Proliferating thymolipoma: ultrastructural, immunohistochemical, and flowcytometric study

A superior-anterior mediastinal tumor was excised from a 50-year-old man. The 207-g mass was encapsulated and multilobulated. It contained adipose tissue and abnormal thymic tissue. In some areas the thymic tissue was characterized by cords and nests of epithelial cells lying within either the adipose tissue or a myxoid matrix. Other areas were characterized by cortical thymic tissue with increased numbers of epithelial cells. Foci of normal medullary tissue were present. The prominent epithelial cells were immunoreactive for cytokeratin and nonimmunoreactive for vimentin, S-100, chromogranin, and parathyroid hormone. Flow cytometry showed that the lymphocyte populations were consistent with a late cortical thymic phenotype. The tumor was diploid. By electron microscopy, the prominent epithelial cells had desmosomes and a few tonofilaments. The cytoplasm contained additional organelles including mitochondria, polyribosomes, and occasional lysosomes. Nuclei were oval and had relatively smooth contours, prominent nucleoli, and moderate quantities of heterochromatin. Basal lamina was present around many nests and cords of cells. This is the first such study of a tumor with this histology.