Histiocytoma - Mar Vista Animal Medical Center

HISTIOCYTOMA

Histiocytoma
(Photocredit: CarolDermoid via Wikimedia Commons)

The histiocytoma is a benign skin growth that usually goes away by itself within a couple of months. The typical histiocytoma patient is a young adult dog (usually less than 2 years of age) with a round eroded growth somewhere on the front half of its body but, of course, not every patient seems to have read the text book. Such growths can be found on rear legs or in older patients as well. Because there are other growths that can look just like the histiocytoma, it is important to get the right diagnosis as the other conditions may not be as benign.

WHAT IS A HISTIOCYTOMA?

All body areas that interface with the environment are rich with immunologic cells constantly sampling assorted organisms and proteins from the environment and presenting them to other cells of the immune system for further processing. Langerhans cells are cells that live in the skin were they happily sample materials entering and contacting our skin for immunologic presentation as described above. A histiocytoma results when Langerhans cells become tumorous and generate a visible growth. This growth is ultimately recognized by the immune system as something to destroy. The histiocytoma becomes infiltrated by T-lymphocytes and is ultimately destroyed by them leaving the skin as it was. This process generates some inflammation, as one might expect, and requires 2-3 months to complete.

Histiocytomas especially common in Labrador retrievers, Staffordshire terriers, Boxers, and Dachshunds. The histiocytoma is not related to the malignant process called “histiocytosis."

WHAT ELSE MIGHT A GROWTH WITH THIS APPEARANCE BE?

There are several possibilities for this type of growth, often termed a "button" growth. The ringworm fungus, for example, can produce some raised round reactions called “kerions” which can look similar. The mast cell tumor, which is highly inflammatory, sometimes malignant, and must be excised widely so as to prevent recurrence, can have an identical appearance. The melanoma can also appear as a similar lesion. If the patient is young, the chances are good that a benign process is afoot but some testing will be needed to be sure.

FINDING OUT FOR SURE:

Most of the time the histiocytoma is small enough for easy removal and the diagnosis comes up on the biopsy report, confirming that the tumor is benign and will not grow back. If the tumor is in a location where removal is not easy (such as the middle of the ear flap or dangerously close to the eye) there are two other options: biopsy of a small piece of growth or cytology exam.

Biopsy may require sedation depending on the location of the growth. Sometimes a local anesthetic is enough. A small piece of tissue is snipped off (or the entire lesion can be removed if it is small enough) and sent to the lab for analysis. In a few days we will know for sure exactly what the growth is.

Less costly but also less accurately, cells can be withdrawn from the lesion by a needle or simply collected from the tumor surface by pressing a microscope slide to the tumor surface. This omits the ability to see the architecture of the tissue and just the cells are available for examination. This may be enough for the lab to confirm the identity of the growth in question.

Canine Histiocytoma Cytology

Cytology sample from a histiocytoma.
The brownish cells are red blood cells.
The purple cells are histiocytoma cells.
(Photocredit: Joel Mills via Wikimedia Commons)

WHAT TO DO WITH A HISTIOCYTOMA

On the average, a histiocytoma undergoes regression by itself within three months. This process may be itchy or may lead to minor infection. Removal of the histiocytoma is the fastest route to resolution but, since they do go away on their own, leaving the growth alone is reasonable (as long as we know for sure it is a histiocytoma). Any growth believed to be a histiocytoma that is still present after 3 months should definitely be removed and may very well turn out not to be a histiocytoma after all. Any histiocytoma that is eroded or seems uncomfortable should be removed rather than waiting through the regression process. Anti-inflammatory medications may be tempting but these can interfere with the regression process.

If the growth in question is in an area not readily amenable to surgery (ear flap, toe, tail or some area where there is not a lot of extra skin for surgical closure for example, cryotherapy (freezing) might be an option. Special equipment is needed for this so if you are concerned about a growth, ask your veterinarian about the best approach.

Page last updated: 8/12/2025