Fine needle and tissue biopsy samples were taken from a subcutaneous mass in the cranial stifle region of a 10-year-old horse. Key cytologic features included numerous mildly to moderately pleomorphic histiocytic cells admixed with mononuclear inflammatory cells, occasional fibroblasts, multinucleated cells, and hemosiderin-laden macrophages. Based upon these features, cytologic differentials included granulomatous inflammation with reactive fibroplasia, histiocytic sarcoma, Giant Cell Tumor of Soft Parts (GCTSP) and Giant Cell Tumor of bone. The tissue biopsy supported a diagnosis of superficial GCTSP. This neoplasm is of controversial origin but generally considered to be a soft tissue sarcoma and classified in the WHO Classification of Tumors under fibrous histiocytic tumors as a Giant Cell Tumor of Soft Tissue with low risk of malignancy. When found in the subcutis, this neoplasm rarely metastasizes in the horse and has a low rate of local reoccurrence. In this case there was no local recurrence of the tumor at six months and the horse was clinically doing well.
Key words: Equine, Giant Cell Tumor of Soft Parts, Cytology, Histopathology
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