Canine Breast Cancer
Canine breast cancer is common among un-spayed middle-aged to older female dogs. The main sign is a lump or mass (usually painless). You can usually detect the lump by gently palpating the mammary glands. This page looks at the symptoms, causes, and conventional treatment of breast cancer in dogs.

There are ten mammary glands in a female dog, five on each side, beginning on the chest and extending all the way down to the groin. The largest glands are those located near the groin.

Tumors of mammary glands are the most common type of tumors in dogs, especially in unspayed females over 6 years of age (the average age is 10).

If a dog has one tumor, she is three times more likely to develop a second one. Multiple tumors are common.

About 55% of canine mammary tumors are benign, and 45% are malignant.

Examples of benign mammary tumors include simple and complex adenomas, fibroadenomas, duct papillomas. There are also “benign mixed mammary tumors”, which are formed by a mixture of different cell types.

Examples of malignant breast tumors include adenocarcinomas, carcinomas, osteosarcomas, fibrosarcomas, and malignant mixed tumors.

Breed-wise, it appears that canine breast cancer occurs more commonly in sporting breeds, Dachshunds, Poodles, and Boston Terriers.

In addition, German Shepherds, Doberman Pinschers, and Nordic breeds seem to do poorly once having developed breast cancer.

Causes of Canine Breast Cancer

Most vets believe that the presence of female hormones (estrogen or progesterone) promotes the growth of mammary tumors. The hormonal cycling of a female dog can also stimulate growth of benign mammary tumors. (This has important implications in that spaying is essential even if a tumor has already developed.)

Symptoms of Canine Breast Cancer

The main sign of breast cancer in dogs is a lump or mass (usually painless). You can usually detect the lump easily by gently palpating the dog’s mammary glands.

The larger glands (the 4th and 5th glands which are closest to the groin) are more prone to develop such lumps.

Although not definitive, there are certain features that distinguish a benign and malignant mammary tumor.

Generally speaking, benign tumors are small, smooth, distinct, and slow growing. Usually they are moveable under the skin.

On the other hand, malignant tumors are fast growing and have indefinite boundaries (irregular shape). They are usually not movable because they are adhered either to the overlying skin or underlying muscle. Sometimes, a malignant tumor causes skin ulceration and bleeding.

Malignant tumors tend to spread widely, primarily to the pelvic lymph nodes and lungs.

Diagnosis of Canine Breast Cancer

A biopsy is the only way to get a definitive diagnosis. The vet usually will remove a part of the tumor or the whole tumor to determine if it is benign or malignant.

Sometimes, the vet will perform a needle aspirate. He uses a syringe to withdraw some cells from the tumor for examination under the microscope.

In addition, a chest x-ray is necessary to rule out lung metastases, which are present in about 30% of canine breast cancer.

To determine if the cancer has spread to the pelvic lymph nodes, he will use ultrasonography.

Conventional Treatment of Canine Breast Cancer

For both benign and malignant mammary tumors, the best treatment is by surgical removal with adequate margins of normal tissue.

How much tissue to remove depends on the type, size, and the location of the tumor.

A lumpectomy is the removal of a tumor (usually a small one) with a rim of normal tissue.

A simple mastectomy involves removing the entire mammary gland.

In more serious cases, a complete unilateral mastectomy is necessary. It involves removing all five mammary glands on one side of the body, usually including the lymph nodes.

In addition, the dog may need post-surgical radiation to help reduce local recurrence.

Use of chemotherapy is largely for minimizing the spread of cancerous cells to the lungs.

Prognosis depends on the size and type of the tumor. Benign tumors are usually curable.

Small malignant tumors (less than one inch across) also have a favorable prognosis.

Large, aggressive tumors that are likely to metastasize. Deeper tumors or tumors that adhere to deeper tissue structures, ulcerated tumors, and mammary cancer that has spread to the lymph nodes all have a poor prognosis.

Prevention of Canine Breast Cancer

Canine breast cancer is easily preventable by spaying the female dog. Take a look at the figures below. You can see how early spaying dramatically decreases the risk of breast cancer in dogs:

  • If a puppy is spayed before her first heat cycle (around 6 months of age), her chance of getting breast cancer is almost non-existence (only about 0.05%).
  • Spayed after one heat cycle, the puppy’s risk of getting breast cancer rises to 7-8%.
  • If the puppy is spayed after her second heat, the risk shoots up to 26%.
  • One out of four unspayed dogs over 4 years of age tend to develop one or more breast tumors in their lives, and about half of such tumors are malignant. Additionally, about half to 75% of these malignant tumors will kill the dog by recurrence or metastasizing to the lungs within one to two years.

If, for whatever reason, you choose not to spay your dog, be sure to examine her mammary glands every month, starting at 4 years of age. If you feel a lump or swelling, take her to the vet immediately.

Do not take a wait-and-see attitude because you may lose the previous opportunity to cure your dog’s disease.

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