Mediastinal Thymolipoma with Striated Myoid Cells: Report of a Peculiar Case

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Thymolipoma is a rare anterior mediastinal tumor composed of mature adipose tissue and benign thymic tissue, and is a benign neoplasm for which complete surgical excision is curative. 1 Literature has reported that the thymic parenchyma of thymoli-poma exceptionally contain myoid cells that are immunoreac-tive for desmin, muscle-specific actin and myoglobin. 2,3 Herein, we describe a very rare case of thymolipoma with myoid cells. To the best of our knowledge, this is the sixth case of thymoli-poma containing striated myoid cells in the English literature since 1972. A 54-year-old man had a large asymptomatic anterior medi-astinal mass incidentally discovered by chest radiograph during a regular health checkup. He had no history of autoimmune diseases including myasthenia gravis, aplastic anemia, hypo-gammaglobulinemia, lichen planus, or Graves' disease. A computed tomography scan of the chest demonstrated a large medi-astinal mass consisting of a mixture of fat and soft tissue contents , radiologically suggesting thymolipoma (Fig. 1). The tumor was surgically removed. Grossly, it was fairly well-circumscribed by a thin fibrous capsule, and measured 12.0× 10.0×3.5 cm. The outer surface was soft, yellow fatty tumor with focal solid areas (Fig. 2). Histologically, the mediastinal mass was characterized by the presence of abundant mature adi-pose tissue admixed with areas containing remnants of thymic tissue (Fig. 3A). The thymic tissue component varied from strands of atrophic thymic epithelium to large areas containing thymic parenchyma with medullary epithelial hyperplasia (Fig. 3B, C). In areas, the tumor showed a frequent occurrence of po-lygonal, striated myoid cells with abundant eosinophilic cyto-plasm (Fig. 3D). These cells showed positive immunoreaction for desmin in immunohistochemical study (Fig. 3E, F), but negative reaction for alpha smooth muscle actin (1:400, Dako). The myoid cells also lacked cytological atypia and mitotic figures. DISCUSSION Thymolipomas are rare, benign mesenchymal tumors of the mediastinum that are often asymptomatic. 1 Thymolipomas may adhere to the adjacent structures and displace organs within the chest cavity, but invasion into adjacent structures has not been documented in the literature. The encapsulated and lobular nature of thymolipomas and the lack of invasion into adjacent structures usually allow for a relatively uncomplicated surgical excision of the tumor. Regarding the unique appearance of my-oid cells in the …