Dermal Melanomatosis Surgery

Tumors arising from melanin-containing cells (melanocytes) in the skin go beyond benign or malignant melanoma. In fact, there are four distinct melanocyte-derived tumor types, each classified according to clinical behavior and cellular appearance. For example, two types--dermal (skin) melanoma and dermal melanomatosis--look identical microscopically and must be distinguished by clinical appearance. While melanoma occurs anywhere on mature gray horses, dermal melanomatosis occurs in old gray horses (older than 15 years) around the perineum--the anus, genitalia, and tail head. Historically, surgical removal of dermal melanomatosis has been avoided if possible because of perceived problems with regrowth and high rate of metastasis.

But Emma Rowe, BVMS, Dipl. ACVS, and colleagues have had success removing these tumors, and so designed a study to evaluate the viability of excising them.

Medical records from the Virginia-Maryland Regional College of Veterinary Medicine at Virginia Tech were reviewed to identify horses admitted between 1994 and 2000 for surgical removal of a melanocytic tumor in the perineal region. Cases were included if the tumor was surgically removed because it was creating an obstruction. In addition, the tumor must later have been classified as dermal melanomatosis. Eleven horses met the study criteria.

No attempt was made to remove any tumor other than the one causing the physical obstruction. Follow-up information was obtained through telephone interviews with owners. Two horses were euthanatized for chronic health problems, but among the remaining nine, there was no evidence of tumor regrowth at the surgical site and no evidence of internal metastasis in any horse. "We looked for clinical signs of metastasis, such as abdominal pain and straining for tumors around the small intestine and rectum," says Rowe. "Metastases usually develop quickly, and with a mean follow-up time of three years, we would have expected clinical signs to appear if metastases had occurred." Two horses did have several small growths appear close to the surgical site. In three horses with small growths near the surgical site that were not removed, the growths expanded in size. Three horses with melanomas present at distant sites experienced an increase in the number of these tumors. Two horses experienced an increase in size of their distant melanomas. Still, none of these changes were judged to be particularly aggressive. "The lesions that increased in size or number at distant sites in our study did not affect the horses' well-being," says Rowe, "and grew at the same rate as they had been previously."

Rowe, E.L.; Sullins, K.E. Journal of the American Veterinary Medical Association, 225 (1): 94-96, 2004.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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