Tumors of the pleura.

Tumors of the pleura may be primary or secondary. Primary tumors are exceedingly rare while secondary tumors are not uncommon. The most important and only primary malignancy of the pleura is the mesothelioma. In the 19th century the term, “tubercle-like lymphadenoma,”6 was used to describe this tumor. Later the name, “endothelioma,”7 was proposed for this malignancy because it was ascribed to a vascular origin. Recent reliable investigations together with the fact that the pleura is a mesodermal derivative have established the term mesothelioma as a more descriptive and appropriate identification of this tumor. Epithelial and mesenchymal elements are both present in the pleura. The free surface Is lined by a single layer of mesothelial cells which rest on a delicate elastic membrane. Beneath the membrane is found the loose connective tissue stroma. This is composed of collagenous and elastic fibers dispersed at various angles but generally parallel to the free surface of the pleura. Fibroblasts and macrophages constitute the predominant cellular elements. The subserous layer of tissue is responsible for the close adherence of the pleura to the related underlying structures. Tumors arising in the pleura will, consequently, display both epithelial and mesenchymal characteristics. Mesotheliomas are ordinarily composed of large epitheliold cells separated by collagenous fibers of tumor cell origin. The cellular elements have a tendency to arrange themselves as alveolar rests or rows. Unfortunately, not all pleural mesotheliomas present this typical histopathologic pattern. Pleomorphism and multidirectional lines of evolution complicate the proper identification and classification of this new growth. This tendency to frequent deviation has been paralleled by equally numerous attempts by well-Intentioned investigators to prove the existence of more than one type of primary pleural malignancy. The problem of pleural malignancies, consequently, is clouded and confusing. Conclusive investigation and authoritative evaluation of this controversial problem have been limited by the paucity of cases available for study. Only one, and probably less than one, case in every one thousand postmortem examinations is a proved pleural mesothelioma.6 If primary malignancies of the pleura do occur, the weight of the available evidence supports the contention that they are all mesotheliomas with inherent possibilities of wide structural variation.