Background Osteosarcoma is the most common primary bone tumor in cats, yet it is typically diagnosed in older animals and rarely metastasizes. Among its histological variants, the fibroblastic subtype—defined by spindle-shaped osteogenic cells that generate osteoid—is infrequently reported in young cats. 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 and a firm swelling on the right hindlimb. Radiographs revealed an aggressive osteolytic lesion of the fourth metatarsal bone with a Codman’s triangle periosteal reaction, as well as a pulmonary mass consistent with metastasis. Fine-needle aspiration cytology demonstrated pleomorphic osteoblasts with anisokaryosis, multinucleated giant cells, and osteoid material, suggesting osteosarcoma. Following humane euthanasia, necropsy revealed extensive bone destruction and metastatic foci in the lung and popliteal lymph node. Histopathology showed a mixture of spindle-shaped and pleomorphic osteogenic cells producing eosinophilic osteoid matrix, confirming a diagnosis of fibroblastic osteosarcoma. Immunohistochemistry showed strong nuclear positivity for SATB2, confirming osteoblastic lineage. Hepatic and splenic changes were reactive and non-metastatic. Conclusions This report describes an exceptionally early-onset fibroblastic osteosarcoma in a one-year-old cat, accompanied by both pulmonary and lymphatic metastasis, a presentation rarely reported in feline patients. Recognition of such atypical presentations broadens the understanding of feline osteosarcoma behavior and underscores the value of integrating radiologic, cytologic, histopathologic, and immunohistochemical findings for accurate diagnosis.

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