A mammary gland tumor is any abnormal growth involving the mammary (breast) tissue. A mammary gland tumor can be benign or malignant. Mammary gland tumors occur in one of every four unspayed female dogs. If a mammary gland tumor develops in a female dog, there is about a 50-55% that the tumor will be benign. If, however, the tumor is cancerous, it tends to spread rapidly and widely to other organs in the body such as the lung and lymph nodes. Early detection is therefore critical with mammary gland tumors. Once your dog has had a mammary gland tumor, she is approximately three times more likely to develop a second tumor.

Although most mammary gland tumors occur in female dogs at least six years old, male dogs have, on occasion, developed mammary gland tumors. Though rare, mammary gland tumors in male dogs are generally very aggressive.

Some common benign mammary tumors include duct papillomas and adenomas. Some common types of malignant or cancerous tumors include tubular adenocarcinomas, papillary adenocarcinomas, papillary cystic adenocarcinomas, solid carcinomas, anaplastic carcinomas, osteosarcomas, fibrosarcomas, and malignant mixed tumors.

The number of mammary glands vary from dog to dog, but a dog typically has ten glands. The number of glands correspond to the number of nipples on your dog, and the largest glands are located underneath the nipples closest to your dog’s groin. Tumors can occur in any gland, but often appear in the largest glands closest to the groin. The tumor will generally be painless for your dog, and it will look like a lump or bump or series of lumps and bumps. The lumps or bumps can be small or large, and can either be movable or seem fixed under the skin.

It is not known what causes mammary gland tumors. However, it is thought that the development of these tumors is related to certain hormones such as estrogen and progesterone.

Tumors are diagnosed by doing a biopsy of the tumor tissue in order to analyze the tissue’s cells and cell structure. Some veterinarians will do a needle aspirate where a syringe will be used to remove cells from the tumor for analysis. However, for a definitive analysis a biopsy is needed.

Since malignant tumors spread rapidly, your veterinarian might also take x-rays of your dog’s chest and inspect your dog’s lymph nodes to see if the tumor has spread. Ultrasound may be used to evaluate the abdomens internal organs for possible spread of the cancer.

For both benign and malignant tumors, your veterinarian will usually remove the tumor and tissue immediately surrounding the tissue. This is known as a lumpectomy. In some cases the veterinarian will perform a radical mastectomy which means your dog’s breasts (mammary glands), the skin over the breasts, and her lymph nodes will be removed. A radical mastectomy is a more major surgery than the lumpectomy with the suture removal and full recovery from the operation taking about two weeks. As of yet, chemotherapy, radiation, and drug therapies have not proved to be successful in treating malignant mammary gland tumors.

Removing the tumor, whether through a lumpectomy or mastectomy will, generally cure benign tumors. For malignant tumors, surgery has been found to eradicate the cancer in just over 50% of cases. Factors that impact your dog’s prognosis include how quickly the tumor has been diagnosed and removed, whether the tumor has spread to the lymph nodes, how deep the tumor has grown into your dog’s tissue, and how large the tumor is – tumors with a diameter larger than 1.5 inches have a less positive prognosis.

Once a dog has had a mammary gland tumor, either benign or cancerous, she is likely to develop more tumors in the future.

Veterinary oncologists are now available to consult, diagnose and treat all forms of cancer including mammary cancer.

Spaying your dog before her first heat is the best prevention against mammary gland tumors.

No, you cannot contract a mammary gland tumor from your dog.