Histiocytoma in Dogs: A Vet‘s Guide to These Benign Skin Tumors
As a veterinarian, one of the most frequent concerns that dog owners come to me with is a new lump or bump on their beloved companion‘s skin. And I totally understand the worry – our minds often jump to the worst case scenarios, like skin cancer. But I‘m always quick to reassure owners that many skin growths in dogs are completely benign. One of the most common culprits? Histiocytomas.
In my 15 years of experience, I‘ve diagnosed countless histiocytomas in my canine patients. And in the vast majority of those cases, the tumor resolved on its own without complication. My goal with this article is to walk you through everything you need to know about these mysterious skin bumps. By the end, you‘ll be armed with the knowledge to tackle a histiocytoma diagnosis with confidence, not fear.
What is a histiocytoma?
First, let‘s define our terms. A histiocytoma is a type of benign skin tumor that arises from a population of immune cells called Langerhans cells. These cells normally live in the outer layer of the skin (the epidermis) and act as sentinels – their job is to capture foreign invaders like bacteria and viruses and present them to the immune system to mount a defense.
For reasons that aren‘t totally clear, Langerhans cells can sometimes start dividing uncontrollably, forming a histiocytoma. The growth itself is considered benign because it doesn‘t invade other tissues or spread to new areas of the body. In fact, the immune system will eventually recognize the abnormal cells and attack the histiocytoma, causing the tumor to shrink and disappear over time.
While histiocytomas can pop up at any age, they are by far most common in young dogs. One large study found that 51% of histiocytomas occurred in dogs less than 3 years old, with a peak incidence at 2-3 years of age.[^1] This early age of onset is a big clue that a new skin growth might be a histiocytoma and not something more sinister.
Certain breeds also seem to have a higher than average risk of developing histiocytomas, likely due to genetic factors. In my experience, I see histiocytomas most commonly in these breeds:
- Bulldogs (English and French)
- Boxers
- Boston Terriers
- Pit Bulls
- Staffordshire Terriers
That being said, histiocytomas can still happen in any breed. I‘ve diagnosed them in everything from Chihuahuas to Great Danes!
What does a histiocytoma look like?
One reason that histiocytomas cause so much concern is that they often seem to appear out of nowhere. An owner may give their dog a belly rub one day and feel a strange new lump that wasn‘t there last week. Understandably, this sudden onset can be alarming.
The classic appearance of a histiocytoma is a solitary, round, raised bump protruding from the skin surface. They are typically 1-2 cm in diameter, though I have seen some "giant" histiocytomas that are more like 4-5 cm across. The overlying skin is usually pink or reddish and hairless. When you touch a histiocytoma, it feels firm but moveable – it isn‘t firmly attached to the tissue underneath.
Histiocytomas are most frequently found on the head, face, ears, neck, and limbs. They can sometimes occur in inconvenient spots, like on the eyelids or lips. Most dogs only develop one histiocytoma at a time. In rare cases, dogs can get multiple histiocytomas, but this is quite uncommon in my experience.
What about other symptoms? The good news is that histiocytomas are typically asymptomatic – they don‘t cause pain, itching, or irritation for most dogs. The tumor might bleed a little if it gets bumped or scraped, since the surface is delicate. But overall, histiocytomas usually don‘t bother dogs.
Diagnosing a histiocytoma
While histiocytomas often have a distinct clinical appearance, there are other skin tumors that can mimic them. This is why it‘s so important to get any new skin growth checked out by your veterinarian. Some common differentials that can look like histiocytomas include:
- Mast cell tumors
- Melanomas
- Squamous cell carcinomas
- Soft tissue sarcomas
The concerning thing about these other tumors is that they have the potential to be malignant cancers. Mast cell tumors, for example, can spread aggressively both locally and to distant parts of the body. So while we never want to cause undue alarm, it‘s always better to be safe than sorry when a new bump crops up.
