The proportion of benign versus malignant components can be quite variable. Occasionally, extensive sampling is necessary to find the benign component and in rare cases, a benign remnant might not be found. But if there is clinicopatho-logic documentation of a previously excised pleomorphic adenoma in the same site, then the malignancy can also be classified as a Ca-ex-PA. The malignant component is most frequently a poorly differentiated adenocar-cinoma (salivary duct type or not otherwise specified) or an undifferentiated carcinoma; however, virtually any form of carcinoma may be found {898,1338, 1491}. An infiltrative, destructive growth pattern is the most reliable diagnostic criterion. Nuclear hyperchromasia and pleomorphism are frequent, although occasional tumours may demonstrate minimal atypia. This latter feature (tumour grade) directly correlates with prognosis. Necrosis is often present and mitoses are usually easy to find.

Ca-ex-PAs should be subclassified into non invasive, minimally invasive (<1.5 mm penetration of the malignant component into extra capsular tissue) and invasive (>1.5 mm of invasion from the tumour capsule into adjacent tissues), as the first two groups usually have an excellent prognosis while the latter has a more guarded prognosis. The distinction between noninvasive and invasive tumours is based on destructive invasion through the capsule into peritumoral tissues.

Non-invasive Ca-ex-PAs are also referred to as carcinoma in-situ arising in a pleo-morphic adenoma, intracapsular carcinoma ex pleomorphic adenoma or pleo-morphic adenoma with severe dysplastic changes. Atypical changes within these tumours range from focal to diffuse often u. , ■ iTviaf; O v .V- .. £ • mhstps* V ;

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