What is Mesothelioma?
Mesothelioma is a rare and aggressive form of cancer that is almost exclusively caused by asbestos exposure. Mesothelioma is specifically a cancer of the mesothelium – a membrane that forms the lining of multiple body cavities, most notably the pleural cavity (which contains the lungs), the abdominal cavity, and the pericardium (which contains the heart). Mesothelioma most often affects the pleural lining (the lining surrounding the lungs). When asbestos fibres are inhaled into the lungs, they become embedded in this lining, causing it to become inflamed. In other cases, asbestos can cause inflammation in the abdominal or cardiac mesothelial membranes. This inflammation irritates the cells of these membranes, causing genetic mutations that develop into tumors over time.[1]
Mesothelioma is one of the most aggressive forms of cancer. This is due, in part, to the location of tumour development. The mesothelium is located in the chest and abdominal cavities, which is close to many of the body’s vital organs. Therefore, when it metastasizes, it may affect these organs quickly.
There are three different forms of mesothelioma, depending on the cell type of the cancer. First, and most commonly, is epithelial mesothelioma, which accounts for 70-80% of all mesothelioma cases.[2] It occurs when the cells that become cancerous are epithelial cells—a type of cellular tissue that consists of sheets of cells that have a uniform size and shape. As it is the most common form of mesothelioma, it is the most widely studied. It is also much more responsive to treatment than the other types. The next type of mesothelioma is sarcomatoid mesothelioma. Here, the cancerous cells have random and irregular cell shapes, typically elongated and without a nucleus. This type occurs in approximately 10-20% of all mesothelioma cases.[2] This type of mesothelioma is often more difficult to diagnose, as it is more difficult to distinguish from healthy tissue and more difficult to biopsy. The last type of mesothelioma is biphasic, and accounts for 15-20% of mesothelioma cases.[2] This type contains a mix of epithelial and sarcomatoid cells. Characteristics of this type depend on the ratio of epithelial to sarcomatoid cells, with a larger proportion of sarcomatiod cells providing a less favourable prognosis. This cell type is more common among pleural (lung) patients.
Symptoms of Mesothelioma
Mesothelioma is a very difficult cancer to diagnose, which makes it very hard to catch at an early stage. One source of difficulty is that its typical symptoms are not specific enough to identify it clearly. These symptoms include chest or abdominal pains, unexplained weight loss, persistent cough, shortness of breath, and pneumonia-like symptoms.[4] None of these symptoms single out a diagnosis of mesothelioma among other diseases, especially since it is such rare cancer. For this reason, it is typically only diagnosed in late stages.[7]
What makes it even more difficult to diagnose is the latency period between exposure to the main risk factor, asbestos, and the onset of symptoms. The latency period between exposure and diagnosis may typically range from 10 to 50 years.[10] The median latency period is about 30 years, and disease very rarely occurs before 15 years.[3] Because of this, it can be very hard to predict if, and when, mesothelioma might develop. Furthermore, the long latency period may mean individuals may fail to make the connection between symptoms and their early exposure to asbestos.
The latency period can be affected by the degree of asbestos exposure, which varies based on the concentration of asbestos fibres in the air, the length of exposure, and the number of times a person is exposed.
Because asbestos-related diseases have such a long latency period, civil procedures can be difficult to prosecute because of rules around statute of limitations. Some countries, like Australia, have adjusted their statute of limitations regarding workplace legal issues to remove this barrier in order to better prosecute companies and corporations who knowingly put their workers in danger of asbestos related diseases.[9]
How is Mesothelioma Diagnosed?
To diagnose mesothelioma, a doctor will begin by taking the patients’ medical history to establish risk factors for mesothelioma. If there are many risk factors present, the doctor will then perform a physical exam to look for fluid in the lungs or abdomen. They may then order medical imaging to determine whether any irregularities are present and/or order a blood test to look for cancer related proteins. Finally, to confirm the diagnosis, the doctor will order a biopsy of the affected area. A biopsy is the only way to definitively confirm a mesothelioma diagnosis.[7] The biopsy can be done using a needle or endoscope or can be performed surgically.
A needle biopsy may be useful as a first attempt at a biopsy, as it is less invasive than other methods. A needle is used to pass between the ribs and the pleura, and a small sample of the tumour is removed. The difficulty in using this method for mesothelioma is that it may not remove a sample large enough for diagnosis. In this case a more invasive biopsy is needed.
An endoscopic biopsy uses an endoscope, a thin tube with a light and camera at the end, to look for the tumour(s). An endoscope can be used in the thorax (chest), abdomen or mediastinum, the area between the lungs. Endoscopic procedures are performed under general anaesthetic, when a surgeon makes a small incision in the desired cavity to guide the endoscope. The surgeon will look for the tumour with the endoscope and will remove small pieces of tissue for microscopic examination.
