Your Guide to Mammary Cancer in Dogs

Mammary cancer in dogs isn’t something you think about when your dog is a puppy. Now that our dogs are getting older, I think we tend to sit up and pay attention when we discover new lumps or bumps.

If you’re worried about a new lump on your dog, please make an appointment and get her to the vet as soon as possible. 

If it is a tumor, it will need to be aspirated or biopsied to get an accurate diagnosis.  Dogs diagnosed with mammary cancer have a 50/50 chance of the tumor being benign or malignant.

By the time you’re finished reading this post, you should have a better understanding of mammary cancer in dogs, how a prognosis is made, and the treatment options available.


According to the Veterinary Cancer Group, mammary tumors are reported to occur in approximately 2 out of 1000 female dogs.
But first…please read my privacy policy and disclaimer. I am not a veterinarian and I never suggest taking any dog health advice unless it comes directly from the mouth of a licensed veterinarian. Also, there are affiliate links on this page. All that means is that if you click on an ad, I will get a small commission. It won’t cost you a cent though.

The Big Question:  What is the Prognosis for Mammary Cancer in Dogs?

Canine mammary cancer has a 50/50 chance of being benign or malignant.  Dogs spayed before six months of age stand the best chance against mammary cancer. 

Once a lump has been removed and analyzed, your veterinarian will be able to determine the grade and stage of the tumor. This system is useful in determining whether the tumor is malignant (spreading) and how advanced it is.  In this case, you want the lowest grade possible for the best outcome.

Grade I:     At this level, the tumor is benign (not cancerous) and has not spread to other parts of the body. This type has the greatest chance of survival, assuming that the entire tumor is successfully removed.

Grade II:   The tumor’s position is deeper and more likely to spread.   Don’t panic, because it may not have spread yet.  At grade II, the chance of the tumor being cancerous is greater.

Grade III:  This is not the grade you want your dog to get.  The tumor is likely aggressive and will require a lot more treatment including surgery, radiation therapy, and chemotherapy.  Note: The veterinarian/surgeon will determine the best treatment options.

After a grade has been established, the next step is to assign a stage.  Stages 0 through 3 have different outcomes as you’ll see below:

Staging Process to Determine How Long a Dog Can/Will Live with Mammary Cancer

Stage 0:   At this stage, no lymph nodes are involved which means the cancer has not begun to spread.

Stage I:   Stage 1 means that the tumor has not spread yet.

Stage II:  Stage II is getting a little more serious and probably involves the lymph nodes.  The veterinarian will need to determine how many lymph nodes are affected and where.  The image below shows you where to find the lymph nodes in your dog’s body.

Stage III:  Multiple tumors with or without lymph node involvement.  This is where things get serious. 

The Science Behind the Spay

Mammary cancer in dogs is more common in older dogs, and dogs who have either not been spayed, or spayed after the first heat. 

I’ve read that females should be spayed well before their first heat, and other reports suggesting that spaying should occur much closer to that 6 month mark. 

At the end of the day, you also have to factor that people are taking excellent care of their dogs these days and they are simply living longer.   So, in some cases, mammary cancer in dogs may develop simply because they have lived longer.

When a female dog is spayed, her reproductive system is completely removed, which eliminates the body’s need to produce estrogen and progesterone for the purposes of pregnancy.  High levels of these hormones are thought to increase the risk of mammary cancer in dogs.  It’s very possible that I don’t have that last statement entirely accurate, although the research I’ve done points to that.

Please remember that I am not a veterinarian.  I highly suggest taking your dog to see a licensed veterinarian if you suspect there is anything wrong.

You’ve got to watch this touching video about a dying dog with a bucket list.

FACT:  According to Fetch a Cure, approximately 6 million dogs in the US are diagnosed with cancer every year.  As stated above, our pets are living longer and that longer life exposes them to various types of cancers.  In addition, when dogs get cancer, the malignancy looks a lot like the human equivalent of the disease in that it spreads to the same organs.

Figuring it Out: A Canine Case-by-Case Study

You can’t tell by touching or palpating the lumps whether they are cancerous or not. The only way is to examine the tissue, which the veterinarian will do through a biopsy. In a biopsy, a small amount of tissue is removed and examined. 

