Carcinosarcoma mimicking a feline injection-site sarcoma in a cat.

A 15-year-old spayed female domestic short-haired cat with cutaneous/subcutaneous well-circumscribed, alopecic mass approximately 25 × 30 mm in diameter, localized to the left shoulder region was brought to the veterinary surgery department. Despite the suggestive location and macroscopic appearance, feline injection-site sarcoma was not suspected based on the cytologic examination of fine-needle aspirates. The tumor was surgically resected, and tissue sections were evaluated microscopically. The tumor was found to be nonencapsulated with a distinct border between the neoplastic parenchyma and surrounding connective tissue. The neoplastic tissue consisted of 2 cell populations: elongated to spindle-shaped cells arranged in bands and cords and malignant epithelial-like cells. Both populations showed microscopic features of malignancy. Multinucleate giant cells with irregular cytoplasm were scattered among the neoplastic cells. The spindle-shaped cells strongly expressed vimentin but did not express α-smooth muscle actin (α-SMA) or cytokeratin. Desmin was strongly expressed in about 0-5% of cells. Epithelial-like cells expressed cytokeratin, but not vimentin, desmin, or α-SMA. Multinucleate giant cells expressed vimentin, but did not α-SMA, desmin, or cytokeratin. Based on microscopic observations and IHC results, the final diagnosis was carcinosarcoma with histologic features compatible with feline injection-site sarcoma, but without the clinical aggressiveness of this tumor.