Aspiration cytology of an enlarged thymus presenting as a mediastinal mass. A case report.

Fine needle aspiration (FNA) cytology was used to evaluate an anterior mediastinal mass in a 12-year-old girl. The smears showed two cellular populations composed of epithelial cells and many lymphocytes, simulating FNA cytologic features of a thymoma. However, histopathologically the mass proved to be an enlarged and microscopically normal thymus. The different shapes and sizes of the epithelial cell nuclei and lymphocytes encircling the individual epithelial cells present in the FNA smears of the enlarged thymus were features that are not seen frequently in FNA smears of thymomas and therefore may be differentiating points. However, studies of FNA smears from more cases of thymoma and thymic hyperplasia are necessary in order to define FNA cytologic criteria to differentiate these two entities.