Malignant Mammary Tumors in Dogs

What is a mammary tumor?

A mammary tumor is an abnormal growth in the mammary gland that develops because of abnormal replication of cells that make up the breast tissue. Mammary tumors can be benign (non-cancerous) or malignant (cancerous). These two forms of the disease have different diagnostics, treatments, management, and prognosis.

In dogs, approximately 50% of mammary tumors are benign and 50% are malignant, although malignant tumors may be more common in older dogs. Malignant mammary tumors can spread to other parts of the body, so they may become life-threatening if they are not detected and treated early.

There are several types of malignant 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, such as osteosarcomas.

What causes mammary tumors?

The exact cause of mammary tumors in dogs is not fully understood. It is known that exposure to hormones, specifically progesterone, increases the risk of mammary cancers in dogs because progesterone stimulates growth factors (molecules that stimulate specific processes in the body) that cause mammary cells to multiply. 

The incidence of mammary tumors is related to hormone status; specifically, whether a dog is intact or has been spayed. It is the most common tumor type in older, unspayed female dogs. Evidence indicates:

• Dogs spayed before the first heat (also called estrus) have only a 0.5% risk of developing mammary tumors.
• Dogs spayed before the second heat have an 8% risk of developing mammary tumors.
• Dogs spayed before the third heat have a 26% risk of developing mammary tumors.

A dog’s age and breed also play a role in tumor development. The risk of developing mammary tumors significantly increases once a dog reaches seven years of age and continues increasing until 11 to 13 years of age. Certain breeds appear to be more commonly affected, including chihuahuas, poodles, dachshunds, Yorkshire terriers, Cocker spaniels, Brittany spaniels, English setters, boxers, 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 (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. Malignant mammary tumors can occur in male dogs but are rare.

Obesity early in life may play a role in tumor development, and exposure to outside sources of hormones (estrogen and progesterone) can also be associated with increased risk. Exposure can occur due to inadvertent contact with human hormone replacement products.

What are the signs that my dog has a malignant mammary tumor?

The most common sign of a malignant mammary tumor is one (or more) masses that you can feel underneath the skin of your dog’s abdomen. 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. Occasionally, the skin over the mass may ulcerate (open) and bleed, and the affected area may feel warm to the touch and become painful. The mammary gland may also produce a discharge.

If the tumor has metastasized (spread to other areas of the body), other signs may become evident. Your dog may generally feel unwell, eat less, become lethargic, and lose weight. If the lungs are involved, your dog may have difficulty breathing or develop a cough.

How are malignant mammary tumors 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 using 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 helps make a diagnosis, it can also indicate whether the tumor is benign or malignant, and how the tumor is likely to behave.

Given the risk of metastasis with malignant mammary tumors, your veterinarian may also recommend staging (searching for potential spread to other locations in the body). Staging may include:

• Blood work and urine testing
• Radiographs (X-rays) of the lungs
• Abdominal ultrasound
• Sampling of lymph nodes associated with the mammary glands by FNA

How do malignant mammary tumors typically progress?

The way malignant mammary tumors progress depends on the type and size of the tumors and whether metastasis has occurred. Large tumors (greater than 3 cm) and those with evidence of spreading have a poor prognosis. Detecting and treating these tumors when they are small, and before spread has occurred, provides your dog with the best chance for long-term control. Tumors smaller than 1 cm have the best prognosis.

What are the treatments for this type of tumor?

For dogs with solitary mammary tumors, surgery is the best treatment. If there is only one small mass, and staging shows no evidence of spread, surgery may be the only treatment your dog receives.

For dogs with multiple tumors of one or both mammary chains, the associated mammary glands, or all the mammary glands, may need to be removed. If your dog is intact, an ovariohysterectomy is recommended, which may be done at the same time as mammary gland removal.

For dogs with larger tumors or evidence of spreading to other areas of the body, chemotherapy is typically recommended. There is increasing evidence that radiation therapy, in addition to chemotherapy, may benefit some dogs with inflammatory carcinomas. Your veterinarian will discuss the options that are best suited for your dog’s particular situation.