MYOEPITHELIAL CARCINOMA ARISING IN A BENIGN MYOEPITHELIOMA - IMMUNOHISTOCHEMICAL, ULTRASTRUCTURAL, AND FLOW-CYTOMETRIC STUDY

A case of myoepithelial carcinoma arising in a benign myoepithelioma o f the minor salivary gland in a 71-year-old patient is reported. The t umor presented initially on the palate and had been diagnosed as ''ben ign lesion'' 40 years before. It recurred 22, 36, and 40 years after i nitial presentation, and a similar histopathological diagnosis was ren dered. One year after the last recurrence, the tumor recurred showing typical changes of malignant transformation, and the diagnosis was mal ignant myoepithelioma. The light microscopy and ultrastructural featur es of the initial tumor were typical of plasmocytoid myoepithelioma. T here were abundant round cells and rare spindle cells with uniform dis persed filaments, sometimes arranged in parallel streams without evide nce of dense bodies. These cells showed micropinocytotic vesicles alon g the cell membrane with poorly developed intercellular junctions and were surrounded by a basal membrane. The malignant counterpart showed fewer plasmocytoid cells and a rather epithelial pattern with marked n uclear pleomorphism and formation of small, or rarely large, glandular lumina. The immunohistochemical features were similar for the benign and malignant tumors, with positivity for S-100 protein, vimentin, cyt okeratins, and CAM 5.2, and were negative for GFAP, muscle-specific ac tin, CEA, and desmin. Flow cytometry showed a change in the DNA conten t profile. The benign myoepithelioma had a diploid DNA content with a low S-phase fraction of 3.9% and proliferative index of 9.1%, while th e myoepithelial carcinoma had an evident aneuploid DNA stem line and a n increased S-phase fraction of 8.3% with a proliferative index of 18. 1%.