Mammary Gland Tumors in Dogs

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Mammary gland tumors are a type of cancer that arise from breast tissues. These tumors are similar to breast cancer in women, and they can be lethal in dogs. Approximately 50 percent of these tumors are malignant, which means they can spread, and 50 percent are benign and do not spread.

The cause of mammary tumors is not well understood. Hormones such as estrogen and progesterone play an elusive role in the development and progression of these tumors. They occur in both intact (non-neutered) and spayed dogs and it is the most common cancer of female dogs, with two cases per thousand dogs at risk, constituting over 50 percent of all cancers. Mammary gland tumors occur most commonly in females; they are rare in males.

The average age that dogs develop these tumors is 10 to 12 years of age.

Any breed of dog may develop these tumors, but breeds that appear to be at increased risk are poodles, terrier breeds, cocker spaniels, and German shepherd dogs.

Timing of ovariohysterectomy, which is removal of the ovaries and uterus and commonly called neutering or spaying, significantly impacts development of mammary gland tumors in dogs. Dogs spayed prior to their first estrus cycle (heat cycle) have less than a one percent risk, those spayed between the first and second estrus have an 8 percent risk, whereas those spayed after their second estrus cycle develop these tumors as commonly as dogs that are not spayed.

Body weight may influence the development and progression of these tumors.

What to Watch For

Masses or lumps within the mammary glands

Bruising of the skin over the mammary glands

Ulceration (open wounds) on the mammary glands

Bleeding of the skin associated with growth of the masses

Difficulty breathing

Coughing

Lack of ability to exercise

Lack of appetite

Diagnosis

A complete physical examination

Fine needle aspirate cytology of the mass, which is a technique where a small needle is inserted into the mass to withdraw some cells. These cells are examined under a microscope by your veterinarian or a pathologist.

Thoracic (chest) radiographs (X-rays)

Blood work, including complete blood cell counts and a biochemical profile

Urinalysis

Fine needle aspirate cytology of local lymph nodes if they are enlarged

Excision of masses and submission for histopathology (microscopic examination)to determine the type of cancer

Abdominal (belly) ultrasound (sonogram)

Abdominal radiographs (X-rays)

Treatment

Mastectomy, which is surgical removal of the mass and associated mammary gland, along with removal of any involved lymph nodes

Ovariohysterectomy. If your dog is intact spaying is generally done at the time of the mastectomy.

Chemotherapy. Drugs that kill cancer may be recommended in certain animals if the cancer has metastasized or is inoperable.

Radiation therapy

Anti-estrogen therapy (anti-hormone therapy)

Home Care and Prevention

If you note a mass in your dog's mammary glands, have her examined by a veterinarian as soon as possible. Malignant masses that have gone undetected for long periods and are large are more likely to spread.

If your dog has a large, ulcerated, bleeding mass keep her indoors to keep the area clean and lessen the potential for infection before seeing your veterinarian.

Have your pet spayed or neutered at an early age to decrease the risk of this type of cancer. Avoid the use of synthetic hormone products to control heat cycles as they may increase the risk of your dog developing this type of tumor.

Take your dog to your veterinarian for regular examinations so that tumors can be detected early when they are more likely to be completely removed. This is especially important if you have an older dog that is at increased risk for this type of cancer.

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