Breast Cancer in Dogs

Breast cancers in dogs are often referred to simply as mammary tumors. This disease usually strikes approximately one in four unspayed female dogs. Females spayed before their first estrus cycle reduce the chances from 25% to .05%. If spayed after their first heat, but before the second, the risk of future breast cancer is approximately 8%. Half of the mammary tumors found in canines are benign, and of the half that are malignant, most can be successfully treated with surgery if caught early enough. Although it is rare, male dogs may also develop breast cancers. Breast cancer in males tends to metastasize aggressively.

Breast cancer in canines is relatively common, occurring in approximately 25% of unspayed female dogs.

Most of the signs of breast cancer are related to the tumors themselves and are located on one of the eight to ten mammary glands present on most female canines. The majority of tumors are found near the mammary glands closest to the back legs. Signs can include:

• Bloody discharge or pus from nipple
• Multiple bumps
• Painful or swollen breasts
• Singular lumps
• Ulceration 
• Yellow discharge or pus from nipple

Systemic symptoms that might indicate cancer could include:

• Breathing difficulties
• Coughing
• Lameness
• Lethargy
• Loss of appetite
• Weakness
• Weight loss
• Enlarged lymph nodes

Types

Several types of cancer can affect the canine breast or mammary gland. These can include: 

Adenoma - A benign tumor of glandular origin. Although adenomas are considered to be a benign tumor, they can occasionally develop into malignant tumors.

Carcinoma - Carcinomas are generally malignant growths made up of epithelial cells.  Common carcinomas in canine breast cancer include adenocarcinomas and inflammatory carcinomas. 

Carcinosarcomas - These are tumors that are made up of both epithelial cells and cells from other tissues. 

Fibroadenoma - Benign, painless tumors that are a combination of skin and connective tissues. These tumors tend to be mobile within the breast tissue. 

Sarcoma - Malignant tumors made up of bone, cartilage, or fat cells. Sarcomas are an infrequent cause of breast cancer in canines.

The causes of cancer are not well understood in either humans or canines. The causes of breast cancer in dogs may have a hormonal component as spaying your female dog before their first heat nearly eliminates the possibility of developing mammary tumors. Genetics also play a factor as certain breeds seem to be predisposed to developing breast cancer. Dog breeds that may have an increased chance of developing mammary tumors include:

• Boston Terrier
• Brittany Spaniel
• Cocker Spaniel
• Dachshunds
• English Setter
• Fox Terrier
• German Shepherd
• Pointer
• Poodle

Although the vast majority of dogs that develop cancer of the breast are unspayed females over the age of 2, this is not always the case. Although it is exceedingly rare, both puppies and male dogs have been known to develop canine breast cancer. When breast cancers arise in either of these demographics, the prognosis is generally grave.

Your veterinarian will most likely start your appointment by palpating the mass or masses and collecting data regarding their size, hardness, and mobility. Information about your dog’s health history, including applicable information about your animal’s last heat cycle, current medications your pet is on, and information about pregnancies or pseudo-pregnancies will be collected. X-rays and ultrasound may be used to visualize the spread, but they may not identify any microscopic dissemination of the cancer cells. 

The veterinary oncologist will usually recommend a fine needle biopsy of any tumors, and may recommend the same procedure for the lymph nodes. General testing is done to check for any concurrent disorders, and a complete blood count, urinalysis, and blood chemistry profile will be used to evaluate the condition of the patient. This is done to ensure that the animal is healthy enough to undergo surgery and the anesthesia required. A biopsy of the tumor, after surgical removal, is usually necessary for a definitive diagnosis.

The first course of action, in most cases, is the surgical removal of the tumor itself. In some situations, just the tumor and a small area around it require removal, but in many cases, the amount of tissue excised is greater. Many veterinary doctors will recommend the removal of all of the mammary tissue, as well as the lymph nodes that they drain into. This is not as invasive a procedure for canines as it is for humans, as the underlying muscle tissue is unaffected in canines. If your female dog is not already spayed, this may be done at the same time as the excision of any mammary tissue. 

Although the role of ovariohysterectomy in reducing further cancers is controversial, it may help prevent related illnesses or infections of the uterus and ovaries and make any new tumor growth more apparent as any remaining mammary tissue shrinks after spaying. Some tumors may be harder to remove than others, and regrowth may appear, particularly with sarcoma type tumors. Surgery itself is generally effective in removing cancer, and chemotherapy and radiation therapies are not generally as effective in canines as in human patients. These treatments are generally reserved for tumors that have metastasized, are inoperable, or have a high chance of spreading. 

Inflammatory mammary carcinoma is usually treated differently than the other tumors of the breast. Neither surgery nor chemotherapy treatments are effective in treating this kind of cancer. The prognosis for dogs with inflammatory mammary carcinomas are poor, but radiation therapy and NSAIDs are helpful in relieving the associated pain.

Recovery from radical surgery is much shorter in canines than it is for humans, with the majority of healing occurring within two weeks from the surgery. It is important to provide the patient with a calm, quiet space to recuperate in when they return home. 

Other than spaying your female dog early, the best way to protect your dog from developing breast cancer in the first place is much the same as in humans. Regular tactile examinations of the 8-10 mammary glands, feeling for lumps or bumps with your fingers, are recommended monthly for any dog with risk factors. This would certainly include any unspayed females over the age of 2, as well as any females that were spayed later in life, or who’s early medical history is unknown.