Mesothelioma is a malignant tumour mainly of pleural origin. Other sites of origin include the peritoneum, pericardium and tunica vaginalis testis.(1)
Most cases, but not all cases are related to asbestos exposure.
Incidence
Chart 1 (above): Death certificates mentioning mesothelioma in the UK from 1968-2009(2)
Age: The long latent period of 15 – 60 years means that mesothelioma presents most commonly between the ages of 50 and 70 years.
Sex: From 1968 to 2009 in the UK the ratio of males to females affected was 17:3.(2)
Geography
There is no racial predilection for mesothelioma.
In the UK and USA it can be seen that the location of industries exposing workers to asbestos determines the geographical variations in incidence.
In Great Britain the areas with the highest incidence of mesothelioma are those areas with a history of shipbuilding.(3) Areas associated with railway engineering also show high SMRs.(3)
Predisposing factors
Asbestos is a fibrous mineral that has been mined on a commercial basis since 1868.
There are two types of asbestos: amphiboles and serpentines.
The amphiboles include:
- Crocidolite (blue asbestos)
- Amosite (brown asbestos)
- Anthophyllite
- Actinolite
- Tremolite
The only serpentine is chrysotile (white asbestos).
The amphiboles are needle-like in shape and have a diameter < 3 μm. They have a high length to diameter ratio and are resistant to dissolution. These properties allow the fibres to penetrate deeply into the lungs.
Chrysotile has a serpentine configuration and a smaller length to diameter ratio. It tends to form clumps and therefore penetrates the lung less well.
There is debate about the risk posed by chrysotile. It is thought by some that this risk is due to tremolite impurities that are not removed during the processing of the chrysotile.(4) This theory is not accepted universally and although the amphiboles are thought to pose the greatest risk, chrysotile is still thought to present a risk of mesothelioma.
Mesothelioma may be induced by erionite fibres found in rocks in Turkey and the USA(5) amongst other places.
The occupations associated with the highest risk of developing mesothelioma were (highest risk first):(6)
Coach & vehicle body builders
Metal plate workers, shipwrights, riveters
Plumbers, heating & ventilating engineers & related trades
Carpenters & joiners
Construction & related operatives
Macroscopic and microscopic appearance
Macroscopically the tumour is greyish and lobulated.
There are three histological types:
• Epithelial 60% (7)
• Sarcomatous 15% (7)
• Mixed 25% (7)
The epithelial type has the best prognosis. Histologically it may appear similar to adenocarcinoma. The sarcomatous type appears similar to true sarcomas.
Investigations
Figure 1 (above): Lobulated pleural thickening on the left.
Radiological investigations may show nodular pleural thickening, pleural effusion, encasement of the hemithorax with reduction in volume, unilateral opacification of the hemithorax with increased volume and rib splaying, chest wall, mediastinal and nodal invasion.
Treatment
There are several options:
• Supportive care
• Surgery
• Chemotherapy
• Radiotherapy
• Trimodal therapy (surgery, chemotherapy and radiotherapy)
Prognosis
Prognosis is poor. The median survival varies from 8-14 months in different studies. Over 90% of patients are dead within two years.(8) Long-term survivors are usually those with an epithelioid tumour that was resected at an early stage. Stephen Jay Gould the evolutionary biologist survived 20 years after having a peritoneal mesothelioma surgically removed.
References
1. Chekol SS, Sun C-C. Malignant mesothelioma of the tunica vaginalis testis: diagnostic studies and differential diagnosis. Arch. Pathol. Lab. Med. 2012;136(1):113-117.
2. HSE. meso01.xls. Death certificates mentioning mesothelioma. Available at: http://www.hse.gov.uk/statistics/tables/meso01.xls. Accessed February 10, 2012.
3. Anon. Mesothelioma mortality in Great Britain: An analysis by geographical area. Available at: http://www.hse.gov.uk/statistics/pdf/mesojune08.pdf. Accessed February 10, 2012.
4. Churg A. Chrysotile, tremolite, and malignant mesothelioma in man. Chest. 1988;93(3):621-628.
5. Carbone M, Baris YI, Bertino P, et al. Erionite exposure in North Dakota and Turkish villages with mesothelioma. Proc. Natl. Acad. Sci. U.S.A. 2011;108(33):13618-13623.
6. HSE. HSE - Science and research - RR696: Occupational, domestic and environmental mesothelioma risks in Britain. 2009. Available at: http://www.hse.gov.uk/research/rrhtm/rr696.htm. Accessed February 10, 2012.
7. Dahnert W. Radiology Review Manual. 6Rev Ed. Lippincott Williams & Wilkins,US; 2007.
8. British Thoracic Society Standards of Care Committee. Statement on malignant mesothelioma in the United Kingdom. Thorax. 2001;56(4):250 -265.