# Early-onset metastatic fibroblastic osteosarcoma of the metatarsus in a young cat: a case report

## TL;DR

A young cat developed aggressive fibroblastic osteosarcoma with metastasis, a rare condition in felines.

## Contribution

Reports an early-onset fibroblastic osteosarcoma with metastasis in a young cat, a rare clinical presentation.

## Key findings

- Fibroblastic osteosarcoma was diagnosed in a one-year-old cat with metastasis to the lung and lymph node.
- Histopathology and immunohistochemistry confirmed the tumor's fibroblastic subtype and osteoblastic lineage.
- The case highlights the importance of multimodal diagnostic approaches for accurate diagnosis.

## Abstract

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.

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.

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.