completely removed, a second surgery (or additional therapy) may be necessary.
    • Radiation therapy: For mast cell tumors that could not be completely removed with surgery, radiation therapy can help eliminate remaining cancerous cells in the localized area.
    • Chemotherapy: This is usually reserved for grade II or III tumors and is often palliative (relieving some discomfort associated with the tumors) rather than able to cure the disease.
    • Other medications: Your veterinarian may recommend additional medications to help relieve some of the secondary effects of these tumors.

    The prognosis can depend on many factors. If the tumor is limited to the skin, shows no evidence of metastasis, and is completely excised with surgery, the prognosis can be very good. For grade III tumors, tumors with evidence of metastasis to other areas of the body, and tumors located around the mouth, muzzle, penis, or anus, the prognosis may not be as good. Your veterinarian may recommend that you consult a veterinary oncologist (cancer specialist) for guidance about all of your options.

    Since early diagnosis and treatment can improve the prognosis, it is important to check your dog’s skin regularly and ask your veterinarian to examine any new lumps and bumps or changes in existing masses.