Fibrohistiocytic osteosarcoma in the metatarsus of a domestic shorthair cat: a case report

Background To our knowledge, this is the first documented case of fibrohistiocytic osteosarcoma in veterinary medicine. Case Presentation A one-year-old intact female domestic shorthair (DSH) cat was referred to the Veterinary Teaching Hospital of Ferdowsi University of Mashhad with a one-month history of progressive lameness in its right hindlimb. The cat had an apparent solid mass on its right foot. Radiology revealed a Codman’s triangle periosteal reaction and destruction of the fourth metatarsal bone, as well as a mass in the right lung suggestive of metastasis. We performed a fine-needle aspiration, and the cytological evaluation of the sample revealed anisokaryosis, multinucleated cells, anisocytosis, and prominent nucleoli, all of which are highly suggestive of osteosarcoma. The cat was euthanized, and samples were taken from the destroyed metatarsal bone, the popliteal lymph node of the affected limb, and the mass in the right lung. Microscopic examination of the bone tissue revealed hyperactive osteoblasts, bone remodeling, and an abundance of pleomorphic tumor cells with bizarre, enlarged nuclei, as well as abnormal spindle-shaped cells. In the lymph node, follicular hyperplasia-associated edema and metastatic spindle-like tumor cells were seen. A microscopic examination of the lung mass revealed metastatic foci of osteoid strands. The case was confirmed to be fibrohistiocytic osteosarcoma. Conclusions This case highlights a rare and aggressive presentation of fibrohistiocytic osteosarcoma in a domestic cat, with evidence of early metastasis to both lymph nodes and the lungs. Its documentation broadens the spectrum of known feline bone tumors and underscores the importance of considering this variant in the differential diagnosis of destructive bone lesions. Early recognition and thorough histopathological evaluation are essential for accurate diagnosis and management.