Immunohistochemical positivity for neuron-specific enolase and Leu-7 in malignant mesotheliomas

The discovery of immunostaining for neuron-specific enolase (NSE) and Leu-7 in a small cell mesothelioma prompted us to study some putative immunohistochemical markers of neuroendocrine differentiation in malignant mesotheliomas and to examine any diagnostically important immunohistochemical distinctions or similarities between malignant mesothelioma and other histologically similar lung tumours. Most mesotheliomas were positive for NSE (96 per cent) and Leu-7 (70 per cent) and positivity for these two markers was also found in small cell carcinomas (NSE 25 per cent, Leu-7 81 per cent) and adenocarcinomas (NSE 28 per cent, Leu-7 28 per cent) but carcinosarcomas were positive for only NSE (44 per cent). Chromogranin A positivity was found only in occasional small cell carcinomas (6 per cent) and adenocarcinomas (6 per cent). No tumour was positive for bombesin. The high incidence of NSE and Leu-7 positivity in mesotheliomas is an important original observation because it guards against the unjustified exclusion of mesothelioma from a differential diagnosis on the basis of positivity for these two markers.