Malignant myoepithelioma of salivary glands: clinicopathological features of ten cases.

Malignant myoepithelioma of the salivary gland is discussed in terms of its clinical behaviour, morphological features and the frequent pre-existence of a pleomorphic adenoma. The study comprised six female and four male patients aged 14-63 years (mean age 38.9 years). Two tumours presented as intraoral lesions and eight were located in the parotid gland. Tumour cells displayed a morphological spectrum ranging from round epithelioid cells to spindle-shaped and stellate cells. Most cells displayed reactivity for high molecular weight keratins and in four tumours there was strong immunoreactivity for smooth muscle actin. Malignant myoepithelioma seems to arise in two different clinical settings: either de novo or in a recurrent pleomorphic adenoma. De novo malignant myoepitheliomas arise in normal salivary gland, tend to be more aggressive and have a short clinical history. Recurrences may not develop or may occur as a single event within a short time interval, and metastases develop in the lungs. Malignant myoepitheliomas arising in recurrent pleomorphic adenomas have a long clinical history, are characterized by multiple recurrences and have to be distinguished from aggressive carcinomas arising in these adenomas. In contrast, the tumours described in the present series arising in pleomorphic adenomas showed local aggressiveness and metastases did not occur until decades after the first treatment. The general opinion that all malignant tumours that arise from pleomorphic adenomas are highly aggressive is not confirmed by the present study.