There are two main ways that veterinarians diagnose histiocytomas:
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Fine needle aspirate (FNA) cytology: This is a quick, non-invasive test where we take a small needle and suction out some cells from the tumor. We smear the sample on a microscope slide and send it to the lab for analysis. A cytologist will look at the appearance of the individual cells and determine if they match the typical features of Langerhans cells. FNA is a great screening tool, but sometimes the sample isn‘t definitive.
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Surgical biopsy and histopathology: The other option is to surgically remove the entire growth and submit it to the lab. There, a pathologist will look at thin sections of the tumor under the microscope to evaluate the cell types and overall tissue architecture. A biopsy gives us the most accurate and reliable diagnosis.
At my practice, I usually start with an FNA to get a sense of what we‘re dealing with. If the cytology is consistent with a histiocytoma, I‘ll discuss the pros and cons of surgical removal with the owner. Factors we consider include the dog‘s age, the location and size of the tumor, and whether it‘s causing any issues for the dog. If the FNA is inconclusive or concerning for another tumor type, I‘m more likely to recommend a biopsy.
Monitoring and treating histiocytomas
So your vet just confirmed a histiocytoma diagnosis – now what? The good news is that for most histiocytomas, the solution is simply time and patience. One study found that 28 out of 30 histiocytomas (93.3%) underwent spontaneous regression within 2-3 months.[^2] The body‘s immune system is really quite amazing at clearing these tumors on its own.
I usually advise my clients to simply monitor the histiocytoma at home. Take pictures and measurements of the growth every week or two. Over time, you should see it slowly shrink, flatten out, and eventually disappear. It‘s okay if the process takes a couple months. Very rarely, a histiocytoma might persist longer than 3 months or even start growing. If that happens, surgical removal is usually curative.
Even if a histiocytoma isn‘t causing problems, some owners elect to remove it proactively. The surgery is typically quick and straightforward. One case series reported a 10% recurrence rate after surgical excision, so regrowth is relatively uncommon.[^3] The main risks of surgery are the usual – bleeding, infection, or failure of the incision to heal properly. But overall, histiocytoma surgery has an excellent prognosis.
It‘s natural to worry about a histiocytoma transforming into a malignant cancer. And I‘ll be honest – there have been a handful of case reports documenting histiocytomas progressing to malignant histiocytic sarcomas.[^4] However, these cases are extremely rare outliers. Please don‘t lose sleep over this possibility. The vast, vast majority of histiocytomas are cured with either time or surgery.
The bottom line
At the end of the day, a histiocytoma diagnosis can be a real relief. These tumors look scary, but they aren‘t anything that you or your dog did wrong. They just represent an overgrowth of normal skin cells that the immune system will clear up. Trust me, I‘ve seen hundreds of these growths disappear on their own without a trace.
If you do notice a new lump or bump on your dog, my biggest piece of advice is not to panic. Make a non-emergency vet appointment to get it checked out. Odds are good that it‘s something harmless like a histiocytoma or skin tag. But on the off chance it‘s malignant, early diagnosis and treatment can make all the difference.
As a devoted dog mom myself, I know how stressful it can be to find an unexpected mass on your fur baby. But please know that you‘re doing a great job by being vigilant and proactive. And as your vet, I‘m always in your corner. Together, we‘ll face whatever challenges come our way and keep your loyal companion happy and healthy.
[^1]: Lamm CG, Stern AW, Smith AJ, et al. Disseminated cutaneous histiocytosis with potential for malignant transformation in a dog. J Am Vet Med Assoc. 2009;234(8):1055-1058.[^2]: Taylor DP. Canine histiocytoma: a retrospective study of 30 cases. J Am Anim Hosp Assoc. 1984;20(5):753-756.
[^3]: Kaim AH, Teifke JP. Cutaneous histiocytoma in dogs and cats. Tierarztl Prax. 1998;26(2):126-133.
[^4]: Moore PF. Canine histiocytoma: a retrospective study of 156 cases with emphasis on prognostic indicators and breed predisposition. Vet Pathol. 1990;27(6):451-452.
