According to The American Heritage® Stedman's Medical Dictionary, carcinoma ex-pleomorphic adenoma is "a carcinoma developing in a benign tumor of a salivary gland and characterized by rapid growth and pain."
So why all of the complicated words?
a malignant and invasive epithelial tumor that spreads by metastasis and often recurs after excision; cancer.*
of, pertaining to, or characterized by pleomorphism - existence of an organism in two or more distinct forms during the lifecycle.*
a benign tumor of glandlike structure.*
a benign, slow-growing, epithelial tumor of the salivary gland, usually of the parotid gland, sometimes serving as a locus for development of a malignant epithelial neoplasm.**
Therefore "ex-pleomorphic adenoma" means that what was once a benign tumor or pleomorphic adenoma has turned malignant.
* According to Dictionary.com Unabridged, based on the Random House Dictionary, © Random House, Inc. 2013.
** According to Medical-Dictionary.TheFreeDictionary.com, Dorland's Medical Dictionary for Health Consumers. © 2007 by Saunders, an imprint of Elsevier, Inc. All rights reserved.