If the previous methods of biopsy prove ineffective, a doctor may then suggest an open surgical biopsy, which allows the surgeon to remove a larger sample of the tumour.
What are the Treatment Options?
Unfortunately, there is currently no known cure for mesothelioma, and remission from mesothelioma is unlikely due to the nature of the cancer. However, life expectancy for patients with mesothelioma has increased in recent years as treatment options have improved.[5]
Treatment is dependent on the type and stage of the cancer, and can include surgery, radiation therapy, and chemotherapy, as well as experimental cancer treatments. The course of action for managing mesothelioma depends on the specific case characteristics.
Surgery, when it is possible, has been found to be the most effective at removing all or most of the cancer. It is generally only viable if the cancer is caught at an early stage, before it has spread to other regions of the body.[7,8]
In some cases a multimodal approach is recommended. This kind of treatment plan combines multiple types of treatment such as surgery, chemotherapy, radiation, or possibly experimental treatments. Typically chemotherapy or radiation therapy is used to reduce the size of the tumor, surgery is used to remove as much of the tumor as possible, and chemotherapy is applied to kill any remaining cancer cells. Recent studies have found multimodal approaches to be more effective than any single type of treatment used on its own.[7,8]
What is the Prognosis?
The prognosis of a patient with mesothelioma depends on specific characteristics of the individual and of the cancer. Currently in Canada, the 5-year survival rate for mesothelioma is 7%.[6] However, it is important to note that survival rate can increase or decrease depending on the stage of the cancer, location of the cancer, type of cancer, and gender and age of the patient.
In general, the earlier the stage of the cancer, the better the prognosis. For example, in stage I, the cancer is localized and any additional tumours are close to the originating tumour, so surgery may be an effective strategy for removing the cancer.
The location of the cancer also affects survival. Those diagnosed with pleural mesothelioma having a better prognosis than those with peritoneal mesothelioma or pericardial mesothelioma.
As previously mentioned, mesothelioma can occur in one of three types: epithelial, sarcomatoid or a mix of the two. Those with epithelial mesothelioma have a better prognosis.
The patient’s gender also makes a difference: Generally women have been found to have a better prognosis than men.[7]
Finally, age can play a part in prognosis, as younger and stronger patients typically have better general health than older patients.
If you are interested in learning more about clinical trials currently being offered for mesothelioma patients in Canada, visit http://www.canadiancancertrials.ca/.
References
1. Park, S., Schalling, M., Bernard, A., Maheswaran, S., Shipley, G.C., Roberts, D., Fletcher, J., Shipman, R., Rheinwald, J., Demetri, G., Minden, M., Housman, D.E., and Haber, D.A. (1993) The Wilms tumour gene WT1 is expressed in murine mesoderm-derived tissues and mutated in human mesothelioma. Nature Genetics 4: 415-420.
2. Mesothelioma Cancer Alliance (2015) Mesothelioma Types. Retrieved July 2015 from http://www.mesothelioma.com/mesothelioma/types/
3. Lanphear, B.P., and Buncher, C.R. (1992) Latent period for malignant mesothelioma of occupational origin. Journal of Occupational Medicine 37: 718-721.
4. Mesothelioma Cancer Alliance (2015) Symptoms of mesothelioma. Retrieved July 2015 from http://www.mesothelioma.com/mesothelioma/symptoms/
5. Mesothelioma Cancer Alliance (2015) Mesothelioma Life Expectancy. Retrieved July 2015 from http://www.mesothelioma.com/mesothelioma/prognosis/life-expectancy.htm
6. Canadian Cancer Society (2015) Survival statistics for mesothelioma. Retrieved July 2015 from http://www.cancer.ca/en/cancer-information/cancer-type/mesothelioma/prognosis-and-survival/survival-statistics/?region=on
7. Ahmed, Ishtiaq, Salman Ahmed Tipu, and Sundas Ishtiaq (2013). Malignant mesothelioma. Pakistan Journal of Medical Sciences 29(6): 1433-1438.
8. Mesothelioma Cancer Alliance (2015) Mesothelioma Treatment. Retrieved July 2015 from https://www.mesothelioma.com/treatment/.
9. Australian Asbestos Network. (2020). The Battles in Australia. Retrieved August 2020 from https://www.australianasbestosnetwork.org.au/asbestos-history/battles-2/battles-australia/
10. The Mesothelioma Center. (2013). Mesothelioma Symptoms. Retrieved August 2020 from http://www.asbestos.com/mesothelioma/symptoms/.