Mammary Tumors in Dogs and Cats

TL;DR. Mammary tumors are common masses in the breast tissue of dogs and cats, requiring prompt veterinary evaluation because a high percentage of these tumors are malignant—especially in cats.

Dogs have five pairs of mammary glands running along their chest and abdomen.

What is it?

Mammary tumors, scientifically known as mammary neoplasia, are abnormal growths of tissue in the mammary glands of pets. These glands, which run in two parallel chains along the underside of the chest and abdomen, are responsible for producing milk to feed newborn puppies and kittens. Dogs typically have five pairs of mammary glands, while cats generally have four pairs.

When cells within these glands begin to divide uncontrollably, they form masses or lumps. These tumors can be either benign (non-cancerous) or malignant (cancerous). In female dogs, mammary tumors are incredibly common, accounting for up to 50% of all diagnosed tumors. Fortunately, about half of these canine mammary tumors are benign, meaning they do not spread to other parts of the body.

In cats, the situation is more concerning. Mammary tumors are the third most common type of tumor found in female cats (queens), representing approximately 17% of all feline tumors. Unlike dogs, up to 80% of feline mammary tumors are highly malignant and aggressive. In both species, the most common type of malignant mammary tumor is adenocarcinoma, which originates in the glandular tissue. While these tumors primarily affect female pets, they can occasionally occur in male dogs and cats, though this is rare.

Causes & risk factors

The development of mammary tumors is heavily driven by exposure to female reproductive hormones, specifically estrogen and progesterone. Each time an intact female pet goes through a heat cycle, the mammary tissues are subjected to hormonal surges that stimulate cell division. Over time, this repeated hormonal stimulation increases the risk of genetic mutations that lead to tumor formation.

Because hormones play such a pivotal role, the timing of a pet's spay surgery (ovariohysterectomy) is the single most important factor in determining their risk. Spaying a female dog or cat before her first heat cycle dramatically reduces her chances of developing mammary tumors later in life.

While some cancers have strong genetic links to specific breeds, our clinical records do not identify any specific breed predispositions for mammary tumors. Any intact or late-spayed female dog or cat is at risk, regardless of breed.

Signs to watch for

The primary sign of a mammary tumor is a palpable lump or mass under the skin of your pet's belly. Because these tumors can start very small—often the size of a pea—regularly feeling along your pet's mammary chains is crucial.

Keep an eye out for the following clinical signs:

• Mammary mass (cardinal): A single nodule or multiple firm lumps within the mammary tissue. These can be free-floating under the skin or firmly attached to the underlying muscle.
• Enlarged lymph nodes (common): Swelling in the lymph nodes near the armpits (axillary) or groin (inguinal), which may indicate the tumor has begun to spread.
• Pain (occasional): Discomfort or sensitivity when the belly area is touched.
• Ulceration (occasional): The skin over the tumor breaks open, bleeds, or oozes, which can lead to secondary infections.
• Inflammation (occasional): Redness, warmth, and swelling of the mammary tissue.

A small, firm nodule under the skin is the most common early sign of a mammary tumor.

Red Flag Emergency: If you notice sudden, rapid swelling of the entire mammary chain accompanied by severe redness, warmth, pain, and skin ulceration, contact your veterinarian immediately. This can be a sign of inflammatory mammary carcinoma, an extremely aggressive and painful form of cancer that requires immediate palliative care.

How vets diagnose it

If you or your vet detect a lump in your pet's mammary tissue, a series of diagnostic steps is necessary to determine exactly what the mass is and whether it has spread.

Your vet will likely start with a physical exam and may recommend a fine needle aspiration (FNA) and cytology. During this quick procedure, a small needle is inserted into the lump to collect cells, which are then examined under a microscope. While cytology can sometimes help rule out other types of skin masses, it has significant limitations for mammary tumors. As noted in a standard veterinary cytology reference:

"Additionally, individual cell pleomorphism is occasionally marked in tumors considered benign because of lack of tissue invasion... Spindled mesenchymal cells are not a definitive cytological characteristic of complex or mixed tumors because they may also be found in some simple tumors."

Because benign mammary tumors can look highly abnormal under a microscope, and because a single mass can contain both benign and malignant areas, cytology alone cannot provide a definitive diagnosis.

Therefore, histopathology is the gold standard diagnostic test. This involves surgically removing the tumor (or the entire gland) and sending it to a veterinary pathologist. The pathologist examines the tissue architecture to determine if the tumor is benign or malignant, identifies the specific tumor type (such as adenocarcinoma), and checks if the surgical margins are clear of cancer cells.

