Histiocytomas are small tumors that arise from the skin of young dogs typically less than 2 years of age. They commonly are round, well demarcated, light to dark red in color, and reamain small (less than 1/2 cm), however, they can occasionally grow ad large as 1-2 cm in diameter. Histiocytomas can grow on skin just about anywhere on the body, but tend to remain confined to the front half of the body.
The good news about histiocytomas in dogs is that they are not malignant (cancerous) tumors, but are infact benign (not cancerous) tumors. The histiocytoma is a tumor originating from what is called a Langerhans cell, a type of cell that resides in the skin as a part of the immune system.
Many experienced vets have seen enough histiocytomas that they are comfortable to recognize them on gross inspection of a patient that fits the typical age and breed of the patient (teh patient is typically a young dog less than 2 years of age, with Labrador retrievers, Staffordshire terriers, Boxers, Cocker Spaniels and Dachshunds most commonly affected breeds). However, they can be seen in patients as old as 5 years, and in just about any breed.
Diagnosis of histiocytoma requires biopsy of the suspect mass. This can sometimes be done with combination of light sedation and local anesthetic. However, given their distinctive appearance and specific signalment, many vets will offer a tentative diagnosis based on gross inspection, and have the owner monitor the growth carefully for rapid changes in color, size, and other gross characteristics. True histiocytomas will regress within 2-4 months, making their eventual disappearance within this time period good evidence for a confirmation of the diagnosis of histiocytoma. Reasons to approach a suspected case of histiocytoma more aggressively and have the suspected lesion biopsied would be if the mass underwent rapid growth, changes in color, bleeding, itchiness, or if the lesion was not typical and resembled other types of skin tumors or diseases.
Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital
Article updated 8/18/2012