Understanding Mast Cell Tumors in Dogs

Mast cell tumors in dogs may appear small and insignificant; however, they account for up to 20% of all dog malignant tumors of the skin. Canine skin cancer occurs in a variety of forms and locations, including when a dog has mouth cancer as one or more lumps in the mouth or skin, which appear red, ulcerated and/or can be itchy. These lymphomas occur in middle-aged to older dogs with no preference to breed.

What Mast Cell Tumors Are

Occurring in tissues throughout the body, mast cells are part of the immune system (defense mechanism of the body) and respond to inflammations, infections, allergies and disease. They can release large amounts of very powerful chemicals including enzymes that break down proteins (proteolytic enzymes), histamine, heparin, prostaglandins and seratonin. Toxic to foreign invaders, such as parasites, these enzymes are released into the body when mast cells are triggered by the immune system. These chemicals are vital to normal body functions, especially immune response. However, they can be very damaging when released in chronic excess, affecting blood pressure, heart rate and other body functions. Because of this, sites where mast cell tumors are surgically removed can sometimes refuse to heal leading to life-threatening diseases, such as gastric ulcers, allergies and internal bleeding.

Mast Cell Tumor Symptoms

Mast cell tumors appear in a variety of forms, making diagnosis difficult. Therefore, proper identification and treatment are very important.

Mast cell tumors can appear anywhere on the body but most commonly on the trunk, genitals, limbs, on the skin or on the tissue underlying (subcutaneous tissue). They can also be benign or malignant. They can be smooth, bumpy or ulcerated as one lump or as multiple bumps as in the case of dog mouth cancer.

Some dogs exhibit systemic signs of the cancer due to the release of histamine. These signs include blood in the stool, vomiting, diarrhea, loss of appetite, enlarged lymph nodes, duodenal ulcers or abnormalities in blood clotting.

Diagnosing Mast Cell Tumors

Since the type of cancer varies in form, it is important for each dog to be evaluated and treated individually. Fine needle aspirates (samples of tumor fluid), blood tests, x-rays, biopsies and ultrasound can all be used to confirm the diagnosis.

After the tumor is diagnosed, it is graded and staged to determine its expected behavior, what treatment is necessary and possible prognosis. Grading refers to the extent of malignancy. More serious tumors have higher grades.

  • Grade I: Most mast cell tumors are Grade I, occurring in the skin. Since they are usually benign, they have not spread even though they can be large and difficult to remove. The estimated long-term survival rate is 90%.
  • Grade II: These tumors can be unpredictable since they extend below the skin and into the subcutaneous tissues. They are usually malignant.
  • Grade III: Since these tumors are very aggressive and have invaded deep below the skin, extensive treatment, such as radiation, is required.

Staging refers to the extent the cancer has spread.

  • Stage I: No lymph node involvement with only one tumor in the skin
  • Stage II: Lymph node involvement with only one tumor in the skin
  • Stage III: With/without lymph node involvement with multiple large, deep skin tumors
  • Stage IV: With lymph node involvement and one or more tumors that have spread in the skin

Canine Lymphoma Treatment

Surgical removal is the first choice for mast cell tumors. The most effective way to treat Stages II to IV is chemotheraphy, using a combination of oral and injectable drugs weekly. Sometimes mast cell tumors do not respond well to chemotherapy so radiation is used to prevent the spread of the cancer. Untreated, the life expectancy is 4 to 6 weeks; however, treatment can extend the quality of life to approximately one to five years.