Different patterns of malignant change occur in pleomorphic adenoma, of which carcinoma ex pleomorphic adenoma is one form; the other 2 forms are true malignant mixed tumor (carcinosarcoma) and metastasizing pleomorphic adenoma.
In addition to the 3 forms listed above, in 2007, an extremely rare case of adenoma characterized by pure sarcomatous change with no epithelial carcinomatous component was reported from the MD Anderson Cancer Center and was named high-grade sarcoma ex pleomorphic adenoma.
Among these types of adenoma, carcinoma ex pleomorphic adenoma is the most commonly encountered; it was first described by Beahrs et al in 1957.
It is defined as a carcinoma that arises in the epithelial and/or myoepithelial component of a pleomorphic adenoma. In most instances (75%), the luminal epithelial cells undergo malignant change. In 19% of cases, a dual epithelial/myoepithelial differentiated carcinoma is seen. Pure myoepithelial malignant change is seen in only 6% of cases.