Benign Mammary Tumors in Dogs

What is a benign mammary tumor?

A mammary tumor is an abnormal growth in the mammary gland that develops due to abnormal reproduction of cells in the breast tissue. Mammary tumors can be benign (non-cancerous) or malignant (cancerous). The two forms of the disease have different diagnostics, treatments, management, and prognosis. Approximately 50% of mammary tumors are benign, and 50% are malignant.

There are several types of mammary tumors, with carcinomas being the most common. A carcinoma is a type of cancer that arises from epithelial (skin) cells, tubules of the mammary glands, or other cells found in the mammary chain (the line of mammary glands down one side of the body). Other cancers can also affect mammary glands, including osteosarcomas.

A benign mammary tumor is non-cancerous and does not spread (metastasize) to other parts of the body. Non-cancerous tumors are not usually life-threatening.

What causes mammary tumors?

The exact cause of mammary tumors in dogs is not fully understood. It is known that exposure to hormones, specifically estrogen and/or progesterone, increases the risk of mammary cancer in dogs because of the hormones’ effects on mammary cells.

The incidence of mammary tumors is related to hormone status; specifically, whether a dog is intact or has been spayed. Evidence suggests that dogs spayed before the first heat (also called estrus) have only a 0.5% risk of developing mammary tumors. The risk increases to 8% once a dog has her first heat, and 26% after the second heat.

A dog’s age is also a risk factor. The risk of developing mammary tumors 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 pinschers.

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. Mammary tumors can occur in male dogs but are uncommon.

What are the clinical signs of benign mammary tumors?

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. The masses may be next to or within the nipple and follow along the mammary chain. The size of the mass(es) and their appearance may vary, but they are usually firm and nodular. They are not typically painful. There may be some discharge from the associated nipple. If the tumor becomes large enough, it may rupture through the skin.

How are benign mammary tumors diagnosed?

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

A common procedure to diagnose this tumor is fine needle aspiration (FNA). FNA involves taking a small needle with a syringe to suction a sample of cells directly from the tumor. These cells are then placed on a microscope slide and a veterinary pathologist 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. For that reason, your veterinarian may recommend a biopsy (surgical excision of a piece of the tumor) or removal of the entire tumor. A veterinary pathologist will examine pieces of the tumor under the microscope, a process called histopathology. Histopathology not only help make a diagnosis, it can also indicate whether the tumor is benign or malignant, and therefore how the tumor is likely to behave.

Dogs can have both benign and malignant tumors at the same time, so surgical removal and histopathology of all tumors is important. Dogs with benign mammary tumors are at an increased risk of developing malignant tumors, and it is thought that malignant tumors develop from benign tumors. If malignancy is suspected, further investigation if required before surgical removal, including chest X-rays, abdominal ultrasound, and FNA sampling of local lymph nodes.

How do benign mammary tumors typically progress?

Some benign tumors 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 the best treatment for dogs with mammary tumors. If a biopsy wasn’t done to confirm the mass is benign, then a surgical excision must be much larger than the visible mass, to try to remove all affected tissue. This tissue should be sent for histopathological analysis to determine the nature of the tumor and whether further treatment is needed. If your dog is intact at the time of surgery, an ovariohysterectomy is recommended at the same time. If your dog is intact at the time of surgery, an ovariohysterectomy is recommended at the same time.