Mammary Tumors in Cats

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.

Mammary tumors are the third most common type of tumor in cats and most are malignant (at least 80%). There are several types of malignant mammary tumors, with carcinomas being the most common. Other cancers can also affect the mammary glands, such as osteosarcomas.

What causes mammary tumors?

The exact causes of mammary tumors in cats are not fully understood. It is known that exposure to hormones, specifically estrogen and progesterone, increases the risk of mammary cancers in cats. It is hypothesized that, under the influence of hormones, the cells of the mammary gland enlarge, progress to a pre-cancerous state, and continue to grow until they transform into a carcinoma.

The incidence of mammary tumors is related to hormone status; specifically, whether a cat is intact or has been spayed. Intact cats have a seven-fold increased risk for mammary tumors compared to spayed cats. Early spaying helps to prevent the development of these tumors.

• Cats spayed before six months of age have only a 9% risk of developing mammary tumors.
• Cats spayed between 7 and 12 months of age have a 14% risk of developing mammary tumors.

A cat’s age and breed also play a role in tumor development. Mammary cancers are seen mainly in middle-aged to older cats (around 10 to 12 years of age). Siamese and Persian cats appear to be predisposed and are typically younger when diagnosed. Obesity may play a role in tumor development as well, though this has not been proven in cats. Male cats rarely develop mammary tumors.

Exposure to outside sources of hormones (progesterone) can be associated with increased risk. Exposure can occur due to inadvertent contact with human hormone replacement products.

What are the signs that my cat has a mammary tumor?

The most common sign of a mammary tumor is one (or more) masses (nodules) that you can feel underneath the skin of your cat’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. Your cat may lick and groom the area excessively, and a strong odor can result as the exposed tissue becomes infected.

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

How are mammary tumors diagnosed?

These tumors are typically found as either singular or multiple nodules along your cat’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). This 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 mammary tumors typically progress?

The way mammary tumors progress depends on the type and size of the tumors and whether metastasis has occurred. Large tumors (greater than 2 cm) and those with evidence of spreading have a poor prognosis. If the histopathology report shows tumor cells invading the local blood vessels, this also carries a poor prognosis. Detecting and treating these tumors when they are small, and before spread has occurred, provides your cat with the best chance for long-term control. Tumors smaller than 2 cm have the best prognosis.

What are the treatments for this type of tumor?

Surgery is the best treatment for cats with mammary tumors. Depending on whether there is a single mass present or several masses, a single chain mastectomy (removal of one side of the breast tissue) or double chain mastectomy (removal of both sides) will be performed. Depending on the tumor size and other factors, chemotherapy may be pursued afterward, especially given the high metastatic rate in these tumors. Your veterinarian will discuss the options that are best suited for your cat’s particular situation.