Histiocytoma Vs. Mast Cell Tumor in Dogs

Histiocytoma vs. mast cell tumor—what’s the difference between these two similar-looking skin tumors in dogs? Integrative veterinarian Dr. Julie Buzby compares and contrasts the appearance, location, treatment, and prognosis for histiocytomas and mast cell tumors. Discover why it is critical to distinguish between the two—for the sake of your dog’s health.

Lumps and bumps can pop up on our dogs and cats at any time. And they run the gamut from benign skin tags to serious cancers. But when it comes to skin masses that are very common in dogs and look similar, like cutaneous histiocytomas and mast cell tumors, how do you know which bump is which? And does it matter?

As it turns out, it is important to know if your dog has a histiocytoma vs. mast cell tumor. The treatment and outlook are very different for these two look-alike tumors.

What is a histiocytoma?

Histiocytomas are a common type of skin tumor in dogs, and they’re almost always benign. As the name would imply, they occur due to abnormal multiplication of histiocytes (i.e. histiocytic cells). These cells are a precursor to macrophages, Langerhans cells, and other cells that play an important role in the immune system. Typically, the cells that form the histiocytoma are Langerhans cells that migrate from the middle skin layer to the surface of the skin and make a mass. However, no one knows why cutaneous histiocytomas in dogs form.

What is a mast cell tumor?

Mast cell tumors are the most common type of skin cancer in dogs. They arise from the uncontrolled replication of abnormal mast cells. Mast cells are also an important part of the immune system. They contain granules filled with substances such as histamine, growth factors, and heparin. When mast cells are part of a tumor, they may release these chemicals in an uncontrolled manner. This can lead to blood-clotting problems, GI ulcers, and anaphylaxis (a severe type of allergic reaction).

Histiocytoma vs. mast cell tumor: Appearance

Cutaneous histiocytomas tend to be round, firm, raised skin masses that can have some or no hair on them. They can be pink or red, and have distinct borders. Typically, histiocytomas are no larger than two to four centimeters wide. Since they sort of look like a pink button on your dog’s skin, histiocytomas have earned the name “button tumors.”

The picture below illustrates the button-shaped appearance of a histiocytoma.

On the other hand, mast cell tumors can be subcutaneous (i.e. under the skin) or cutaneous (i.e. on the skin). This is unlike histiocytomas, which only occur on the skin, not under it. Mast cells tumors on the skin have many of the same characteristics as histiocytomas. They tend to be pink or red hairless tumors with distinct borders.

However, mast cell tumors in dogs can also be other colors like white or tan. In fact, they can look like almost any other kind of skin bump, including lipomas in dogs. This is part of what makes mast cell tumors so dangerous.

Where do histiocytomas tend to occur?

Histiocytomas can occur anywhere on the dog’s body. But one study demonstrated that 29% of histiocytomas occurred on the head, and almost half of those were on the ears. The most common locations for dog histiocytomas to form are:

• On the muzzle, lips, or bridge of the nose
• On a dog’s paw or leg
• On the ear flaps (i.e. pinnae)
• On the chest
• Anywhere on the neck
• Around the eyes and on the eyelids

Where do mast cell tumors arise most often?

Mast cell tumors can also occur anywhere on the body. Unlike histiocytomas, there is no tendency to form in any particular location. They can be on the dog’s head, neck, chest, trunk, belly, legs, or tail.

Are there breed or age predispositions for each tumor type?

While any dog of any age could technically develop a histiocytoma or mast cell tumor, certain dogs do tend to be prone to each tumor type.

Histiocytoma

Overall, histiocytomas tend to occur in young dogs (less than three years of age). And they are the most common skin tumor of dogs under a year of age. However, middle-aged and older dogs do sometimes develop histiocytomas too.

Histiocytomas are more likely to form in purebred dogs such as Scottish Terriers, Bulldogs, Great Danes, and Cocker Spaniels. Flat-coated Retrievers are four times more likely to form histiocytomas when compared to other breeds. And the Chinese Shar Pei is the most likely breed to develop multiple histiocytomas.

Mast cell tumor

In contrast to histiocytomas, middle-aged or older dogs are more commonly affected by mast cell tumors. But younger dogs are not immune. The following dog breeds have a higher risk for developing mast cell tumors:

• Beagles
• Boston Terriers
• Boxers
• Chinese Shar Peis
• Cocker Spaniels
• English Bulldogs
• Golden Retrievers
• Labrador Retrievers
• Weimaraners

Histiocytoma vs. mast cell tumor: How does each behave in the body?

These two tumor types may act very differently in the body. This is one of the reasons it is so important to know which tumor your dog is dealing with.

Histiocytomas—usually benign but can be cancerous in rare cases

Thankfully, almost all histiocytomas are benign and are unlikely to spread to the internal organs. Histiocytomas usually grow rapidly for the first few weeks but then resolve on their own after about one to three months. They generally aren’t itchy or painful, although histiocytomas that develop on the eyelid may cause slight discomfort.

Typically, the dog will develop one histiocytoma at a time. But sometimes dogs can develop cutaneous Langerhans cell histiocytosis (LCH). In this condition, the dog may have multiple histiocytomas at once and the skin is ulcerated and irritated. LCH may also impact the mouth and the junction between haired skin and mucous membranes, which is different from typical histiocytomas.

