Light and electron microscopic findings in a patient with orbital myolipoma

Background: This report describes the pathology of myolipoma, which is a benign soft tissue tumor and is a very rare tumor of the orbit. Methods: A 41-year-old woman complained of exopthalmos of her right eye. By MRI ( magnetic resonance imaging) examination, the tumor ( 30 x 20 mm) was located in the extraconus of four recti muscles. A part of the tumor was excised and examined with both light and electron microscopes and immunohistochemically. Results: Light microscopic inspection revealed the tumor was formed by spindle-shaped cells, which were adipocytes and smooth muscle. Electron microscopy showed the cytoplasm contained parallel filaments associated with fusiform densities. Immunohistochemical staining showed smooth muscle stained with SMA ( smooth muscle actin). Angiomyolipoma was excluded by negative staining for HMB45, Melan A. Conclusion: To our knowledge, only one case of orbital myolipoma has been reported. The intimate relationship between the two cell types led us to conclude that the tumor was dimorphic and could be classified correctly as a myolipoma.