Presumed myofibroblastic hepatic sarcoma associated with a retained surgical swab in a 3.5-year-old dog

A 3.5-y-old spayed female Labrador Retriever was presented with inappetence, weakness, vomiting, and a tense abdomen. Via abdominal ultrasonography, radiography, and diagnostic laparotomy, we identified a hepatic mass. Thereafter, we performed a lobectomy and sent the resected specimen for histologic examination. The dog was euthanized given the lack of clinical improvement and the formation of an intrathoracic soft-tissue mass identified with radiography. Histologically, the thoracic mass was an infiltrative, highly cellular mesenchymal neoplasm. Its spindle cells were arranged in interwoven fascicles and bundles with marked anisocytosis, anisokaryosis, and nuclear atypia, and a high mitotic count. The tumor tissue surrounded large necrotic areas with numerous cotton fibers originating from a retained surgical swab, which had elicited a marked granulomatous inflammatory response. The neoplastic cells uniformly expressed strong cytoplasmic vimentin immunolabeling. Approximately 50% of the neoplastic cells had strong cytoplasmic "tram-track" α-SMA and moderate cytoplasmic calponin immunolabeling. Tumor cells did not have desmin, S100, CD31, factor VIII-related antigen (von Willebrand factor), HepPar1, or IBA1 immunolabeling. The microscopic and immunohistochemical features were consistent with a soft-tissue sarcoma, most likely a myofibroblastic sarcoma. Hepatic myofibroblastic sarcoma related to retained surgical swab has not been reported previously in dogs, to our knowledge, nor has the use of HepPar1 immunohistochemistry to investigate the possible hepatocellular origin of the tumor.