Benign Mammary Tumors in Dogs

What is a mammary tumor?

A mammary tumor develops because of abnormal reproduction of the cells that make up the breast tissue. Mammary tumors can be benign (non-cancerous) and malignant (cancerous). The two forms of the disease have different diagnostics, treatments, management, and prognosis.

There are several different types of mammary tumors, with carcinomas being the most common. Carcinomas arise from epithelial (skin) cells, tubules of the mammary glands, or other cells found in the mammary chain.

There are other cancers that can affect mammary glands, including osteosarcomas (see handout “Osteosarcoma in Dogs” for further information on this type of cancer).

What causes mammary tumors?

The exact causes for the development of mammary tumors in dogs are not fully understood. It is well known that exposure to specific hormones, namely progesterone, increases the risk of developing mammary cancers in dogs. This is because progesterone stimulates growth factors (molecules that stimulate specific processes in the body) that cause mammary cells to multiply (i.e., replicate or reproduce).

"Dogs spayed prior to their first heat have only a 0.5% risk of developing mammary tumors."

The incidence of these tumors is related to hormone status, specifically whether a dog is intact or has had an ovariohysterectomy (has been spayed). Dogs spayed prior to their first heat (also called estrus) have only a 0.5% risk of developing mammary tumors. However, after a first or second heat, this dramatically increases to a risk of 8% and 26%, respectively. Age also appears to play a role; the risk of tumor development significantly increases once a dog reaches seven years of age and continues increasing until 11-13 years of age. This increased risk is breed-dependent, indicating that there is also a genetic component.

Certain breeds appear to be more commonly affected, including Chihuahuas, Poodles, Dachshunds, Yorkshire Terriers, Cocker Spaniels, Brittany Spaniels, English Setters, Boxer Dogs, and Doberman Pinchers. Research in veterinary medicine continues but, to date, little is known about which genes and mutations can lead to the development of mammary tumors in dogs. In human medicine, research has established a definite relationship between the BRCA gene (or breast cancer gene) and the development of breast cancer. Research is ongoing but there may be a link between mammary tumors and similar genes in dogs.

How can I tell if my dog has this type of tumor?

The most common clinical sign of benign mammary tumors is one (or more) masses that you can feel underneath the skin of your dog’s lower chest or belly. They may be next to or within the nipple and follow along the mammary chain (see illustration). The size of the mass(es) and their appearance may vary but they are usually firm and nodular. They are not typically painful.

How is this type of tumor diagnosed?

These tumors are typically found as either singular or multiple nodules along your dog’s abdomen during a physical exam. Other procedures are then performed to determine the type of tumor and whether it is benign or malignant.

A common procedure to diagnose this tumor is a fine needle aspiration (FNA). FNA involves taking a small needle with a syringe to suction a sample of cells directly from the tumor and placing them on a microscope slide. A veterinary pathologist then examines the slide under a microscope.

FNA can help distinguish between mammary and non-mammary tumors, however, it typically does not distinguish between benign and malignant tumors. Because of this, your veterinarian may recommend a biopsy (surgical excision of a piece of the tumor) or removal of the entire tumor. Pieces of the tumor are then examined by a veterinary pathologist under the microscope. This is called histopathology. Histopathology is not only helpful in making a diagnosis but can also indicate whether the tumor is benign or malignant, and therefore how the tumor is likely to behave.

Because dogs can have both benign and malignant tumors at the same time, surgical removal and histopathology of all tumors are very important. Dogs with benign mammary tumors are at an increased risk for developing malignant tumors and it is thought that malignant tumors develop from benign tumors.

"Dogs with benign mammary tumors are at an increased risk for developing malignant tumors..."

How do benign mammary tumors typically progress?

Some benign tumors may never change, remaining small and benign over time. Other benign mammary tumors may progress, become malignant, and develop new tumors in new locations in the mammary chain. Detecting and treating these tumors when they are small and before spread has occurred provides your dog with the best chance of long-term control.

What are the treatments for this type of tumor?

Surgery is by far the best treatment for dogs with mammary tumors. If biopsies are not taken prior to surgery to determine if the tumor is benign or malignant, a large surgical excision is the treatment of choice to ensure complete removal of the tumor. 

If your dog is intact at the time of surgery, an ovariohysterectomy is recommended at the same time.

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