Joe is a 10 month old, male, neutered Boxer puppy. He is very happy and energetic. He is here today for a small lump on the face that has appeared over the past week. Over the weekend it became raw with some serous to slightly bloody discharge. He is acting fine and does not seem to be scratching at the lesion. On physical examination Joe appears to be generally healthy. He is a wiggly boxer puppy. The only abnormal finding is the mass on his face. It is raised, firm and ulcerated. It is about 1cm diameter, flat and about 4mm thick. It is located just under the left eyelid. He does not mind that it is touched, just that he has to hold still.

What could this be and what should we do about it?



Answer: Cutaneous Histiocytoma

There are a number of things this lump could be. Given its rapid formation and current appearance, the most likely answer is a cutaneous histiocytoma. However, other problems are possible and should be ruled out. These include mast cell tumor, malignant histiocytoma, infection, and other skin tumors.

Cutaneous histiocytomas are benign inflammatory lesions. The exact cause has not been determined, but it does not seem to be infectious.

We most commonly see these in young dogs, often less than 2 years old and it seems most commonly in dogs less than 1 year old. These masses can form anywhere on the body but seem to be more common about the face. They form rapidly, usually as a firm, raised, skin mass that then loses its hair and the entire surface ulcerates. It then weeps a small amount of serous to slightly bloody fluid, as most open wounds will do. It can form a large scab if it is in a location with lots of hair. It can also become infected secondarily. Usually they are small (less than 1cm diameter) and round, but they can be several centimeters diameter and somewhat irregular.

The problem comes with accurately diagnosing what these are. Given the small thickness of these lesions, it can be difficult to get some cells with a needle. Touch preparations with microscope slides are not always rewarding due to the serum. When a cytology sample is collected, we can rule out a mast cell tumor and some other tumors, but determining the benign or the malignant forms of this tumor is not possible. However, patient age is strongly suggestive.

                                Typical appearance of a histiocytoma cytology.

                                Typical appearance of a histiocytoma cytology.

There are really 2 ways to determine whether this is benign or not. The most rapid way is to remove the mass and send it in for histopathology. The other way is to wait and watch. The benign histiocytomas usually will regress in a few weeks with only minor treatment, mainly keeping it clean. Sometimes it is better to monitor the mass than to jump into surgery. As with Joe, we could cause functional problems by removing skin so close to the eyelid. Joe would need sedation to attempt to get cells with a needle.

Joe's owners elected to watch the mass initially and keep it clean. Two weeks later the mass was quieter and nearly epithelialized. It was also getting smaller. Joe's mass went on to resolve completely.

Cutaneous histiocytomas run a very predictable course both in appearance and in microscopic changes. By the ratio of cell types in this benign inflammatory lesion, pathologists can determine whether it is forming or regressing.