In horses, giant-cell tumors of soft parts are rare neoplasms, with the majority of reported cases occurring within the hind limb muscles and soft tissues in older horses. The following article documents 21 cases of equine giant-cell tumors of soft parts clinically examined within the state of Colorado from 2000 to 2007. The majority of cases occurred in male horses aged 10 years or older. Nine (43%) arose within the hind limbs. Key histologic features included numerous multinucleated giant cells and hemosiderin-laden macrophages admixed with a spindle-cell proliferation. The majority demonstrated liposarcomatous change, variable areas of necrosis and hemorrhage, and an intermediate number of mitotic figures. Immunohistochemical results demonstrated 2 distinct cell populations: vimentin-expressing neoplastic mesenchymal cells and CD18 (histiocytic marker) expressing multinucleated giant cells. These results suggest a mesenchymal origin of the neoplasm with possible recruitment of the secondary histiocytic population. Surgical excision was attempted in the majority of horses and was considered clinically complete. A recurrence of the neoplasm was documented in 1 horse and 1 mule. In 18 horses, surgical excision, regardless of margin integrity, appeared successful with no recurrence of disease documented. Unfortunately, 10 of 21 horses were lost to follow-up within approximately 3 months of surgery. Of the 11 remaining horses that were available for follow-up evaluation, there has been no evidence of metastasis. A larger case series with more controlled follow-up is necessary to evaluate malignant potential and the importance of complete surgical excision.
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