Mucoepidermoid carcinoma arising in Warthin's tumour of the parotid gland: report of two cases with histopathological, ultrastructural and immunohistochemical studies.

Abstract

Malignant transformation of Warthin's tumour (WT) is a rare event. We present two cases of mucoepidermoid carcinoma (MEC) arising in WT in the parotid gland. Two cases of MEC arising in WT, which were found in 185 cases of WT of the parotid gland, were investigated by light and electron microscopy, and immunohistochemistry. Both cases had largely similar macroscopic and histological features with some differences. Histologically, the tumours consisted mainly of WT with multilayered hyperplastic arrangements of oncocytic cells and focal squamous and goblet cell metaplasia. In the same tumour mass, however, the features of MEC were observed with invasion to adjacent adipose tissue. A transitional zone between WT and MEC was evident. Both patients were alive and well without evidence of recurrence 30 and 34 months after surgery, respectively. Electron microscopy revealed that cyst lining epithelial cells in WTs had abundant mitochondria whereas some of the MEC cells contained numerous tonofilaments and mucinous granules. Immunohistochemically, oncocytic cells of WTs were strongly positive for mitochondria and SalylTn was extensively stained in MECs. The labelling index for Ki67 was obviously higher in the carcinoma cells than the epithelial cells of WT. Our cases confirmed the possibility of malignant transformation of the epithelial component in WT to MEC.