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Cornell Richard P. Riney Canine Health Center

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Mammary cancer

Mammary cancer in female dogs

About half of the tumors are malignant

Breast cancer is an omnipresent topic in human medicine these days, but what about our canine companions? The risk is even higher for female dogs than it is for women. Mammary tumors in female dogs account for 42% of all diagnosed tumors, with a lifetime risk of 23-34%, according to the Veterinary Society of Surgical Oncology (VSSO). Compare that to women, who have a 12.4% lifetime risk according to Susan G. Komen, an American breast cancer organization. It’s sobering.

“At a cellular level, mammary tumors in dogs and cats are very similar to human breast cancer tumors,” says Dr. Scott Coonrod, the Judy Wilpon Professor of Cancer Biology and director of the Baker Institute for Animal Health. “And they may occur in pets even more often than they do in humans.”

That’s because intact (un-spayed) female dogs have fully-developed mammary tissue. A dog spayed prior to sexual maturity (around 6 months old) is at a reduced risk of mammary cancer because the spay removes the ovaries, which produce the estrogen that is significant to the full development of the mammary glands.

According to the American College of Veterinary Surgeons, the risk of your dog getting mammary cancer is:

  • 0.5% risk for female dogs spayed before their first heat cycle
  • 8% risk for female dogs spayed after their first heat
  • 26% risk for female dogs spayed after their second heat
  • Additionally, 50% of mammary tumors are malignant

Obesity, especially at a young age, as well as breed, may also influence the risk.

Detection and diagnosis

Tumors usually are found by owners who feel a lump. Other symptoms include:

  • Swollen glands
  • Painful abdomen
  • Discharge from one or more glands
  • Ulceration of the skin on the abdomen
  • Lethargy
  • Weight loss (at later stages)

Upon examination, your veterinarian may be able to determine if the mass is just within the skin or if the mammary gland is involved. A benign tumor is usually small and firm, with a well-defined border. Malignant tumors are often fast-growing, with ragged edges, fixated to the skin or tissue. They may cause ulcers or inflammation. After it is determined that a mammary mass is present, the next step is to stage the cancer to determine the type of tumor and if it has spread.

Tumor staging is based on the tumor size, lymph node involvement and any metastasis. Stage 1 is the earliest phase (small tumor, no lymph node involvement, no metastasis), and Stage 5 is the worst phase (tumor size may vary, but lymph nodes are affected and metastasis is noted).

Diagnostic tests include:

  • Blood-work, with a complete blood cell count and chemistry panel, possibly a clotting profile
  • Urinalysis
  • Chest x-rays, abdominal ultrasounds or CT scans to look for metastases (spreading)
  • Fine needle aspiration (FNA) to see what type of cells are present
  • FNA of lymph nodes to look for signs that cancer cells have spread
  • Biopsy and histopathology to determine the type of tumor


The gold-star treatment for mammary tumors is surgery. In most cases, the surgeon will remove the tumor itself or the entire affected mammary gland. It is uncommon to remove the entire mammary chain in dogs. Surgery usually is not recommended with inflammatory or metastatic cancer.

Some surgeons may recommend spaying at the same time as surgery to remove the tumor, but it’s controversial with disagreement on tumor recurrence and survival time. The notable exception is secretory carcinoma, which depends on the hormone estrogen. For dogs with these tumors, spaying is recommended to prevent recurrence or continued growth. (Note: If you want your dog spayed, discuss it with your veterinarian, as it may be better to do it all at once.)

Your veterinarian may refer you to an oncologist to determine the best treatment for your dog. The VSSO states that hormonal therapy is controversial and may not be effective due to a lack of steroid receptors in malignant mammary tumors.

With inflammatory mammary carcinoma, surgery doesn’t improve the dog’s survival rate, so instead of putting the dog through surgery, the recommended treatment is radiation therapy in conjunction with nonsteroidal anti-inflammatory medication.

Chemotherapy is often reserved for cases of advanced disease.

After surgery

For malignant tumors, the best prognosis is if it is caught early while the tumor is small, and then it is removed completely. Larger tumors, lymph node involvement, and existing spread to other parts of the body make for a poorer prognosis. Some tumor types, such as ductal carcinomas and sarcomas, have a poor prognosis. The tumor specimen is sent for a histopathology examination to see if the surgeon achieved good margins, which assesses the ability remove the entire tumor.

It will remain important to watch for tumor recurrence or spread with malignant tumors, so the recommendation is to check for metastasis with chest x-rays or abdominal ultrasounds every 3-6 months, to examine the surgery site and to check local lymph nodes.


The prognosis for your dog’s recovery depends upon many factors and is unique to each dog. Not surprisingly, the size of the tumor when it’s found is important. The larger the tumor, the poorer the prognosis. Other factors include any ulceration, the histologic grade, lymph node involvement and risk of metastatic disease.

According to the blog Canine Cancer Awareness, “Half of all tumors are malignant and unfortunately, 50-75% of them will kill the dog by recurrence or spreading (metastasizing) to the lungs within 1-2 years.”

Your decision on treatment depends upon your veterinarian’s advice, cancer stage and your choice on what’s best for your dog. The most important thing is to ensure your dog does not suffer pain unnecessarily.

Mammary cancer facts

According to VSSO:

  • Mammary tumors account for 42% of all tumors in female dogs and 82% of reproductive tumors 
    • 41-53% of these are malignant
  • The lifetime risk of a female intact dog developing a malignant mammary tumor is 23-34%
  • Common metastatic sites include: regional lymph nodes, lungs, adrenal gland, kidney, heart, liver, bone, brain and skin
  • The mean age for development of a mammary tumor is 6-10 years 
    • They rarely occur in dogs younger than age 4
  • Mammary tumors in male dogs are usually malignant
  • Multiple mammary tumors are common

This article has been reprinted with permission from the Cornell University College of Veterinary Medicine’s DogWatch newsletter, published by Belvoir Media Group. When you become a member of the Riney Canine Health Center, you will receive a free subscription to DogWatch.