Mast-cell Tumors

Mast-Cell Tumors in Cats

  • Cancerous (known as “malignant”) round cell tumor; round cell tumors are made up of cells that appear round or oval on microscopic examination; mast-cell tumors are one type of round cell tumor
  • Tumor arising from mast cells
  • “Cutaneous” refers to the skin
  • Mast cells are connective tissue cells that contain very dark granules; the granules contain various chemicals, including histamine; they are involved in immune reactions and inflammation; mast cells can be found in various tissues throughout the body
  • Differentiation is a determination of how much a particular tumor cell looks like a normal cell; the more differentiated, the more like the normal cell
  • Mast-cell tumors of the skin in cats are classified as “compact” (more benign behavior) or “diffuse” (more undifferentiated and aggressive)
  • Mast-cell tumors also may be found in the tissue immediately beneath the skin (that is, the subcutis), spleen, liver, and intestines
  • Mast-cell tumors are the most common tumor found in the spleen of cats
  • Mast-cell tumors can release histamine, leading to the development of hives, reddening of the skin (known as “erythema”), bleeding and bruising, which can be seen around the tumor


  • Increased likelihood of developing mast-cell tumors is seen in certain breeds, indicating that a genetic likelihood exists

Signalment/Description of Pet


  • Dogs
  • Cats

Breed Predilections

  • Cats—Siamese

Mean Age and Range

  • Cats—middle-aged, 8–9 years of age for mast-cell tumors affecting the skin (known as “cutaneous mastocytoma”) and older cats for mast-cell tumors found in the intestines or spleen
  • Cats—histiocytic form of mast-cell tumors of the skin (cutaneous mast-cell tumor) occurs in young cats, mean age of 2.4 years

Predominant Sex

  • Cats—male Siamese

Signs/Observed Changes in the Pet

  • Depend on the location and grade of the tumor


  • Lack of appetite (known as “anorexia”)—most common complaint with mast-cell tumor of the spleen
  • Vomiting and diarrhea—may occur secondary to mast-cell tumors of the spleen or gastrointestinal tract
  • Long-term (chronic) weight loss
  • Sluggishness (lethargy)
  • Mast-cell tumor of the spleen—enlarged spleen (splenomegaly)
  • Intestinal mast-cell tumor—firm, segmental thickenings of the small intestinal wall; spread (metastasis) to the mesenteric lymph nodes, spleen, liver, and (rarely) lungs


  • Genetic mutation has been identified in up to 30% of mast-cell tumors

Risk Factors

  • Hereditary
  • Certain breeds of dogs and cats are more likely to develop mast-cell tumors (see “Breed Predilections”)


Health Care


  • Surgery—treatment of choice for mast-cell tumors of the skin; only narrow margins around the removed tumor are necessary, as the majority of tumors do not regrow following narrow surgical margin tumor removal
  • Surgical removal of the spleen (known as “splenectomy”)—treatment of choice for mast-cell tumors of the spleen; splenectomy is recommended in cats with large tumor burden in the abdomen, despite spread of the cancer (metastasis)


  • Limit activity for pets with heavy tumor burden (such as cats with mast-cell tumors of the intestines or spleen)


  • Medications presented in this section are intended to provide general information about possible treatment. The treatment for a particular condition may evolve as medical advances are made; therefore, the medications should not be considered as all inclusive


  • Lomustine
  • Vinblastine


  • Symptomatic treatment—Benadryl; famotidine or other histamine-blocking (H2) agents; omeprazole and sucralfate should be considered for any dog or cat with gross mast-cell disease

Follow-Up Care

Patient Monitoring

  • Evaluate any new masses microscopically
  • Evaluate regional lymph nodes at regular intervals to detect spread (metastasis)
  • Check complete blood count (CBC) at regular intervals, if the pet is receiving chemotherapy
  • Check liver enzymes on serum biochemistry profile, if the pet is on certain chemotherapy drug protocols (such as lomustine therapy)


  • Abdominal, including mast-cell tumor involving the intestine or spleen—abdominal ultrasound every 3 months for 1 year

Possible Complications

  • Low white blood cell count (known as “leucopenia”) due to suppression of the bone marrow (known as “myelosuppression”) secondary to chemotherapy
  • Liver toxicity (known as “hepatotoxicity”)

Expected Course and Prognosis

  • complete surgical removal (excision) of low-grade mast-cell tumors in most locations is curative
  • Complete surgical removal (excision) of high-grade mast-cell tumors or those located in areas associated with a poor prognosis (mucocutaneous junctions [areas where skin and moist tissues of the body come together; for example, the lips]), possibly inguinal regions) often require chemotherapy; median survival times averages approximately 11–12 months
  • Incomplete surgical removal (excision) of a low-grade mast-cell tumor may require additional local therapy with another surgery (often cured) or radiation therapy (85% disease-free at 3 years)
  • Incomplete surgical removal (excision) of a high-grade mast-cell tumor requires additional local therapy, in addition to chemotherapy; median survival times range from 6 to 12 months
  • Spread of the cancer to lymph nodes in the area of the tumor (regional metastasis) should be treated with surgical removal (excision) of the affected lymph node(s) at the time of the primary tumor removal; chemotherapy is necessary; median survival times are typically less than 9 months
  • Spread of the cancer to lymph nodes located away from the tumor or other organs (known as “distant metastasis”) often is treated with chemotherapy or ancillary therapies alone with a median survival of 4 months or less

Key Points

  • All new masses should be evaluated by a veterinarian
  • Fine-needle aspiration and microscopic examination should be performed as soon as possible on any new mass
  • Surgical removal (excision) should be done as soon as possible on any new mast-cell tumor

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