Definition

A histologically benign pleomorphic adenoma that inexplicably manifests local or distant metastasis.

 ICD-O code 8940/1

 Synonyms

Metastasizing benign mixed tumour, malignant mixed tumour.

Epidemiology

To date, approximately 40 cases have been described {406,2768}.

Etiology

 It has been postulated that multiple recurrences and surgical procedures allow some tumours to gain venous access and metastasize.

 Localization

Greater than three-quarters arise in the parotid gland, 13% in the submandibular gland and 9% in the palate.

 Macroscopy

Tumours are well-circumscribed in primary and metastatic sites.

 Histopathology

Characteristically, the primary salivary gland tumour and metastases are composed of the typical mixture of benign appearing epithelial and mesenchymal components of a pleomorphic adenoma. The histology is not predictive regarding its ability to metastasize. Mitotic figures and nuclear pleomorphism may be seen, but the tumour is not overtly histologically malignant.

Prognosis and predictive factors

The treatment of choice is surgical excision. Metastasizing pleomorphic adenomas are characterized by multiple local recurrences and a long interval (1.5-55 years) between development of the primary tumour and its metastasis. Half of the tumours metastasize to bone, 30% to lung and 30% to lymph nodes; rarely tumours spread to other body sites. Forty percent of patients died with disease; 47% were alive and well, and 13% were alive with disease {899}.