Photo courtesy of Dr. Carol Foil
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. 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 a histiocytoma, it is important to get the right diagnosis as the other conditions may not be as benign.
The histiocytoma is a tumor originating from what is called a Langerhans cell. These cells live in the skin and serve as part of the immune system by processing incoming antigens and presenting them to other immunologic cells. 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 get rid of. It becomes infiltrated by T-lymphocytes and is destroyed.
Histiocytomas are especially common in Labrador retrievers, Staffordshire terriers, Boxers, and Dachshunds. The histiocytoma is not related to the malignant process called histiocytosis.
What 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 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.
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). Topical therapy with a product containing DMSO and a cortisone derivative is helpful in controlling symptoms of irritation. Any growth believed to be a histiocytoma that is still there after 3 months should be removed. Any histiocytoma that is eroded or seems uncomfortable should be removed rather than waiting through the regression process.