Malignant mesothelioma eight years after a diagnosis of atypical mesothelial hyperplasia

The separation of mesothelial hyperplasia from early malignant mesothelioma remains one of the most difficult problems in histopathology. Inconclusive cases are termed "atypical mesothelial hyperplasia" and treated expectantl y. A 49 year old male pipeline engineer was diagnosed as having atypical me sothelial hyperplasia in appendiceal serosa by the US-Canadian Mesothelioma Panel. Eight years later, he developed overtly malignant peritoneal and pl eural mesothelioma. In hindsight, histological similarities between the dif fuse malignant mesothelioma and the atypical mesothelial proliferation sugg ested malignancy from the outset. The most important of these features were the degree of mesothelial proliferation, micronodularity, architectural co mplexity, superficial invasion, uniform mild cytological atypia, and the ab sence of a clinical cause for a benign mesothelial proliferation. Ancillary investigations including immunohistochemistry were of no benefit in determ ining whether the atypical mesothelial hyperplasia was benign or malignant. Careful histological examination remains the mainstay of the diagnosis of early mesothelioma.