Dedifferentiated epithelial-myoepithelial carcinoma of the parotid gland: a rare case report of immunohistochemical analysis and review of the literature.

Dedifferentiation of salivary gland neoplasms is a rare event, unlike bone and soft part sarcomas, which was first described by Stanley et al. in 1988. An additional case of dedifferentiated epithelial-myoepithelial carcinoma (EMC) is reported here. The patient was a 70-year-old Japanese man who requested examination of the rapid growth of a mass in the right parotid region, which he had first noticed 25 years previously. Clinical examination showed an ill-circumscribed, 6.8 x 4.7 x 7.0-cm lesion. Histologically, most parts of the lesion were high-grade carcinoma (HGC) with sheetlike and nestlike growth of markedly atypical cells and comedonecrosis, whereas the minor part consisted of typical EMC. The outer clear cells of EMC were positive for alpha-smooth muscle actin (ASMA), p63, cytokeratin (CK) 14, and vimentin, and the inner ductal cells of EMC were positive for CKs and epithelial membrane antigen. HGC was negative for ASMA, CK14, and vimentin, but diffusely positive for p53 protein and cyclin D1. The Ki-67 labeling index of EMC was 11.5%, whereas that of HGC was 67.1%. These findings and a review of literature indicate that HGC arose from preexisting EMC, and this phenomenon is the dedifferentiation of EMC. Dedifferentiated EMC is extremely rare.