Mesothelioma
Created | Updated Nov 7, 2005
Mesothelioma is a tumour of the pleura, which can be involved in both primary and secondary tumours, although the secondary metastatic involvement is the most common, from metastasis from primary neoplasms normally originating from either the lung or breast. alternatively highly metastatic neoplasms can travel from distant origins and implant at the pleura and hence lead to a mesothelioma.
gross structure of the lungs.
the lungs are applied to the inner chest wall surface by the plural membranes, between which is the plural fluid, holding the lungs firmly to the inside aspect of the chest wall, both the chest wall and the lungs are elastic, containing elastic fibres, those in the lung tend to collapse the lung whilst those in the chest wall are pulling outwards. The pleura, and the pleural fluid are important in reducing friction in the lung tissue as the lungs expand and deflate during inspiration. The most common clinical presentation of pleura related pathology is pleurisy; inflammation of the pleura (pleuritis), typically caused by bacterial infection of the pleural fluid laying between the two pleural membranes.
The lungs are enclosed in a “box”, at the base of which is the diaphragm; a muscular membrane separating the thoracic and abdominal cavities, connected to the inner wall by tendons, important in inspiration and expiration.
the 12 pairs of ribs provide protection and the method for inspiration, the intercostal muscles are vital in there relation to the ribs to provide a means of changing the thorax volume. there are 2 sets of internal intercostal muscles, the intercostalis intimus, nearest to the lungs and the intercostalis intermus on the inside of the ribs, being a bigger set. the single external set of intercostal muscles, the intercostalis externus are furthest from the lungs on the outer side of the ribs.
Benign mesothelioma.
This may also be termed a benign pleura fibroma.
This form of mesothelioma has no relationship to asbestos exposure, being more easily treated and hence much less severe than the malignant state.
Benign mesothelioma is a localised growth that is often attached to the pleural surface by a pedicle. The tumour may be small- 1-2 cm in diameter, or may reach a far larger size, however, in all cases the tumour remains confined to the surface of the lungs, no pleural effusion is produced.
On gross examination a Benign mesothelioma consists of dense fibrous tissue with occasional cysts containing viscous fluid. Microscopically they demonstrate walls of reticular and collagen fibres amongst which spindle cells resembling fibroblasts are distributed, hence benign mesothelioma is often termed fibroma.
Malignant mesothelioma.
The Malignant mesothelioma can arise from either the visceral or the parietal pleura. The most common cause of primary malignant mesothelioma is exposure to asbestos, statistically approximately 1,500 cases of malignant mesothelioma occur annually, and 80% of these are caused by asbestos exposure. Although mesothelioma is most common on the pleura it may also occur on the peritoneum and the tunica vaginalus.
On gross examination of malignant pleural mesothelioma the lung is seen to be encased by a dense pleural tumour that extends into the fissures and the interlober septa. It is a diffuse lesion that spreads widely in the pleural space, being associated with extensive pleural effusion and with direct invasion of thoracic structures, effected lung is covered in a thick layer of soft gelatinous greyish-pink tumour tissue. Lobules of white tissue encapsulate the lung, and a diffuse band of similar tissue, completely surrounds the lungs surface. The thickness is usually most extreme at the bottom and lower portions, particularly the right.
Microscopically classic mesothelioma exhibits a biphasic appearance namely epithelial and sarcomatus patterns, glands and tubules that resemble adeno-carcinoma are admixed with sheets of spindle cells that are similar to fibro sarcoma.
In histology the cells of the mesothelioma have several nuclei, which appear large, in excess and variable in nature and which demonstrate many abnormalities. In the malignant mesothelioma many cells are seen to be undergoing cell division, representing the growth of the tumour.
Patients with pleural mesothelioma present with a pleural effusion or with a pleural mass, chest pain and dyspnoea. The pleural effusion is, however usually only present when the tumour is malignant; benign mesothelioma does not produce pleural effusions in the majority of cases.
Pleural effusion.
An abnormal accumulation of excess fluid in the pleural cavity, varying from a few mil, to a massive accumulation that shifts the mediastinum and trachea to the opposite side.
The lesion may be limited to the thorax but in about 25% of the cases the metastasis appears elsewhere. treatment is ineffective and the prognosis is poor.
Patients with mesothelioma are usually more than 50 years old and the neoplasm appears with a latency period of 20 years or more, therefore often appearing in asbestos workers many years after exposure.
Long, thin asbestos fibres have the greatest carcinogenic potential. They may be capable of penetrating from central airway to the peripheral lung and pleura. It is unclear whether ongoing inflammation, a feature of many asbestos related diseases, is important for the pathogenesis of mesothelioma as with asbestosis, but it is probable that both play a role.
