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Mammary Cancer in Dogs: Understanding the Diagnosis

Mammary tumors in dogs are one of the most common tumors found in intact females. These tumors can be benign or malignant. Approximately half are reported to be malignant, but the proportion can vary among studies and populations.

Thus, understanding the difference between a mammary mass and mammary cancer, recognizing warning signs, and looking for veterinary evaluation early may help pet parents make informed decisions along with their veterinarians.

What Are Mammary Tumors in Dogs?

Extending from the chest to the groin, your dog typically has four to five pairs of mammary glands. A mammary tumor can develop when cells in this tissue grow abnormally, forming a mass.

Benign tumors typically remain localized, while malignant tumors (cancers) can invade surrounding tissues and, in some cases, spread or metastasize to other areas of the body.

What Causes Mammary Tumors in Dogs?

There is no known specific cause of mammary cancer. However, several factors may influence risk. 

Ovarian hormone exposure is one of the well-known risk factors for this type of tumor. Female dogs that are not spayed and/or exposed to ovarian hormones for longer periods have a higher risk of developing mammary tumors. But it is important to note that the decisions about when to spay should be individualized and based on breed, age, and overall health.

Mammary tumors are not common in young dogs and are more commonly seen in middle-aged and older female dogs. However, genetics, breed, reproductive history, and other biological or environmental factors may also play a role.

Signs of Mammary Cancer in Dogs

One of the most common signs is a lump or mass on one or more mammary glands. Other signs may include one or more firm nodules around the nipples or mammary glands, a lump that increases in size,  irregular or fixed masses, redness, swelling, warmth, discomfort, skin changes over the mass, ulceration or bleeding, enlarged nearby lymph nodes, etc.

Some dogs with more localized mammary tumors may appear completely healthy, particularly during the earlier stages of the development of the tumor.

Therefore, if you observe any new mammary lump, a veterinary evaluation is very important, even when it is very small.

Why Does Early Evaluation Matter?

In the modified TNM staging system, tumor size is one factor associated with prognosis and is categorized as:

  • T1: less than 3 cm
  • T2: 3 – 5 cm
  • T3: greater than 5 cm

However, it is important to emphasize that size alone does not determine prognosis. Tumor type and grade, lymphatic/vascular invasion, lymph-node involvement, and metastasis are also important factors to be considered.

Thus, finding a mammary mass while it is still small may provide more options for surgical management and treatment.

Mammary Cancer vs. Mastitis

Not every swollen or painful mammary gland is cancer. For instance, mastitis is inflammation of the mammary gland, usually associated with bacterial infection and commonly seen during lactation. The gland may become swollen, warm, red, and painful. Some mammary cancers can also cause similar signs, such as inflammatory mammary carcinoma, an aggressive form of mammary cancer that may cause diffuse redness, swelling, warmth, and pain. Therefore, persistent mammary inflammation, particularly in a female dog that is not breastfeeding, should be evaluated as soon as possible by a veterinarian.

How Is Mammary Cancer Diagnosed?

Diagnosis begins with a physical examination by your veterinarian that will assess the number, size, location, and characteristics of the masses and may also evaluate the nearby lymph nodes.

A fine-needle aspirate (FNA) may also be performed. FNA collects cells from the mass for microscopic evaluation. However, for a precise diagnosis, histopathology is needed. It occurs after surgical removal, and a veterinary pathologist examines the tumor and can determine if it is benign or malignant, tumor type and grade, cellular characteristics, mitotic activity, lymphatic or/and vascular invasion, and the surgical margins. This information is important for understanding how the tumor is likely to behave and determining if more treatment is needed.

Staging Mammary Cancer

If mammary cancer is confirmed, your veterinarian may recommend staging to determine whether it has spread using, for example, the TNM system, which evaluates:

  • T – Tumor: size of the primary tumor
  • N – Node: regional lymph-node involvement
  • M – Metastasis: spread to distant locations

Staging can also include bloodwork, urinalysis, lymph-node assessment, chest radiographs, and additional imaging when needed, as the lungs and lymph nodes are important metastatic sites for canine mammary cancer.

Treatment of Mammary Tumors in Dogs

  • Surgery: For most dogs with localized mammary tumors, surgical removal is the primary treatment. The procedure depends on the number, size, and location of the tumors. Surgery provides local tumor control and tissue for definitive histopathology. However, not for all mammary cancer is recommended for surgery. For example, inflammatory mammary carcinoma is diffuse and has aggressive behavior, and surgery is not recommended.
  • Spaying: For females that are intact, spaying may sometimes be performed at the same time as the surgery to remove the tumor. However, the potential benefits should be considered individually based on age, reproductive status, tumor characteristics, and overall health of the female dog.
  • Chemotherapy: Female dogs that have aggressive tumors or a higher risk of metastatic disease may be referred to a veterinary oncologist, as chemotherapy may be considered. 

Life Expectancy

For some benign tumors, they can be effectively managed with surgery, and some localized, low-grade malignant tumors may have favorable outcomes after complete surgical removal. However, other mammary cancers can behave clinically aggressive and metastasize.

Once the pathology and staging results are available, your veterinarian or veterinary oncologist can provide a more meaningful prognosis.