[Differential diagnosis of preneoplastic lesions of the pleura and of early mesothelioma: immunohistochemical and morphological findings].

The morphological diagnosis of small mesothelial lesions in pleural biopsies is gaining importance in view of more aggressive, multimodal therapeutic options and of medicolegal aspects if a malignant mesothelioma is diagnosed. We present a light microscopically and immunohistochemically based morphological classification for the planning of further clinical follow-up procedures. A reactive mesothelial hyperplasia heals without scars. Mesothelial inclusions in pleuritic scars are common in recurrent pleuritis and must not be confused with an epithelioid component of a desmoplastic mesothelioma. In case of atypical mesothelial proliferations, further diagnostic procedures have to be performed to obtain a clear diagnosis of malignancy. Mesothelioma in situ is the first morphological step in neoplastic mesothelial changes, also with regard to medicolegal aspects for the unambiguous diagnosis of a mesothelioma. Early infiltrative growth is characteristic of so called early mesothelioma. A useful immunohistochemical panel for the differential diagnosis consists of anti-cytokeratin, Ck 5/6, calretinin, EMA and MiB-1, whereas the immunohistochemical detection of telomerase is not helpful.