In very rare circumstances, a malignant type of histiocytoma can form. Also known as systemic histiocytosis or histiocytic sarcoma complex, malignant histiocytosis is a disorder found primarily in Bernese Mountain Dogs. But it has also occurred in some Golden Retrievers, Flat-coated Retrievers, and Rottweilers.

In malignant histiocytosis, multiple histiocytomas may form on the skin, subcutaneous tissue, or joints. But they also occur in major internal organs like the lungs, liver, and spleen, as well as regional lymph nodes and the nervous system. Sadly, malignant histiocytosis is a fatal disease with a very short survival time.

Mast cell tumors—cancerous

Unfortunately, mast cell tumors in dogs are cancerous. And they can spread to the lymph nodes and bone marrow. Mast cell tumors may also affect the spleen and liver, but rarely spread to the lungs. Additionally, they may result in systemic clinical signs such as vomiting, diarrhea, fever, swollen limbs, and collapse.

How do you know if your dog has a histiocytoma or a mast cell tumor?

With most histiocytomas being no big deal and mast cell tumors being very much a big deal, it is important to be able to know which tumor your dog has. As you have learned, both can look similar, so there really isn’t a good way to visually distinguish between the two. Plus, mast cell tumors have earned the moniker “Great Pretender” because they can look like anything—not just like a histiocytoma.

Sure, histiocytomas usually affect young dogs, and they can resolve on their own after a few months. However, without an official diagnosis, you might wait several months only to find out that the growth you assumed was a histiocytoma didn’t go away because it was actually a mast cell tumor. And now you are months behind in getting your dog the treatment he or she needs.

Knowing all this, the best course of action if you find a mass on your dog’s skin is to make an appointment with your vet. He or she can examine the mass and perform a fine needle aspiration (FNA). This is one of the main ways to identify the various skin tumors dogs may get. Performing an FNA involves inserting a small needle into the mass to withdraw some cells. Then the veterinarian will analyze the cells under the microscope or send the slide to a veterinary clinical pathologist for evaluation.

On cytology (i.e. when examined under the microscope), histiocytomas tend to have moderate numbers of histiocytes and some other immune system cells like lymphocytes. And mast cell tumors are composed of mast cells—round cells which usually have prominent granules.

The two images below show the difference between a histiocytoma and degranulated mast cells when viewed under a microscope.

What is the treatment for histiocytomas in dogs?

If the result of the fine needle aspiration shows that the tumor is a histiocytoma, benign neglect (i.e. leaving it alone) is a reasonable treatment option. The histiocytoma will typically start to regress on its own in a few months without the need for any veterinary or home treatment.

However, your veterinarian may recommend cryotherapy (i.e. freezing the tumor) or surgical removal if the histiocytoma doesn’t go away within three months, becomes ulcerated, swollen, infected or starts bleeding, or is painful and growing rapidly.

When a histiocytoma is ulcerated or infected, your veterinarian may also prescribe a course of antibiotics. However, he or she will usually avoid using immunosuppressants because the dog’s own immune response to the histiocytoma is what causes it to go away. However, dogs with multiple histiocytomas may need immunosuppressive or chemotherapeutic therapies.

What is the treatment for mast cell tumor in dogs?

Because they are cancerous, the treatment for mast cell tumors is much more aggressive than for histiocytomas. When possible, the vet will recommend a mast cell tumor be surgically removed. During surgery it is important to take wider margins around the mast cell tumor than are necessary when removing a histiocytoma. This is the case because it is important to ensure all of the mast cell tumor is removed due to its potential to spread.

If the dog has metastatic disease, the vet may recommend radiation and chemotherapy too. Alternatively, an intratumoral injection called Stelfonta may be an option for some mast cell tumors.

Histiocytoma vs mast cell tumor: What is the prognosis?

As you can imagine, dogs with solitary histiocytomas have an excellent prognosis. The tumor almost always resolves within one to three months without any treatment interventions. If surgery is needed, complete removal of the mass is usually curative.

Dogs with Langerhans cell histiocytosis have a more guarded prognosis. Regression of the tumors usually takes about 10 to 12 months. And if LCH spreads to other organs or the skin is severely affected, humane euthanasia may be necessary.

The prognosis for malignant histiocytosis is guarded to grave, unfortunately. It doesn’t respond well to traditional therapies and in a study of histiocytic sarcoma in Bernese Mountain dogs, average survival time was only 49 days after diagnosis.

Thankfully, lower-grade mast cell tumors can behave benignly and surgical removal is often curative. However, higher grade mast cell tumors have a guarded to poor prognosis, depending on their location. And mast cell tumors that form on the muzzle or near a nail bed tend to be associated with a poorer prognosis.

Head to the vet if your dog has a skin tumor

If you only take away one thing from this article, let it be this…

If you find a new lump or bump on your dog, the best thing you can do is to make an appointment with your vet. Without some diagnostic testing (usually a fine needle aspiration) there really isn’t a way to definitively know what type of mass your dog is dealing with.

When it comes to cancerous skin masses, the sooner you find and remove them the better. If you are looking at a mass on your dog’s skin right now and Googling “histiocytoma vs mast cell tumor,” please give your vet a call so you can find the answers that your dog needs.

Has your dog had a histiocytoma or mast cell tumor?

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