In an autopsy series of 191 patients who had malignant tumors, 55 (28%) were found to have pleural metastases and 30 (15%) had pleural effusions.[2] In all 24 cases with lung cancer, the visceral pleura was involved with tumor, whereas in those with extrapulmonic primaries, 27 (87%) of 31 cases had visceral pleural involvement. Of the 24 cases of lung cancer, 16 (67%) had parietal pleural tumor involvement, while in the 31 cases of extrapulmonic origin, 15 (48%) had parietal pleural involvement. There were no cases where the parietal pleura alone was involved except when there was direct extension of the tumor. Neoplastic vascular invasion was seen in 43 (78%) of the 55 cases. Retrograde lymphatic spread from the mediastinum was seen in 2 cases and direct pleural involvement from a peripheral tumor in 10. In agreement with Meyer,[3] tumor emboli in the lungs and their spread to the visceral pleura is an important mechanism in pleural metastases with subsequent involvement of the parietal pleura.
A series of processes is required for pleural metastasis to develop.[4,5] Several sequential events lead to pleural seeding and independent tumor growth at the focused site. The malignant cell must initially detach from the tumor and then must attach to and migrate through the blood vessel wall. Next, there must be vascular migration to the visceral pleural surface. Autocrine growth factors must be present to keep the cell viable and, lastly, angiogenesis is necessary to potentiate both local growth and spread of the tumor cells.
Pleural metastases result in progressive pleural effusions from several mechanisms, one of which includes increased vascular permeability due to angiogenic factors, such as vascular endothelial growth factor (VEGF),[6] which not only causes new vessel formation but an alteration of the permeability of the mesothelium. A blockage of lymphatic drainage at any point from the stoma of the parietal pleura to the mediastinal lymph nodes will cause pleural fluid to accumulate and is an important mechanism in pleural fluid accumulation in malignancy