Two malignant pleural mesotheliomas with unusual histological features.

Morphologically, mesotheliomas may be composed of epithelial and/or sarcomatous elements with various patterns, such as tubular, papillary, tubulopapillary, and diffuse epithelial or mixtures of these. Two cases are descirbed in which, in addition to typical mesothelioma, there was cartilage with foci of calcification and ossificated with the mesothelioma, which suggested that they formed an integral part of the tumour. One of the cases also showed a cuff of cartilage and bone round blood vessels and bronchioles in the lung parenchyma. The patholgenesis could be explained if the mesothelial cell is considered to be totipotent and able to give rise to epithelial and connective tissue elements. Other theories that must be considered are: that there are two separate neoplasms; that there is a circulating substance, perhaps induced by the mesothelioma, which stimulated the cartilage and bone formation; and that the cartilage and bone were due to a previous or associated infection such as tuberculosis. Calcification is also common in asbestotic pleural plaques.