Histiocytoma

Photo courtesy of Dr. Carol Foil

A histiocytoma is a benign skin growth that usually resolves on its own within a couple of months. The typical histiocytoma patient is a young adult dog (usually less than two 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 textbook. Such growths can be found on rear legs or in older patients. Further, an association between the use of oclacitinib (more commonly known as Apoquel®) and histiocytoma development is emerging. The point is that, while there is a "classic" clinical picture, not every patient fits the classic mold. 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?

The histiocytoma is a tumor originating from what is called a Langerhans cell. 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. In the skin, cells called Langerhans cells reside and happily sample materials contacting and entering through the skin. A histiocytoma results when Langerhans cells become tumorous and generate 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 two to three months to complete. Langerhans cells form a histiocytoma, the histiocytoma is detected by the immune system, the histiocytoma is attacked and removed by T-lymphocytes.

Histiocytomas are 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 that can look similar. The mast cell tumor, which is highly inflammatory, sometimes malignant, and must be excised widely so as to prevent a recurrence, can have an identical appearance. The melanoma can also appear as a similar lesion and even T-cell lymphoma can be hard to distinguish from a histiocytoma. 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: a biopsy of a small piece of growth or a cytology exam.

A 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 accurate, 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.

Treatment of Histiocytomas

Histiocytomas heal by themselves within three months and may not need any treatment, depending on the case. The healing process may be itchy or may lead to secondary bacterial or fungal infections, which need to be treated for the area to fully heal. There have been some studies looking at immunomodulating drugs, or drugs that calm down immune responses, for the treatment of these growths. More research is necessary, and there are not any specific immunomodulating drugs recommended by veterinarians for histiocytomas at this time.

On average, a histiocytoma undergoes regression by itself within three months. This process may be itchy or may lead to a 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). If the patient is taking oclacitinib, consider discontinuing it. Any growth believed to be a histiocytoma that is still present after three 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 (e.g., the ear flap, toe, tail, or an area with not much extra skin for surgical closure), 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.

Removal of Histiocytomas

Surgical removal of the histiocytoma is the fastest route to healing, but since they do go away on their own, leaving the growth alone is reasonable if: 1) your veterinarian has confirmed it is a histiocytoma, and 2) it is not very large, painful, or interfering with your pet’s daily activities. 

Cryotherapy, where your veterinarian freezes the area and removes the histiocytoma, is another option. Usually, this method is recommended for tumors that need to be removed in places where surgery would be difficult, such as an eyelid, where the skin is more delicate.

The prognosis (outlook) for histiocytomas of the skin is usually excellent, as most of them will resolve on their own or be completely cured by surgical removal, with a very small percentage (less than 1%) potentially growing back. Discuss treatment options and what to look out for with your veterinarian so your pet can hopefully get back to being comfortable as soon as possible.