Definition: Pleomorphic lipoma are benign tumors of the orbit.
Incidence/Prevalence: Pleomorphic lipomas are rare in the orbit.
Etiology: The possibility that pleomorphic lipoma may represent a reactive change in the fibroadipose tissue related to herniation of fat rather than a neoplasm has not been excluded. Dr. Heathcote's group chose to entitle the entity as pleomorphic lipoma while Dr. Grossniklaus prefers prolapsed orbital fat (see references 1 and 2, respectively). The etiology is unknown.
Clinical Findings: Most patients reported are male with a mean age of about 70 years. There is usually a history of swelling in the eyelid, bulbar or forniceal conjunctiva caused by a freely mobile lesion. The lesion appears yellow in color and has the appearance of prolapsed fat. Prolapsed or herniated fat is almost always the profferred clinical diagnosis from our colleagues. Herniated orbital fat occurs generally in older obese males.
Gross Pathology: Pleomorphic lipomas are well circumscribed and more firm than normal adipose tissue and yellow in color. The lesions are not encapsulated.
Histopathology: The key characteristics are the fairly uniform appearance of fibroadipose tissue with a well developed vasculature. There are dark nuclei that are evident at low power but otherwise the appearance is rather inconspicuous.
At greater magnification there are large pleomorphic cells with characteristic multinucleated floret cells (arrow 1). The floret cells have overlapping nuclei arranged in a petaloid pattern (arrow 2). The nuclei are hyperchromatic and the cytoplasm is generally quite eosinophilic in the floret cells. The nuclei frequently have clear inclusions (nuclei of Lochern). Individual spindle cells may have hyperchromatic nuclei and are associated with collagen bundles rather than fat. There may be multivacuolated cells that mimic lipoblasts. There are very few if any mitotic figures. The lesions can resemble a myxoid liposarcoma; immunohistochemistry may be helpful.
Immunohistochemistry: CD 34 and S100 may be extremely helpful in difficult cases and also to confirm that the lesion is not a myxoid liposarcoma.
The floret cells react strongly with CD34. This is contradistinction to lipoblasts of a liposarcoma and normal adipocytes which are CD34 negative (see arrow4).
S100 is usually weak in the floret cells (arrow 5) whereas in lipoblasts it is often strongly reactive.
Treatment: Excision is often performed for these lesions.
Prognosis: Excision is curative.
1. Daniel CS et al: Pleomorphic lipoma of the orbit: A case series and review of literature. Ophthalmology 2003;110:101-105.
2. Schmack I et al: Subconjunctival Herniated Orbital Fat: A Benign Adipocytic Lesion That may Mimic Pleomorphic Lipoma and Atypical Lipomatous Tumor. Am J Surg Path 2007:31:193-198.