Primary pulmonary sarcomatous and mixed epithelial-mesenchymal neoplasms

The vast majority of primary pulmonary neoplasms arise from the epithelial cells of the airway conductive system. The high proportion of epithelial neoplasms in the lung is easily explained by the large surface area of the lung and the continued renewal of its epithelial lining and the exposure to carcinogens. In contrast, the supporting tissues only make up a small fraction of the lung parenchyma and their renewal turnover is low. This is especially relevant in the interstitial alveolar tissue of the gas-exchange compartment where the amount of connective tissue is minimal. Thus, primary mesenchymal tumours in the lung are very rare and most commonly arise in the larger airways. Most tumours with morphological mesenchymal phenotype (pure or mixed) are likely to be of epithelial derivation.