Subconjunctival herniated orbital fat: A benign adipocytic lesion that may mimic pleomorphic lipoma and atypical lipomatous tumor.

Prolapse of subconjunctival intraconal orbital fat is a rare cause of an intraorbital mass lesion. Over the past several years, we have seen a number of cases in which this prolapsed fat was confused pathologically with a neoplasm of adipocytic lineage, specifically pleomorphic lipoma and atypical lipomatous neoplasm (well-differentiated liposarcoma). We report the clinical, histopathologic, and immunohistochemical findings in 21 specimens from 17 patients, all of whom presented with prolapsed intraconal orbital fat. All specimens were routinely examined and processed for light microscopy. Immunohistochemistry for CD34, CD68, S100 protein, vimentin, alpha-smooth muscle actin, and Ki-67, and Giemsa, Masson trichrome, and alcian blue histochemical stains were performed. Clinical and follow-up information was extracted from a chart review. The mean age (+/-SD) of the patients was 65.6+/-11.9 years (range: 41 to 85 y); 2 were women and 15 were men. Subconjunctival prolapsed orbital fat was localized in the superotemporal quadrant or lateral canthus around the rectus muscle below the lacrimal gland. The lesions were unilateral in 10 and bilateral in 7 patients. No recurrence was clinically evident over a mean (+/-SD) follow-up time of 2.5+/-3.2 years (range: 1 mo to 13.5 y). Histopathologically, all specimens showed an admixture of mature fat, fibrous septae lacking hyperchromatic cells, adipocytes with intranuclear vacuoles (Lochkern cells), multinucleated giant cells with a wreathlike configuration of normochromatic nuclei (floret cells), and varying numbers of histiocytes, lymphocytes, plasma cells, and mast cells. "Control" sections of normal orbital fat showed occasional Lochkern cells but lacked floret cells. By immunohistochemistry, the floret cells expressed only CD34 and vimentin, whereas the Lochkern cells expressed CD34, S100 protein, and vimentin. We conclude that subconjunctival herniated orbital fat commonly contains multinucleated floretlike giant cells, fibrous septae, and Lochkern cells, features that may result in diagnostic confusion with pleomorphic lipoma and atypical lipomatous neoplasms. Importantly, specific diagnostic features, such as aggregates of bland spindled cells associated with wiry collagen, as seen in pleomorphic lipoma, and enlarged hyperchromatic cells within fibrous septae, as in atypical lipomatous neoplasms, are entirely absent in herniated orbital fat. Multinucleated floret cells present in prolapsed orbital fat likely represent a reactive phenomenon, as they are not present in normal orbital fat.