Before performing surgery, your vet will also recommend "staging" tests to see if the cancer has spread to other parts of the body:

• 3-view chest radiographs (X-rays): This is a critical step to check for metastasis (spread) to the lungs, which is a common site for mammary cancer spread.
• Abdominal ultrasound: This imaging technique allows the vet to evaluate the internal organs and regional lymph nodes in the abdomen for any signs of cancer.
• Advanced imaging (CT or MRI): For large, invasive, or multiple tumors, a CT scan or MRI may be recommended to help the surgeon plan the exact boundaries of the surgery.

Chest X-rays are essential to ensure the tumor has not spread to the lungs before starting surgery.

Treatment options

Surgery is the primary and most effective treatment for the vast majority of mammary tumors. The goal of surgery is to remove the tumor completely with clean margins to prevent local recurrence.

Surgical Therapy

The extent of the surgery depends on the species, the size of the tumor, and the number of masses present. In dogs, your vet may remove just the lump (lumpectomy), the affected gland (simple mastectomy), or multiple connected glands (regional mastectomy).

In cats, because the disease is highly aggressive and tends to spread rapidly through the lymphatic system, a more radical approach is required. As detailed in a leading veterinary surgery textbook:

"Good oncologic surgical principles still apply, however, regardless of the procedure used, invasive tumor should be widely resected with deep and centrifugal 2 cm en bloc margins of normal tissue with early ligation of blood vessels performed. In cats, complete unilateral mastectomy is the surgical procedure of choice for all mammary tumors."

This means that for cats, the entire chain of mammary glands on the affected side is removed, even if only one small lump is visible.

Medical & Chemotherapy Options

If the tumor is highly malignant, has spread to the lymph nodes, or cannot be completely removed with surgery, medical oncology treatments may be recommended:

• Mitoxantrone: This is an antineoplastic drug belonging to the anthracenedione class. It is a form of chemotherapy used to slow down or stop the division of cancer cells.
• Toceranib: This is a tyrosine kinase inhibitor, a targeted antineoplastic therapy that works by blocking signals that tell cancer cells to grow and by inhibiting the blood supply to the tumor.

Note on Inflammatory Mammary Carcinoma: Surgery is strictly contraindicated for inflammatory mammary carcinoma. This specific type of cancer causes widespread inflammation and clotting in the local blood vessels, making surgery highly dangerous and ineffective. Treatment for this condition focuses on pain management and palliative care.

Prognosis

The prognosis for pets with mammary tumors varies widely depending on whether the tumor is benign or malignant, its size, and whether it has spread. For benign tumors, surgical removal is curative, and the long-term prognosis is excellent.

However, for malignant tumors, the prognosis is generally guarded to poor. According to a leading veterinary surgery reference:

"Prognosis for both dogs and cats with malignant tumors is guarded to poor. Although length of survival is inversely correlated with the growth rate of the tumor, the extent of local infiltration, and the status of regional lymph nodes and lungs, the major statistically significant survival factor is tumor volume. Both dogs and cats with large (> 3 cm) malignant mammary tumors have significantly [shorter survival times]."

Pets with smaller tumors (less than 3 cm in diameter) that are caught early and surgically removed with clean margins have a much better outlook and longer survival times than those with larger or invasive tumors.

If the cancer has already metastasized to the lungs or regional lymph nodes at the time of diagnosis, survival times are typically shorter, and treatment focuses on maintaining a high quality of life for as long as possible. Inflammatory mammary carcinoma carries an extremely grave prognosis; it is invariably fatal, usually within a month of diagnosis.

Prevention

The most effective way to prevent mammary tumors is to have your female dog or cat spayed at an early age. Spaying before the first heat cycle virtually eliminates the risk of developing this disease. While spaying later in life still offers health benefits—such as preventing life-threatening uterine infections (pyometra)—the protective effect against mammary tumors decreases with each subsequent heat cycle.

In addition to early spaying, performing regular "belly rubs" to check your pet's mammary glands is an excellent habit. Gently palpate the tissue around each teat once a month. If you feel even a tiny, firm nodule, schedule an appointment with your veterinarian right away. Early detection and prompt surgical removal of small masses remain the most effective ways to achieve a positive outcome.

When to call your vet

You should contact your veterinarian if you notice any new lump, bump, or thickening in your pet's mammary tissue.

Seek immediate, emergency veterinary care if you notice:

• A mammary mass that is rapidly growing or changing shape.
• The skin over a mammary mass becomes red, warm, swollen, or painful.
• An open sore, ulcer, or discharge coming from the mammary area.
• Your pet is showing signs of lethargy, loss of appetite, or difficulty breathing (which can be a sign of advanced metastatic disease).

Sources

• Current Techniques in Small Animal Surgery, 5th Edition, p. 612
• Cowell and Tyler's Diagnostic Cytology and Hematology of the Dog and Cat, 5th Edition, pp. 122-123