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The World Health Organization (WHO) classification differentiates between pleural tumors of mesothelial and mesenchymal origin as well as lymphoproliferative disorders, with malignant mesotheliomas forming the most common pleural primary tumor. Histologically, epithelioid (40-60 %), sarcomatoid (20-40 %), and biphasic mesotheliomas (20-40 %) are distinguished. The certain morphological diagnosis of a malignant pleural mesothelioma requires the establishment of mesothelial differentiation by means of an appropriate panel of antibodies to exclude pleural dissemination of a pulmonary or extrapulmonary epithelial malignancy and also requires the establishment of at least focal invasive growth to distinguish from reactive mesothelial proliferation. The exclusion of a malignant pleural mesothelioma may induce further differential diagnostic considerations, e. g. concerning the assignment to a certain primary tumor after the establishment of carcinomatous pleuritis.

The definitive diagnosis of malignant mesothelioma (MM) in effusion cytology is often avoided or reluctantly made by cytology alone. The most probable reason for this skepticism is the lack of expertise in cytology among many pathologists and clinicians. When an effusion specimen is composed of cells with unequivocal cytological features of malignancy that have the morphology and immunophenotype of mesothelial cells, the cytological diagnosis of MM is straightforward. However, in the daily routine difficult cases of atypical mesothelial cells are often encountered and additional methods are required to establish an accurate diagnosis. In contrast to reactive mesothelial cells cells of MMs often harbor chromosomal aberrations, most frequently a polysomy in combination with a 9p21 deletion. These chromosomal aberrations can easily be detected by multitarget fluorescence in situ hybridization (FISH); therefore, FISH allows a reliable distinction between reactive mesothelial cells and MM cells. In order to be able to discriminate between MM and adenocarcinoma, an immunocytochemical panel consisting of different mesothelial and epithelial markers is very helpful. In most inconclusive cases of atypical mesothelial cells the combination of morphology, immunocytochemistry and FISH allows a better distinction between reactive mesothelial cells and MM in effusion cytology.

Of a total of 4455 persons with mesotheliomas listed in the German register of mesotheliomas, 72 patients have an academic education in whom exposure to asbestos would not necessarily be assumed at first. Only one patient in this group was female. The tumors were located in the pleura in 94% ( n=68) and in the peritoneum in 6% ( n=4). The mean age at the time of diagnosis was 59.1 years. The mean survival time was 10.2 months. The epitheloid subtype predominated (57.7%, n=41) followed by the biphasic subtype (33.8%, n=24). Development of the sarcomatoid subtype was the least common (8.5%, n=6). Lung dust analysis provided proof for elevated pulmonary asbestos load in 78.2% of the patients. Work-related or non-work-related exposure to asbestos was cited or at least suspected by 68% ( n=49) of the patients. The mean latency period was 36.8 years. Thus, in those patients with an academic career the majority of mesotheliomas can be considered associated with asbestos.