Diagnosis and Discussion:
The ultrastructural features, particularly the nuclear characteristics
of the larger, partially fragmented tumor cells and the numerous
desmosmes, which are sometimes in a row, confirm that the diagnosis
of an anterior mediastinal thymoma is indeed correct.
The case illustrates one of the limitations
of immunohistochemistry and a situation that resulted in a false
negative immunostain with cytokeratin antibodies. Probably as
a result of somewhat excessive aspiration pressure during the
FNAB, the more senstive epithelial cells ruptured. This dispersed
the cytoplasmic contents, including intermediate filaments, into
the supernatant fluids, which were lost during the preparation
of smears and the cell block. Cellular fragments, including cell
membranes and their the desmosmes, however, were retained and
these had features typical for the epithelial cells of thymoma.
Immunohistochemistry is not sufficiently sensitive to visualize
intermediate filaments associated with such desmosomes.
