Cat Mammary Gland Tumors

Published Oct. 20, 2021

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What Are Cat Mammary Gland Tumors?

Cat mammary gland tumors are formed by an abnormal mass of cells in the mammary (breast) glands. They can be benign (noncancerous) or malignant (cancerous).

Mammary gland tumors are most common in middle-aged and older female cats, but they can occur in younger female cats, as well as male cats.

Symptoms of Cat Mammary Gland Tumors

The most common symptom of a cat mammary gland tumor is a lump or swelling along the cat’s mammary chain, which is the line of breast tissue and nipples along your cat’s belly.

The mass may be soft or firm, widespread or clearly defined. You may also see ulceration (sores) as well as discharge from the nipples.

In cases of metastasis, where the tumor has spread, you may see other signs of illness, such as difficulty breathing when cancer is also in the lungs. Your vet may also see lymph node swelling.

In the later stages of malignant disease, cats lose weight, and a weakened immune system can lead to systemic infections. 

Causes of Cat Mammary Gland Tumors

The reason why a particular cat may get a mammary gland tumor is not entirely understood. Some cats have a genetic predisposition to develop cancer.

The risk of cancer also increases with age and exposure to carcinogens, which are cancer-causing agents like pesticides.

While we aren’t always able to fully control our cat’s environment and genetics, we can control the largest single factor that increases our cat’s risk of developing mammary tumors. That factor is exposure to the sex hormone estrogen.

Early spaying reduces your cat’s exposure to hormones such as estrogen and progesterone. Too many of these hormones can cause the abnormal growth of mammary tumors in cats.

When your veterinarian surgically removes your cat’s ovaries and uterus by spaying at an early age, it significantly decreases the risk that your cat will develop mammary cancer.

However, spaying is not a treatment for cat mammary gland tumors if cancerous disease has already spread throughout your cat’s body. This makes early spaying—before your cat’s first heat cycle—so important.

How Vets Diagnose Mammary Gland Tumors in Cats

Your veterinarian will be suspicious of this tumor based on appearance alone, but accurate diagnosis of cat mammary gland tumors requires microscopic examination of tissue.

A fine-needle aspirate (collection of fluid from the tumor) and cytology (test for abnormal cells) are noninvasive procedures performed in the vet’s office that yield quick answers.

However, these are not reliable diagnosis tools for this type of tumor, so a biopsy (tissue sample) will be required regardless of the cytology results. A pathologist (doctor who studies disease) will interpret the tissue after your veterinarian submits it to a specialized laboratory.

This biopsy report typically tells the veterinarian the following information:

  • The type of the tumor – malignant or benign

  • The origin of the tumor – what cells the tumor started from

  • The grade of the tumor –  how normal its cells appear, with a higher grade being more cancerous

  • The stage of the tumor – its size and spread

The stage of the tumor is also determined by further diagnostic testing, such as chest X-rays, abdominal ultrasound, and occasionally lymph node needle sampling. These tests investigate if the primary tumor has spread to other parts of the body. 

Treatment for Cat Mammary Gland Tumors

Almost all cat mammary gland tumors are potentially or already malignant when detected. That’s why prompt surgery to remove all affected tissue is critical in preventing the cancer from metastasizing, or spreading, to other parts of the body.

On top of that, while one mass may stay benign, another may have already become cancerous, so picking and choosing which mass to remove won’t do any good.

The veterinary surgeon may even recommend removing the whole mammary chain, along with the accompanying lymph nodes. 

Recovery and Management of Cat Mammary Gland Tumors

After surgery, chemotherapy and/or radiation therapy may be recommended. If a cure is the goal, chemotherapy following complete surgical removal is likely to be the best course, but it is important to realize that this type of tumor is rarely cured once it has become malignant.

Recurrence or metastasis (spread) is likely, and survival times are usually not longer than a year.

Spaying before the cat’s first heat cycle is the single best way to prevent the possibility of your cat developing mammary cancer.

Cat Mammary Gland Tumors FAQs

How long can cats live with mammary cancer?

Most cats with malignant (cancerous) mammary gland tumors will experience recurrence of the tumor, despite surgical removal.

High-grade (more aggressive) tumors are associated with shorter survival time, which can range from weeks to around one year. Benign and low-grade (less aggressive) tumors can be cured.

Is mammary cancer painful in cats?

Yes, it can be, as the tumors can grow large and ulcerate (make sores). Furthermore, metastasis (spread) to the lungs can make breathing difficult.

Laci Schaible, DVM, MSL, CVJ


Laci Schaible, DVM, MSL, CVJ


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