Traditionally, diffuse epithelial mesotheliomas are mainly identified at the ultrastructural level by the numerous, long, wavy-appearing surface microvilli. By electron microscopy of a series of diffuse mesotheliomas of varying subtype (epithelial, biphasic, sarcomatous, and poorly differentiated), it can be demonstrated that the differentiation of this specialized surface organelle is quite variable even in well-differentiated lesions. The presence of only a few, scattered, short microvilli does not exclude a diagnosis of epithelial mesothelioma, particularly if historical, surgical, and radiologic findings support this diagnostic conclusion. Indeed, even the complete absence of surface microvilli is compatible with a diagnosis of diffuse epithelial mesothelioma. It is important to become aware of the spectrum of tumor cell differentiation in serosal tumors, as all of the fine structural diagnostic criteria in mesotheliomas are expressed to varying degrees in individual cases.

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We report the clinicopathologic features of a 22-year-old patient with aneurysmal bone cyst and fibrous dysplasia of the orbit. The patient was evaluated clinically with computed tomography of the orbit before surgery. An orbital biopsy specimen was examined histologically with conventional light microscopy. The lesion was treated with combined neurosurgical and orbital intervention. Clinical evaluation revealed axial and inferior displacement of the globe. Computed tomography revealed a cystic mass in the superotemporal left orbit with adjacent bone erosion. "Ground-glass" thickening of the adjacent frontal bone and sphenoid bone was observed. Microscopic examination showed fibrous stroma with giant cells and hemosiderin-laden macrophages with adjacent trabeculae of woven bone and osteoblast cells. A fronto-orbital craniotomy was performed, the cystic cavity was excised, and the hyperostotic bone was debulked. After treatment, the globe position and patient appearance have improved. There has been no sign of recurrence of the aneurysmal bone cyst. Aneurysmal bone cyst should be considered in patients with fibrous dysplasia that has a cystic component, or in patients with fibrous dysplasia who present with sudden expansion of their lesion.