Gazette Vet: Mast Cell Tumors in Dogs


By John Andersen, DVM

“Casey” is an 11-year-old Golden Retriever who came to me for an annual wellness exam.  During the physical, his owners asked me to check out a few lumps they had noticed growing recently. All told, Casey had about 15 lumps and bumps affecting his skin, but fortunately they all seemed perfectly benign. I recorded them in the chart and told the owners to call if any of them seemed to be changing.

“Bailey” is a 6-year-old Boxer mix who came in right after Casey for the same thing–an annual wellness visit and “Hey, can you check out this bump?” Bailey’s bump was a little different than the ones Casey had. It was slightly pink, raised, and hairless, and had been there for a couple of months. I told Bailey’s owner that I could not write this one off as benign by the looks of it and suggested we do a quick “fine needle aspirate” to get an idea if it’s a problem or not. While Bailey held still like an angel, I stuck a small gauge needle into the mass and pulled back on the syringe a few times. Then, over a microscope slide, I sprayed out the small amount of cells and fluid that I was able to aspirate. We stained the slide and put it under the microscope. It took all of five seconds to see what I suspected–a mast cell tumor. I headed back to the room needing to break the news.

I often get asked to check lumps and bumps that people find on their dogs. Just like us, as our pets age, it’s not uncommon to find a variety of skin lesions that pop up over time. Fortunately, most of the skin masses that dogs acquire as they age are benign–skin tags, adenomas, cysts, and moles. These tend to be small growths on the surface of the skin that have a very slow rate of growth and usually are not bothering the dog and never will.

How can we tell if the lump on your dog is benign or something more concerning?

Experience plays a big role. If I walked into a dermatologist’s office and asked him to look me over, I hope he wouldn’t recommend a surgical biopsy for every little freckle or mole he sees. At the same time, I sure as heck don’t want him to miss anything, especially if something could be done now to keep me alive longer!

With dogs, there are a lot of skin masses that I can tell are benign just by my naked eye.  However, we do see malignant skin tumors on a very regular basis, so it is good to keep a cautious approach if you have noticed a new lump on your dog.

“Malignant” tumors are tumors that can spread to other parts of the body or that invade and destroy local tissues. When people refer to “cancer,” they are typically referring to a malignant type of tumor.

The number one (by far) malignant skin tumor we see in dogs is a mast cell tumor. Mast cells are a certain type of white blood cell associated with our immune system. They contain compounds such as histamine, and are usually associated with allergic response. When you get a mosquito bite, the reason it turns into a little hive and itches is because mast cells have released histamine and other compounds to create redness, swelling, and itching.

Though rare in people, mast cell tumors are very common in dogs, and are a type of tumor that occurs in younger animals as well as old. It is not uncommon to find three- and four- year-old dogs with mast cell tumors.

Mast cell tumors are also among the most perplexing skin cancers because while some dogs die quickly from mast cell cancer in their body, other dogs will have had a mast cell tumor on their skin for years without incident. There are not many other cancers that have such a wide range of behavior. Because mast cell tumors can spread, we always recommend prompt removal.

Fortunately, we cure 95 percent of the mast cell tumors we see. By diagnosing them and removing them while they are new and small, we almost always will cure them.

Then there are dogs like Godiva–my boss’ dog. Godiva had a very subtle mast cell tumor pop up on her chest. We removed it promptly, and while it never recurred in that same spot, a few weeks later she came in with her entire chest and armpit area swollen and bruised. A few tests showed that this, sadly, was mast cell cancer that had spread to a local lymph node and was aggressively spreading in that area, causing swelling and bruising along the way.  Godiva got the best care, yet lost the battle to a very aggressive mast cell tumor right before our eyes.

Although mast cell tumors are easily the most common skin cancer in dogs, we do see other types, all of which behave in slightly different ways. Surgery is definitely the first line of treatment for all skin tumors and fortunately is very successful–“a chance to cut is a chance to cure.”

So if your older (or younger) dog has a new lump or bump pop up, it’s best to get it checked out. And although there’s nothing wrong with being “old and lumpy,” if we can extend the quality and quantity of your pet’s life, it is definitely worth it.


  1. My little Shia poo had a round pink mast cell tumor, pop up on her bottom two years ago.
    I bathed her today, and felt nothing. After being gone most if the afternoon, u returned home to pick her up and felt a hard bump on her back.
    I immediately thought it was a tick bite, as I had removed one earlier before her bath.( bad year in Florida with ticks!)
    I looked closer and freaked out a bit, because I saw a round reddish nodule, more than a bite.
    Is it possible a tumor popped up that quickly? It’s Sat. Night, but we will be in the vet first thing Monday morning. Any input is appreciated. Thank u

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