Metastatic benign pleomorphic adenoma. Report of a case and review of the literature.

Pleomorphic adenoma (PA), originally called mixed tumour, is the most common neoplasm of the salivary glands and is generally accepted as benign biologically. Occasionally PA may give rise to metastasis. The metastasis may develop in a PA in which a malignant transformation occurs, either arising a carcinoma in the PA (carcinoma ex-mixed tumour) or as a carcinosarcoma (so-called true malignant mixed tumour). However, very rare benign PA eventually metastasise, usually after having a previous recurrence, displaying benign histological features as well in the primary tumour as in the metastasis. These tumours have been termed metastatic PA or metastatic mixed tumours. The aim of this paper is to report one case of metastatic histological benign pleomorphic adenoma, and to consider the clinical, pathological and therapeutic consequences of these rare tumours as well as its possible causes and mechanisms for its behaviour.