Mammary Gland (Breast) Tumors in Dogs


Overview of Canine Mammary Gland Tumors

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

Signs of mammary gland tumors in dogs may include: 

  • 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 of Mammary Gland Tumors in Dogs

  • 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 of Mammary Gland Tumors in Dogs

  • 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.

    In-depth Information on Mammary Gland Tumors in Dogs

    Swelling of the breast tissue can be related to a number of conditions – both normal and abnormal. For example, normal hormonal changes associated with the female reproductive cycle in nonspayed females lead to enlargement of the mammary glands. Pregnancy is of course related to glandular development. Inflammation, hyperplasia (excessive growth), and cancers are examples of abnormal growth. When mammary glands are enlarged or swollen, a veterinarian will consider a number of diagnoses.

  • Mammary gland tumor (neoplasia). Approximately 50 percent of mammary gland tumors are benign. The other 50 percent are malignant, and of these, about 50 percent will metastasize elsewhere in the body.
  • Mastitis is an infection of the mammary glands that causes swelling of the glands and mimics cancer. The glands are usually warm to the touch, painful, and can discharge discolored milk. Mastitis is most often observed in association with the female estrus cycle (heat), during pseudocyesis (false pregnancy) or after a dog has given birth. Animals with mastitis frequently are very ill with fever or other signs of infection. In contrast, most dogs with breast cancer are not ill unless the cancer spreads.
  • Cysts, papillary cystic hyperplasia, and lobular hyperplasia are benign conditions affecting the mammary glands in which proliferation or growth of normal tissue structures is accompanied by fluid production forming cysts (pockets of fluid).
  • Inflammatory carcinoma is a more aggressive form of mammary gland tumor seen primarily in the dog in which all glands in one or both chains are affected. Severe bruising, and ulceration of the overlying skin are accompanied by an animal that feels very sick. This particular form of mammary gland cancer is very likely to spread throughout the body and it carries a very poor prognosis.
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