Periocular sarcoid with bone invasion in a Thoroughbred mare.

A periocular nodular sarcoid was diagnosed on the right upper eyelid and medial canthus of a 12-year-old Thoroughbred mare. Enucleation was performed and during the procedure the mass was noted to be firmly adhered to the underlying frontal bone. Partial ostectomy of the dorsal orbital rim was performed. Histopathology revealed invasion of the cortical lamellar bone and the bone marrow by neoplastic spindle cells and extension of these cells to multiple surgical margins. Recurrence at the level of the surgical site and its surroundings occurred 3 months after the procedure. The horse was euthanized 12 months later. Key clinical message: Invasion of the underlying bone occurs in some cases of equine periocular sarcoids. The case highlights how this bone invasion might affect the surgical planning and shows potential aggressiveness of this type of tumor.