Pleural metastatic tumours and effusions. Frequency and pathogenic mechanisms in a post-mortem series

We examined the post-mortem records of 191 patients who had one or more malignant tumours (196 neoplasms in total). We found 55 cases with pleural metastases (28%) and 30 of these presented pleural effusions (15% of the total number of neoplasms). The visceral pleural was involved in all 24 cases of lung cancer with pleural metastasis and in 27 out of 31 of those of other origins. The parietal pleura was affected in 16 out of 24 cases of the lung tumours and in 15 out of 31 of those of other origins. There were no cases in which only the parietal layer of the pleura was involved, with the only exception of neighbouring tumours. We found neoplastic vascular invasion in 43 out of 55 cases, retrograde lymphatic spread from the mediastinum in two cases, and direct pleural involvement from a neighbouring tumour in the remaining. We conclude that malignant pleural implantation fundamentally arises from the spread of tumour emboli to the lungs and the visceral pleura, with secondary seedings to the parietal pleura. Therefore, this phenomenon leads to the usual finding of lower involvement of parietal pleura as compared to the visceral pleura in our series.