Breast cancer in dogs

Breast cancer occurs most frequently in older, female pets that have not been spayed. While the cause of breast cancer is unknown, hormones are thought to play a role. Any suspicious lump in the mammary area should be examined by a veterinarian as soon as possible.

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Breast cancer is the uncontrolled growth of abnormal mammary gland (breast) cells. If left untreated, certain types of breast cancer can metastasize (spread) to other mammary glands, lymph nodes, the lungs, and other organs throughout the body.

While any pet can develop mammary tumors, these masses occur most often in older, female dogs and cats that have not been spayed. 50% of mammary masses in dogs are malignant.

The exact cause of mammary gland cancer is unknown. However, dogs that are spayed before their first heat cycle are less likely to have breast cancer, so hormones may play a role. Treatment with hormones for other conditions may increase the risk for this type of cancer.

Genetics may also play a role in canine breast cancer. Recent findings show that certain genes are overexpressed in dogs with this condition.

Signs of breast cancer include firm nodules in the tissue around the nipples, ulcerated skin, and swollen, inflamed nipples with or without discharge.

There’s no way to determine if a lump is cancerous simply by feeling it. Any lump in the mammary area has the potential to be cancerous, so it’s a good idea to check your pet regularly.

Mammary tumors tend to be firm, nodular masses that feel like BB pellets under the skin. Tumors may be located in a single mammary gland (the area around one nipple), or they may be in several mammary glands at once. The skin covering the tumor may be ulcerated or infected. Nipples may be swollen or red, and there may be discharge from the nipple itself.

Inflammatory mammary cancer is a more aggressive form of the disease which presents as painful red ulcerated masses that progress quickly over the course of few days to weeks.

Breast cancer is best diagnosed with a surgical biopsy. Blood work, radiographs (x-rays or CT), ultrasound, and sampling of the draining lymph node are usually recommended to help determine if the cancerous cells have spread to other parts of the body.

In dogs with large masses, it may be possible to obtain a fine needle aspirate of the tumor, which involves placing a needle into the mass and extracting cells for examination under the microscope. This procedure may be more difficult with smaller masses. Since a biopsy usually provides a larger tissue sample (likely to yield a more definitive diagnosis), this is the best option. Biopsies generally require some form of anesthesia or sedation, so your veterinarian may recommend a preanesthetic evaluation and/or blood work.

Early detection and surgical removal of the masses is the best treatment option. Before performing surgery, your veterinarian will most likely recommend blood work and staging with imaging of the chest and abdomen (radiographs and ultrasound or CT scan) as well as sampling of the draining lymph node(s). Chest radiographs are important to check for metastases to the lungs, and abdominal ultrasound may show enlarged lymph nodes. If staging shows no evidence of metastasis, the pet has a better prognosis. Submission of the tissue for microscopic examination will determine if the tumors have been completely removed.

If your dog still has her ovaries and uterus, your veterinarian may recommend spaying her at the time of mammary surgery. Spaying female dogs before their first heat cycle is the best way to reduce the likelihood of breast cancer.

Following surgery, your veterinarian may recommend radiation therapy or chemotherapy. Radiation therapy is designed to kill any potentially cancerous cells in a focused area. Chemotherapy involves systemic drugs that treat cancerous cells that may have travelled to other parts of the body.

The best way to reduce the likelihood of breast cancer is to have your dog spayed before her first heat cycle. Even spaying your dog by 1 year of age can help reduce breast cancer risk. Dogs that are spayed later in life will be at higher risk for breast cancer.