Mucoepidermoid carcinoma arising in Warthin's tumor of the parotid gland: Clinicopathological characteristics and immunophenotypes.

Mucoepidermoid carcinoma (MEC), an extremely rare tumor, arises from the epithelial component of preexisting parotid Warthin tumors (WT). Among the 309 cases of surgically resected WTs in Chinese PLA General Hospital and Beijing Shijitan Hospital of Capital Medical University, 5 cases (1.6%) fulfilled the criteria for MECs transformed from WTs. Clinicopathological characteristics of MECs was demonstrated in order to avoid misdiagnosis of this rare type of tumor. All the 5 patients, 3 males and 2 females, presented painless masses in the parotid gland. MECs were located inside or at the edge of WTs, with an obvious transitional zone between WT and MEC. Basal cells of WTs and epidermoid cells of MECs were strongly positive for cytokeratin CK5/6, CK34βE12, and P63; whereas negative for CK7, CK20, and CEA. Mucous cells of MECs were positive for CK7, CEA, as well as periodic acid-Schiff (PAS), whereas negative for CK5/6, CK34βE12, CK20, and P63. MECs patients were followed up for 25-69 months after surgery, presenting no evidence of recurrence or metastasis. Collectively, MECs arising from WT is very rare. The pathological diagnosis was based on histological morphology, especially the transitional zone between WT and MEC.