Pleural malignant mesothelioma with invasive micropapillary component and its association with pulmonary metastasis

The micropapillary pattern (characterized by papillary structure with tufts lacking a central fibrovascular core) is a predictor of aggressive carcinoma. The purpose of the present study was to review 34 pleural malignant mesotheliomas (21 epithelioid, five sarcomatoid, seven biphasic and one lymphohistiocytoid), with special reference to the presence of invasive micropapillary component. Two invasive micropapillary pattern‐positive tumors were identified. The invasive micropapillary pattern was seen to have a focal distribution in 15–20% of the tumor tissues. The majority of the invasive micropapillary clusters expressed MUC1 along the outer cell surface. Analysis of pleural malignant mesotheliomas with epithelioid features and with or without invasive micropapillary pattern (21 epithelioid and seven biphasic subtypes) indicated pulmonary micrometastases in only the invasive micropapillary‐positive tumors (P < 0.015), and the spread was probably via the lymphatics. Lymphatic involvement (confirmed on immunohistochemistry with D2‐40 antibody) and lymph node metastasis were found in both of the invasive micropapillary‐positive tumor patients, whereas they were noted in only one of 10 (10%, P < 0.046) and three of nine (30%) invasive micropapillary‐negative patients. To the authors’ knowledge this is the first study to indicate the presence of invasive micropapillary component in pleural malignant mesothelioma. This component can predict more aggressive lymphatic spread, similar to that of carcinomas in other organs with micropapillary pattern.