Mammary tumors (breast cancers) are the most commonly diagnosed tumor in intact female dogs older than 7 years of age.  Male dogs can also develop mammary tumors, but rarely. Several breeds are prone to developing mammary cancer including Poodles, English Spaniels, English Setters, and Terriers.  About 50% of mammary tumors are malignant (invasive to surrounding tissue with a high risk of spreading) and 50% are benign—this is very similar to the statistics for breast cancer in women.  Read about Mammary Cancer in Cats.

What causes this type of cancer in dogs?

• A hereditary genetic component (which has been found in humans with breast cancer) has not been established as a risk factor for canine mammary cancer.
• Hormonal influences have been found to be involved in the development of canine mammary cancer. In fact, it has been well documented that spaying (ovariohysterectomy) before the first heat cycle (typically before 6 months of age) dramatically decreases the risk of developing mammary tumors to a less than 1% risk. If spayed after the first heat cycle, the risk of getting mammary cancer increases to 8%. The risk increases to 26% if a dog is spayed after the third heat cycle.

What are the most common signs?

• Many owners will notice a lump somewhere along the mammary chain on their dog. The 4th and 5th mammary glands (closest to the groin or inguinal region) are most commonly affected. These masses can be red, purple, or fleshy in color. They can be either soft or hard and sometimes ulcerated. Many dogs have multiple masses present at one time.
• One of the more aggressive types of mammary cancers (called inflammatory carcinoma) can make a dog’s entire mammary chain very inflamed, ulcerated, and painful. These tumors are very rare. 

How is it diagnosed?

• The best way to diagnose a malignant mammary tumor (as well as determine the type of mammary tumor) is with a biopsy. Needle aspirates (cytology) can be used to rule out other lesions, such as mast cell tumors or benign cysts, but typically cannot definitively diagnose mammary tumors.
• There are many different types of mammary tumors; each can behave in a biologically different manner (in regards to aggressiveness-risk of spreading or recurring). These include carcinomas, sarcomas, and carcinosarcomas. Most mammary tumors in dogs are classified as carcinomas.
• It is important to perform blood work to evaluate a dogs’ general heath status.
• Radiographs are useful in evaluating for spread of these tumors to the lungs.
• Abdominal ultrasound can be used to look for spread of the cancer to the internal abdominal organs.

How is mammary cancer treated in dogs?

• Surgery is the treatment of choice for all dogs with mammary gland tumors. The type of surgery performed depends on the extent of disease. Veterinarians try to remove the tumor along with a large zone (about 2-3cm) of normal surrounding skin, fat, and sometimes muscle to reduce the risk of local tumor recurrence. If only one or two tumors are present, removal of just those tumors is typically adequate. If there are multiple tumors present, a more aggressive surgery  (removal of an entire chain, or both chains, of mammary glands) may be recommended.

     Will chemotherapy be recommended to treat my dog?

• Chemotherapy may be recommended as part of your dog’s treatment for mammary cancer if the tumor is of a high grade (determined when the sample is evaluated microscopically by a pathologist), aggressive tumor type, is very invasive and unable to be completely surgically removed, or there is metastatic disease (spread to other organs or lymph nodes). The chemotherapy protocols used to treat dogs with mammary cancer are typically very well tolerated.

     Will radiation therapy be recommended?

• Radiation therapy may be recommended for certain types of mammary cancers. This treatment is not as routinely used in veterinary oncology as it is for the treatment of breast cancer in people.

What is the prognosis for dogs with mammary tumors?

• The average survival time for dogs with mammary tumors is variable and depends on several factors including the grade of the tumor (obtained on the biopsy report after surgery), tumor type, the presence of metastatic disease, and whether there is invasion of the tumor into the lymphatics or blood vessels as well as what treatment regimens are used.
• The average survival times range from less than a month (for aggressive inflammatory carcinomas) to several years.

What is on the horizon for this cancer?

• As in women, the use of targeted therapies like Herceptin—a treatment that targets certain molecular and genetic defects found in cancer cells-- holds great promise for improving the outcome of therapy for the treatment of mammary cancer. In veterinary medicine there are currently two targeted therapies-Palladia and Masivet. There is currently a great deal of research looking for targeted therapies.