Canine Mammary Cancer Explained

Mammary tumors—breast tumors—are the most common tumors in female dogs. Mammary tumors are three times more common in dogs than in humans. About one percent of canine mammary tumors occur in male dogs, which is about the same percentage as in people.

The basics of canine breasts and canine breast cancers

Most dogs have 10 mammary glands, five on each side. These glands are made of several different kinds of cells, and there are several types of canine mammary tumors, not all of them cancerous. 

The ratio of benign canine mammary tumors to malignant (cancerous) mammary tumors in dogs is about 50-50. The most common benign canine mammary tumors are:

  • Adenomas, which arise in the linings of the milk ducts,
  • Cystadenomas, which originate in the tissues that produce milk, and
  • Mixed mammary tumors, which start in both kinds of tissues.

The most common malignant mammary tumors in dogs are:

  • Carcinomas,
  • Adenocarcinomas, which are cancerous versions of adenomas, and
  • Cystadenocarcinomas, which are cancerous versions of cystadenomas.

Dogs can also develop a particularly aggressive form of canine breast cancer called inflammatory mammary carcinoma, also known as IMC. It is known for rapid spread and difficult treatment.

Mammary cancer in dogs can metastasize to the lymph nodes, lungs, liver, and bones.

Predisposing factors in canine breast cancer

Some breeds have a greater risk of canine breast cancer than others. The condition is more common in Brittany Spaniels, Cocker Spaniels, Dachshunds, English Springer Spaniels, German Shepherds, Maltese Pointers, Poodles, and Yorkshire Terriers. Breast tumors and breast cancers are less common in Boxers and Collies.

But a bigger factor in the development of canine breast cancer in dogs is whether she has been spayed and when. A female that has been spayed before her first heat has only a one-half of one percent chance of developing mammary cancer. The risk jumps to 8 percent if the dog is spayed after the first heat before the second, and 26 percent after the second heat cycle. Dogs that are spayed after the age of nine months are seven times more likely to develop breast cancer than dogs spayed before they are six months old.

The longer a dog is reproductively capable, the more its breast tissues are exposed to the estrogen and progesterone that can encourage the growth of breast cancers. However, spaying even at the age of two years still reduces the risk of canine breast cancer.

What are the signs of breast cancer in dogs?

Most dogs don't seem sick during the early stages of breast cancer. Most canine breast cancers are discovered during a routine trip to the vet.

Breast tumors near the nipple are firm, while breast tumors in the soft tissue of the breast tend to be squishy. Because a dog's blood tests will come back with normal values during the early stages of cancer, direct examination of each breast is the primary way canine mammary cancer is detected.

Tumors are most likely to form in the fourth or fifth breasts in either chain, closer to the tail than to the head. In the early stages of breast cancer, tumors feel superficial and unattached to any other tissue. But malignant tumors feel attached to underlying tissues. Once a tumor has become malignant, it may increase in size very rapidly.

Other signs of malignancy include poorly defined borders, pain, swelling, and ulceration. However, these symptoms can also occur with benign tumors.

Canine breast cancers can also spread to the dog's lymphatic system. Lymph nodes become enlarged when lymph can no longer pass through them. The most common places canine breast cancer will spread to the lymphatic system are the inguinal (inner thigh) and axillary (armpit) nodes.

Inflammatory mammary carcinoma causes redness, swelling, and ulceration of the breast. They make the breast painful and warm to the touch. The advanced stages of any form of breast cancer can cause your dog to lose interest in play, lose interest in food, and lose weight.

What you can expect when you take your dog to the vet

When you take your dog to get checked by the vet, the first thing your vet will do is to look for an explanation of symptoms that aren't cancer. The vet will use a fine needle to take a sample of fluid from the breast to look for markers of infection. This process is called aspiration.

Aspiration alone, however, can't rule out cancer. That requires biopsy, surgical removal of tissue to be stained and viewed under a microscope to look for cancerous cells. If the pathology report shows cancer, then surgery is usually required. It will probably be a lumpectomy, removal of just the cancerous tumor if your dog has been spayed, or a mastectomy, removal of the entire breast and some surrounding tissue, if she hasn't.

About 50 percent of cancerous tumors and 100 percent of benign tumors can be cured with surgery. Veterinarians use some of the same chemotherapy drugs, especially doxorubicin (Adriamycin) and 5-fluorouracil (5-FU) for dogs that doctors use to treat cancer in people, but most dogs don't get chemotherapy for canine breast cancer.

Diet for canine mammary cancer is a complex subject

Diet plays an important role in preventing cancer.

Obese dogs have a greater risk of canine breast cancer than dogs of normal weight. Extra weight put on during adolescence, at the age of nine to twelve months, is correlated to higher probabilities of breast cancer. Dogs that eat a homemade diet are more likely to develop breast cancer, as are dogs that eat more red meat and less poultry and fish.

Why should this be? Red meat is high in a compound called arachidonic acid, which a dog's body uses to increase inflammation. Inflammation is a tool cancer can use to build its own blood supply and eventually to spread. Fish, especially wild-caught Alaskan salmon and wild-caught Alaskan salmon oil, provide eicosapentaenoic acid which stops this process. Feeding human-grade food for dogs ensures that your dog is getting maximum nutrition without any carbohydrate fillers that digest into sugar and without any mystery byproducts.

What about premium supplements for dogs with cancer? It's tempting to give your dog every herb and supplement that could possibly fight cancer in people, because you care. However, you shouldn’t. Rely on veterinarian-formulated supplements like Canine Biologics to give your dog the best fighting chance for remission and recovery. Give your dog good veterinary care, Canine Biologics Integrated Nutrition System, and lots of love.