title: Tail-base mass from a "horse of a different color".
abstract: A 14-year-old bay Thoroughbred gelding was presented for evaluation of a mass at the base of the tail. The mass had been present for 1 year, and recently had begun to increase in size. Additional masses were found around the eye and shoulder. A fine-needle aspirate of the tail-base mass revealed highly anaplastic round to polyhedral cells containing dark green to black cytoplasmic granules interpreted to be melanin. Histologically, the mass was composed of pleomorphic, poorly pigmented, round to polyhedral cells interpreted to be neoplastic melanocytes. With immunohistochemistry, the cells were positive for vimentin and S-100, but negative for pancytokeratin and Melan-A. The cytologic and histopathologic diagnoses were amelanotic melanoma. The horse was treated with cimetidine, but the tumor continued to progress. In this report, we describe the cytopathologic features of an aggressive amelanotic melanoma in a non-grey horse and emphasize the unique correlation between cytologic and histologic findings.
authors.affiliation: Department of Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA, USA. bleroy@vet.uga.edu
authors.forename: Bruce E
authors.initials: BE
authors.lastname: LeRoy
authors.forename: Miranda C
authors.initials: MC
authors.lastname: Knight
authors.forename: Randy
authors.initials: R
authors.lastname: Eggleston
authors.forename: Fernando
authors.initials: F
authors.lastname: Torres-Velez
authors.forename: Barry G
authors.initials: BG
authors.lastname: Harmon
mesh_terms: D000818:Animals; D006734:Horse Diseases / Q000473:pathology*; D006736:Horses; D008297:Male; D018328:Melanoma, Amelanotic / Q000473:pathology / Q000556:secondary / Q000662:veterinary*; D012878:Skin Neoplasms / Q000473:pathology / Q000662:veterinary*