Mammary Gland (Breast) Tumors in Dogs

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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.
  • Veterinary care should include diagnostic tests and subsequent treatment recommendations. Medical tests are needed to establish the diagnosis, exclude other diseases, and determine the impact of the mammary gland tumor on your dog.

    In-depth Information on Diagnosis 

  • A complete medical history should be obtained and your veterinarian should complete a thorough physical examination.
  • Fine needle aspirate cytology of the tumor is a simple and safe technique in which a small needle is briefly inserted into the tumor mass to withdraw some cells. This sample is examined under a microscope by a pathologist. The results help to determine if the mass is a mammary gland tumor or some other form of cancer. Results from a fine needle aspirate test can be difficult to interpret. If the results indicate a malignant tumor, the tumor is removed. However, sometimes the cells appear benign yet the mass may still represent a malignant form of cancer. Thus, decisions to remove a mammary gland tumor surgically cannot be made solely on these results. Other factors, including the age, physical appearance, a more complete biopsy, or associated clinical findings, may determine the recommendation for surgery or not.
  • Chest X-rays allow your veterinarian to look for evidence of metastatic cancer that has spread. The lung is a frequent site of metastasis for this type of cancer.
  • Fine needle aspiration cytology of enlarged regional lymph nodes is appropriate. This test would help stage a cancer and provide a more accurate treatment plan and prognosis. This can be a very important test to decide if your dog needs to receive chemotherapy in addition to surgery.
  • Histopathology (biopsy) is done when the cause of a swelling is uncertain or whenever mammary gland tissue or adjacent lymph nodes are removed. The tissue is examined by a pathologist who determines the type of cancer, if the cells are benign or malignant, and if the cancer has been removed to the extent of the surgical margins. This information is crucial to your veterinarian for making recommendations about further therapy. Biopsy results may also determine if your dog should be evaluated by a veterinary oncologist, which is a specialist in the treatment of cancer.

    Your veterinarian may recommend additional diagnostic tests to ensure optimal medical care. These are selected on a case-by-case basis.

  • A complete blood cell count (CBC) is a standard test to screen your pet’s general health and to assure that it is safe to perform other procedures.
  • A blood biochemical profile is a simple blood test that helps to assess the general health of the body organs such as the liver and kidneys.
  • A urinalysis is often obtained to assess kidney function and the lower urinary tract for a hidden infection that might cause trouble if the immune system becomes suppressed.
  • Abdominal X-rays can be used to evaluate intra-abdominal organs such as the liver, internal lymph nodes, and visualize parts of the spine.
  • Abdominal ultrasound is a noninvasive study that permits visualization of soft tissue body organs such as the liver, kidneys, spleen, intestines, and lymph nodes. This test is often done to determine if the cancer has spread. There is no risk to your dog with this procedure, unless a biopsy procedure is also planned, in which case the risk is small. This test requires that the hair over the abdomen be clipped.
  • In-depth Information on Treatment

  • The treatment of cancer in dogs is almost identical to the treatment of cancer in people. Surgical removal of the cancer is the optimal treatment. A simple procedure called a lumpectomy suffices in some cases, while more radical mastectomy is required in others. If the type of cancer is confined to the mammary gland, mastectomy can be curative.
  • In the dog, the type of surgery does not seem to influence overall survival, so that less radical surgery – a simple mastectomy with removal of the adjacent glands – is often recommended
  • Any enlarged lymph nodes should be removed as well. If lymph nodes contain cancer cells, an oncologist should be consulted. Chemotherapy is generally necessary.
  • Because of the influence that hormones have on breast tumors, it is recommended that intact dogs be spayed at the time of surgery for the mammary gland tumor. Recent evidence suggests that removal of the ovaries may impact favorably on long term survival. Spaying can be performed at the same time as mastectomy.
  • Chemotherapy using drugs that kill cancer may be recommended in certain cases such as when the cancer has metastasized or it has a high potential to metastasize. Your veterinarian is likely to consult with you or refer you to a veterinary oncologist for this. There are many different types of chemotherapy drugs, including doxorubicin (Adriamycin), a commonly-used drug in mammary gland tumors.
  • Radiation therapy may be recommended to treat large tumors that are not amenable to surgical removal.
  • Anti-estrogen therapy with drugs like Tamoxifen, an anti-hormone agent used in women with breast cancer, has been used experimentally in dogs with mammary gland tumors. This therapy is rarely recommended in dogs as it does not seem as effective and can cause vaginal discharge, urinary incontinence (leaking urine), and pyometra (infection in the uterus).
  • Follow-up Care for Dogs with Mammary Gland Tumors 

    Optimal treatment for your dog requires a combination of home and professional veterinary care. Follow-up can be critical. Administer prescribed medications as directed, and be certain to alert your veterinarian if you are experiencing problems treating your dog.

    Specific optimal follow up veterinary care for mammary gland tumors in dogs and cats involves the following:

  • Initially, your dog will be recovering from surgery. Activity during this period should be restricted to allow for proper healing of the surgery site. Your dog is likely to be on a short 10 to 14 day course of antibiotics to prevent any infections from developing at the site of the mammary gland tumor removal. Your veterinarian may also prescribe a short course of anti-inflammatory pain medication. Less often, opiate pain medication, given by mouth or via a skin patch, may be prescribed.
  • Sutures (stitches) must be removed in 10 to 14 days after surgery, after skin healing has occurred.
  • The pathologist’s biopsy report that comes after surgery is very important. With this information, your veterinarian or oncologist will determine with you if your pet needs further therapy.
  • Even if no follow up treatment is recommended, you should have your dog checked regularly for recurrence or spread of the cancer. Re-evaluation is recommended every two to three months for the first year, then every six months thereafter. Your veterinarian should perform a complete physical exam at these visits and obtain chest radiographs (x-rays) to be sure the cancer has not spread to the lungs.
  • You should examine your own dog routinely for signs of recurrence at the site of tumor removal or for the presence of new breast tumors in the remaining mammary glands.
  • Signs that may indicate the cancer has spread include decreased activity, weight loss, shortness of breath, coughing, decreased appetite, vomiting, or diarrhea. If you note any of these symptoms, contact your veterinarian.
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