In the best case scenario, all of the lumps are benign.  However, you could be faced with other scenarios including:

  1. Some of the lumps are benign, but some are also malignant, meaning the cancer may have spread to the lungs and lymph nodes.
  2. The lumps are all malignant and have likely spread to other parts of the body.
  3. All the lumps are benign.

The Most Important Thing is Not to Delay an Appointment with The Veterinarian

By using fine-needle aspiration, the veterinarian is able to take a small tissue sample for further evaluation. 

If there is more than one lump, all lumps must be tested because it is very possible for some to be benign (non-moving) and some to be malignant (moveable and likely to spread within the body). They are not all necessarily the same. In fact, there are different sub-types of malignant mammary cancer in dogs that will determine how aggressive the cancer actually is.

What Are The Signs and Symptoms of Mammary Cancer in Dogs?

In addition to seeing or feeling lumps in and around the breast tissue, you may also notice:

  • an unusual secretion draining from the nipple(s).
  • ulceration (in the case of large tumors)
  • inflammation
  • secondary infection which will leave your dog feeling under-the-weather

Again, the key is to be aware of what’s happening in your dog’s body and having any unusual lumps and bumps examined by a veterinarian. Even experienced veterinarians can (and have) missed the signs of cancerous tumors because they don’t always present the same way.

Everything I’ve read on the topic suggests that nobody (not even your veterinarian) can tell what a lump is just by looking at it. The only way to get an accurate diagnosis is through aspiration and/or removal of the lump.

Risk Factors for Mammary Cancer in Dogs

Female dogs not spayed are more likely to develop mammary cancer than any other dog.  The risk of a dog developing a mammary tumor is 0.5% if spayed before their first heat (approximately 6 months of age), 8% after their first heat, and 26% after their second heat.

If you suspect your dog might have mammary cancer, it’s important to understand that they are:

  • usually slow-growing
  • inflamed
  • moveable (implying a benign tumor)
  • affixed to skin or body wall

The motto used by veterinarian specialists is IF YOU SEE SOMETHING, DO SOMETHING.  That means if your dog has a mass that is the size of a pea (1 cm), and it’s been there a month, get it checked! 

Veterinarians should be aspirating more suspicious lumps rather than suggesting a wait-and-see approach.  In fact, this just happened to me. I brought my golden retriever in for a suspicious lump on her torso and was told it “looks like a fatty tumor”.  Then I was told to come back if the growth “doubled in size within a month”.

Let’s hope the veterinarian is right.

The following YouTube video shows you how to check your dog’s lymph nodes. However, please note that enlarged lymph nodes do not necessarily point to cancer and can mean any number of things.  If you suspect your dog does have enlarged lymph nodes, you should bring your dog to the vet.  The video explains it all!

Treatment is Not a Cure, But No Treatment is Deadly

Most mammary tumors are removed surgically, taking a wide margin to determine whether or not the cancer has spread. Chemotherapy is not recommended in dogs with mammary gland tumors.

The best course of action post-surgery is to maintain regular check-ups so that the veterinarian can intervene if any tumors come back. Mammary cancer in dogs is not necessarily a death sentence if the tumor is found early.

Quality of Life for Mammary Cancer in Dogs

If you get the worst-case scenario news, ask your doctor how surgery will affect your dog. You may have to decide if the post-surgery quality-of-life is worth the procedure. Some questions to ask the veterinarian might include:

  • Is my dog strong enough to undergo anesthesia?
  • What will my dog’s quality of life be after surgery?
  • What is a realistic life expectancy with the surgery?
  • What is a realistic life expectancy without the surgery?
  • Is my dog in pain?
  • How can I keep my dog as healthy as possible through this process?

Living With the Prognosis

I wish I could have given you a definitive answer to your question on how long a dog can live with mammary cancer. As you can see, the answer is a moving target that is dependent on a lot of different things.

You should know that new treatments are being studied all the time and researchers are getting closer to finding longer-term solutions for dogs with cancer. I realize that doesn’t help you much right now.

My best advice is to try and stay positive and work with your veterinarian to find the best possible treatment plan for your dog. Your beautiful dog is going to need a lot of care and love through this, and nobody is better equipped for that job than you are.

I want to wish you the best of luck as you go through this process. Please share your story with others so that everyone can benefit. Let us know what works and what doesn’t. What was your experience with the veterinarian and what challenges did you face?