Histologic features indicative of mesothelioma:
(1) definite evidence of invasion into underlying tissues
(2) presence of atypical cells throughout the full thickness of the pleura (absence of zonation)
(3) presence of tumor/bland necrosis (no or rare inflammatory cells)
(4) definite histologic features of malignancy (severe pleomorphism, atypical mitoses, etc)
(5) cellular tumor nodules showing expansion into the stroma
(6) complex branching tubules or glands
(7) papillary excrescences with true fibrovascular cores (atypical mesothelial hyperprlasia may have papillary tufts of cells without fibrovascular cores)
where:
• Immunoperoxidase staining has limited value in distinguishing atypical mesothelial hyperplasia from mesothelioma. A pankeratin stain may help to identify invading tumor cells.
• Simple, non-branching glands can be seen in atypical mesothelial hyperplasia, mesothelioma and metastatic carcinoma.
