Tail-base mass from a "horse of a different color"

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 hig...

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Bibliographic Details
Published in:Veterinary clinical pathology 2005-01, Vol.34 (1), p.69-71
Main Authors: LeRoy, Bruce E., Knight, Miranda C., Eggleston, Randy, Torres-Velez, Fernando, Harmon, Barry G.
Format: Article
Language:English
Subjects:
Animals
Cytology
equine
Horse Diseases - pathology
Horses
Male
melanoma
Melanoma, Amelanotic - pathology
Melanoma, Amelanotic - secondary
Melanoma, Amelanotic - veterinary
metastasis
Skin Neoplasms - pathology
Skin Neoplasms - veterinary
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Summary: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.
ISSN:0275-6382
1939-165X
DOI:10.1111/j.1939-165X.2005.tb00014.x