Canine Mammary Cancer

October is Breast Cancer Awareness month, and we’ve all seen everyone going pink to support the cause.  Even the NFL has its players wearing pink shoes, sweatbands, and mouth guards.  But how much do you know about canine mammary tumors?  Did you know, for instance, that mammary tumors are the most common tumors in female dogs who have not been spayed?  Just knowing that one simple fact provides you with a very easy way to prevent your dog from becoming a victim of what can be a very deadly disease.

Which dogs are at risk for mammary tumors?

Yes, male dogs, just like male humans, can get breast cancer, and usually it has a worse prognosis in males than in females.  Fortunately, male dogs account for less than 1% of mammary tumors.

The incidence of breast cancer in dogs is three times higher than in humans, and higher than the incidence in any other companion animal.

In female dogs, mammary tumors are most often seen between the ages of five and ten years.  One out of four un-spayed females will develop breast tumors and half of them are usually malignant.  Malignant tumors generally take the dog within one to two years, usually after the cancer has spread to the lungs.

In dogs spayed before their first heat cycle, mammary tumors are very nearly non-existent.  Veterinary scientists postulate that the hormones produced by a dog’s ovaries sensitize the breast tissue to make it more receptive to the growth of cancer cells.  Removing the ovaries by spaying the dog before the first heat cycle prevents the release of hormones, thus leaving the breast tissue as a hostile environment for the tumor cells.

As far as breeds, Poodles, Terriers, and Cocker Spaniels have a higher incidence of breast tumors, but German Shepherds, Dobermans, and the Nordic breeds including Eskies, Buhunds, and Spitz-type dogs have a worse prognosis.

A little anatomy lesson

The mammary tissues of a dog are located all up and down the belly, best located by finding the two parallel rows of five nipples.  Breast tumors are most common in the 4^th and 5^th glands (closer to the pelvic area).

The mammary gland itself is a system of ducts that conduct milk to the nipple, surrounded by fat cells and connective tissue.  Interwoven in the soft tissue are the lymph vessels which carry away debris from the tissue to lymph glands in the chest wall.  It is these same lymph vessels that carry tumor cells from breast tumors to the lungs or to other organs in a process called metastasis.

In a healthy dog, the mammary glands should be soft and supple.  When a tumor arises, it often feels like a piece of pea gravel.  Tumor lumps start out very small, but grow quickly, sometimes doubling in size each month.  It is not uncommon for tumors to grow in several places at once, producing lumps in several of the mammaries simultaneously.

Benign vs. Malignant

Only about half of the lumps you find in your dog’s mammary tissue will be cancerous.  Benign lumps are generally softer, smoother, and slower growing.  Malignant tumors grow rapidly, have an irregular shape, and are firmly attached to the surrounding tissue, making them feel immovable under the skin.  They often break through the skin, creating an ulcer on the skin surface.

That said, it is impossible to differentiate a malignant tumor from a benign one without a biopsy.  Sooner is better than later, as malignant tumors often metastasize to the body’s other organs very quickly, making it tough to successfully treat the dog.  Your vet may stick a needle into the breast tissue and withdraw some of the tumor cells, or he may do a more invasive biopsy, involving actual surgery to remove the lump for pathological analysis.

Treatment of Mammary Cancer

Removing the lump for biopsy is not as bad as it sounds, however, since the first line of treatment of malignant mammary tumors is surgical removal.  Even benign tumors are generally removed as a matter of course.  If you have found a malignant lump early enough, surgery can be curative in about 50% of dogs.

Depending on your veterinarian’s judgment, your dog may have just the lump removed, or may have the lump, the surrounding tissue, and the associated lymph nodes removed to reduce the chance of metastases.

For example, if a tumor is found in the 4^th gland, the vet may decide to remove the 3^rd, 4^th, and 5^th glands on that side of the abdomen, as well as the inguinal lymph node into which the lymph vessels from those glands drain.  If the tumors are widespread, the vet may choose to do a radical mastectomy, removing all ten glands, as well as the skin covering them, and the lymph system draining them.

Removal of the mammary glands in dogs is less drastic than in humans.  In a dog, there is no loss of underlying muscle tissue because the mammary glands are located between the skin and the muscle wall.  Thus the surgery and subsequent recovery are much easier.

Chemotherapy and radiation therapy have produced inconsistent results.  The most common chemotherapeutic cocktail is Adriamycin and Cytoxan.  However, some oncologists prefer using mitoxantron, followed by Adriamycin or Carboplatin if the tumor doesn’t respond to the mitoxantron.  Be sure to consult a veterinary oncologist for a second opinion before you make a decision on your dog’s care.

Cancer treatments can be very expensive, and it may be in your dog’s best interest to simply provide pain relief or even euthanasia rather than treatment.  You would be well advised to get at least two opinions before you make this very painful and personal decision.

Some nutritional supplements have also been shown to be effective.  Inositol hexaphosphate and 1 -3 beta glucan can provide nutrients which may be deficient in cancer patients.  Supplements are usually recommended for the remaining life of your dog, as they may be helpful in reducing the recurrence of tumors.

Prognosis for mammary tumors

Lifespan following the discovery of a malignant mammary tumor depends on many things such as the type of tissue involved (ducts or connective tissue in the breast), the grade of the tumor (how dissimilar the tumor cells are from normal cells when viewed under a microscope), the tumor size (smaller than 3 cm is best), and the tumor stage (whether or not there are metastases and where they are located – regionally or scattered throughout the body).

To completely evaluate your dog, your vet will likely take chest X-rays to check for metastases, as well as a blood work and a urinalysis.  He will check the dog’s lymph nodes by feeling them, or possibly by doing a needle biopsy or even an open biopsy.

Prevention of mammary malignancies

The single most important thing you can do to protect your dog from breast cancer is to get her spayed before her first heat cycle.  In dogs spayed before their first heat, the risk of malignant mammary tumors is 0.05%.  If the dog is spayed after one heat cycle, her risk jumps to 8%, while if you wait until after the second heat cycle, the risk skyrockets to 26%.

Other risk factors include obesity and a high dietary intake of red meat.  Older dogs often do worse than younger dogs, but it is not clear as to whether this is cancer-related or simply due to the fact that older dogs are more likely to die from any number of reasons than are younger dogs.

Factors that have been shown not to be associated with the prognosis are the number of pregnancies your dog has had, her age at her first pregnancy, and whether or not she has had false pregnancies.

For further information

Canine Cancer Awareness

Canine Cancer

And our own series on canine cancer from the doggies.com dog den