Breast Cancer In Dogs

Awareness campaigns and the fact that hundreds of thousands of women are diagnosed with it each year have made breast cancer among the most well-known types of cancer in humans. Less known, perhaps, is that dogs can get breast cancer, too. That’s important to know, because awareness can lead to early detection, and treatment, while not a cure, can then extend a dog’s life and enhance the dog’s quality of life.

So, what to look for? One of the most visible signs of breast cancer in dogs is a lump on the underside of the dog’s belly. If your dog is having complete physical exams regularly, your vet may already be palpating the mammary glands, since mammograms are not performed for animals. However, a lump can develop from other causes, too — and it may be nothing more than a noncancerous fatty tumor. Thus, before you make any presumptions, have your veterinarian examine your dog.

It might help to know that if your beloved pet is young or was spayed early, a breast cancer diagnosis is unlikely. Dogs that get breast cancer tend to be 10 or older. Also, fewer than 1% of dogs that were spayed before their first heat (typically at 6-8 months old) develop breast cancer. Compare that with a 26% chance if she was spayed at age 2 or older. By then, a female dog’s mammary glands have more fully developed. That explains why breast cancer is more common in females, but a male dog also can get breast cancer. It’s rare, but one possibility that could put a male dog at higher risk is being on medication that contains estrogen. No particular breed is more susceptible than another.

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Types Of Mammary Tumors In Dogs

Not all mammary gland tumors in dogs are cancerous, and those that are breast cancer can vary in terms of aggressiveness and other characteristics. Tumors are typically classified based on where the cancer originated. Here are some of the different types:

  • Mammary gland adenoma: Adenomas are benign tumors that start in the epithelial tissue (cells that form linings within the body). In dogs, 50% of mammary tumors are benign.
  • Carcinoma/Adenocarcinomas: Canine mammary carcinoma — also called adenocarcinoma — is a malignant form of cancer that originates within mammary tissue.
  • Carcinosarcoma: A carcinosarcoma is a type of malignant mammary cancer that typically begins in bones or connective tissue.
  • Inflammatory carcinoma/inflammatory adenocarcinoma: With this very aggressive type of malignant cancer, a lump on the belly might also be warm or hot to the touch. The area tends to be red, and on occasion the cancer can metastasize (spread) to the skin. These tumors tend to be very fast growing.

Signs Of Breast Cancer In Dogs

Dogs have 10 mammary glands, with five on each side. Thus, the aforementioned lumps that might present on a dog’s underbelly can be anywhere from near the front legs to back by the groin area. If it is breast cancer and it has spread to the lymph nodes, the nodes in axillary region (up under the front legs) or inguinal area (groin) also may be enlarged. A dog can develop a nodule in just one mammary gland or multiple glands. In each instance, there is a 50% chance that it’s cancerous. The number of nodules is not an indicator of whether they will be benign or malignant.

In early stages of breast cancer, there may not be other clinical signs. However, as the cancer progresses, a dog might lose weight or become lethargic. This can occur even if the dog has a healthy appetite, as cancer competes for the nutrients. If the cancer has spread to the lungs, the presence of nodules might cause the dog to breathe more rapidly or more shallowly.

Workup & Diagnosis

A number of tests are used to determine whether a dog has breast cancer. A veterinarian will usually start with bloodwork (chemistry and a CBC — complete blood count) and urinalysis. As there is the possibility of spread to the lungs, taking chest X-rays is an important step. An abdominal ultrasound might also be recommended. If a mammary mass (or multiple masses or nodules) is noted on palpation of the entire mammary gland chain, further testing may be necessary to determine whether it’s truly a mammary gland tumor or something benign, such as a lipoma (fatty tumor) or cyst. With some cancers, a fine-needle aspiration is used to draw just a tissue sample, but with a mammary mass, a full biopsy is needed to get the most accurate result. Unlike with other cancers, a biopsy (or piece of tissue) is needed to determine if a mammary mass is benign or in fact malignant.

Treatment Options

The primary goal of treatment is to prolong the time before the breast cancer comes back. For benign adenomas, surgery is curative. Depending on how aggressive the malignant tumor was, cancer may not come back for many months to a couple of years if the dog has undergone treatment.


In the majority of canine breast cancer cases, surgery designed to remove the cancer is the treatment of choice. Typically with a lumpectomy in a dog, the entire gland is excised. If there are multiple nodules in different glands, then more than one is removed. It is sometimes easier to remove the entire chain of five mammary glands on the side with the malignancy, depending on the location of the cancer. The removal of lymph nodes in the underarm area and groin area is typically recommended, as well, if the entire mammary chain is being excised or if there is any concern that the original mass is aggressive. If the dog has not been spayed, spaying can be performed at this time to prevent further hormonal activity.

A benign tumor also can be removed. Otherwise, it may keep growing. These tumors don’t spread and often don’t get very large, but if they do, they can affect a dog’s quality of life.


Circumstances sometimes dictate that surgery be followed with chemotherapy. If the cancer is very aggressive or has a high mitotic rate (mitosis is division of one cell into two), if the tumor has ulcerated or if the cancer has spread, chemotherapy can help. Chemo is often intravenous but may be in the form of a pill. Dogs typically handle chemotherapy fairly well. About 15% of dogs will have an upset stomach, nausea or diarrhea within the first four days, and that could last about four days.

Other Drugs

Non-steroidal anti-inflammatory drugs (NSAIDs) or estrogen antagonist drugs also may help canine cancer patients.


Only in rare cases is radiation potentially helpful for a dog with breast cancer. There are two primary types: conventionally fractionated radiation therapy (CFRT) and Stereotactic Radiation. The former typically requires 16 to 18 treatments, while the more innovative version, Stereotactic Radiation, usually needs just one to three treatments. The PetCure Oncology team is highly qualified to provide stereotactic radiation therapy.


The prognosis for dogs with breast cancer is closely tied to the aggressiveness of the tumor and whether and how far the cancer has spread. A dog that has been treated may remain cancer-free for from six months to multiple years. However, if the cancer has spread to the lymph nodes or lungs or if it is inflammatory in nature, the dog may decline after a just a couple of months. If all treatments are declined, the dog’s quality of life and remaining time also depends largely on how aggressive that cancer it and whether and to where it has metastasized.

Find A PetCure Oncology Location Near You

PetCure Oncology provides innovative treatments for dogs with many types of cancer. We care deeply about you and your dog and have made it our mission to not only do everything we can to extend your time together but provide good quality of life. For more information about PetCure Oncology and our treatment options, find a location near you today.

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Types of Cancers in Dogs

If your dog is displaying any symptoms of cancer or has been diagnosed with cancer, sort below by cancer type or tumor location to learn more about the most common types of cancer in dogs and available treatment options. Click on the links for more specific information on treatment and real patient stories.


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