Subcutaneous extraskeletal osteosarcoma in a metatarsal footpad in a cat

Extraskeletal osteosarcomas (ESOSAs) are rare neoplasms in humans and animals. In cats, ESOSA has been reported to arise from orbital, ocular, intestinal, mammary and subcutaneous locations. Subcutaneous ESOSA occurs most commonly at sites used for vaccination including interscapular, dorsal lumbar or thigh areas. Previous reports of feline cases have not documented the use of advanced diagnostic imaging to exclude a primary bone tumour.

To describe the clinicopathological and advanced imaging findings of a subcutaneous ESOSA occurring in a metatarsal footpad of a cat and to report the one year follow-up status.

A 9-year-old neutered male domestic short hair cat.

Physical, abdominal ultrasonographic and computed tomographic examinations, and excisional biopsy for histopathological and immunohistochemical evaluation.

The cat presented with mild focal erythematous swelling of the left metatarsal pad. ESOSA was diagnosed through advanced diagnostic imaging and histopathological examinations. Histopathological findings were consistent with osteosarcoma. No primary bone disease was observed on computed tomography. The owners declined limb amputation. One year after diagnosis, the cat was alive without disease progression.

Extraskeletal osteosarcoma should be considered in the differential diagnosis of soft tissue swelling in footpads in cats. Advanced diagnostic imaging is recommended to exclude primary bone tumours.