Primary Meningeal Extraskeletal Osteosarcoma in a Domestic Shorthaired Cat: Case Report and Review of Comparative Pathology

This case describes the presentation and diagnosis of an extraskeletal osteosarcoma arising in the cranial meninges of a domestic shorthaired cat. Clinical signs were compatible with a space-occupying cerebral lesion, and magnetic resonance imaging revealed a multilobular mass with extracranial and intracranial components, which asserted marked forebrain mass effect and apparently originated from the frontal and parietal bones. Due to clinical deterioration and poor prognosis, the cat was euthanized and a post-mortem examination performed. Gross lesions confirmed a solitary neoplasm, firmly attached to the dura mater, overlying and compressing the left cerebral hemisphere. The neoplasm was not adherent to bony structures of the skull, but local osteolytic destruction of the frontal bone and cribriform plate allowed its extension outside the calvarium and into the nasal sinuses. The tissue was diagnosed by histological assessment as an osteosarcoma. Primary extraskeletal osteosarcomas of the meninges are exceedingly rare in all species, and to the authors' knowledge, have never been reported in cats. As the clinical behaviour is very different from more typically encountered neoplasms, this possibility, although rare, may be considered as a differential diagnosis for feline intracranial tumours.