Mediastinal Thymolipoma with Striated Myoid Cells: Report of a Peculiar Case
A 54-year-old man had a large asymptomatic anterior mediastinal mass incidentally discovered by chest radiograph during a regular health check-up. He had no history of autoimmune diseases including myasthenia gravis, aplastic anemia, hypogammaglobulinemia, lichen planus, or Graves' disease. A computed tomography scan of the chest demonstrated a large mediastinal mass consisting of a mixture of fat and soft tissue contents, radiologically suggesting thymolipoma ().\nThe tumor was surgically removed. Grossly, it was fairly wellcircumscribed 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 (). Histologically, the mediastinal mass was characterized by the presence of abundant mature adipose tissue admixed with areas containing remnants of thymic tissue (). The thymic tissue component varied from strands of atrophic thymic epithelium to large areas containing thymic parenchyma with medullary epithelial hyperplasia (). In areas, the tumor showed a frequent occurrence of polygonal, striated myoid cells with abundant eosinophilic cytoplasm (). These cells showed positive immunoreaction for desmin (1:200, clone D33, Dako, Glostrup, Denmark) and myoglobin (1:800, RB-9068-R7, Thermo, Fremont, CA, USA) in immunohistochemical study (), but negative reaction for alpha smooth muscle actin (1:400, Dako). The myoid cells also lacked cytological atypia and mitotic figures.