What is mesothelioma and what are its causes?

Mesothelioma, or malignant mesothelioma, is a relatively rare type of cancer developing from mesothelium. Mesothelium is a thin layer of cells of mesodermal origin, lining several of the body’s cavities: the pleura (thoracic cavity), the peritoneum (abdominal cavity), the pericardium (heart sac). Mesothelium is also found in male and female internal reproductive organs. Mesothelium has two layers – one of them surrounds the organ and the other forms a sac surrounding the cavity.

The main (but not the only) purpose of mesothelium is the production of a slippery, lubricating fluid. This fluid is then released between the two layers of mesothelium, allowing the organs (like heart, intestines and lungs) to move, expand and contract repeatedly within the body cavity, without any damage (for example, due to friction). The most common (but not the only possible) site for mesothelioma is pleura. Pleural cavity (which itself is a part of thoracic cavity) is a body cavity surrounding the lungs. Pleura is a serous membrane, and the space between the two layers of pleura is called pleural cavity.

The outer (or parietal) pleura is attached to the chest wall. The inner (or visceral) pleura covers the lungs. Humans have two lungs, so there are two pleural cavities – left and right, with no anatomical connection between them. This lack of connection allows one lung to function normally in case when the other is unable to function due to pneumothorax (collapsed lung) – the collection of air in the pleural cavity. The pleural cavity is usually filled with small amount of fluid, continuously produced and reabsorbed.

Micrograph of benign mesothelial cells
Micrograph of benign mesothelial cells.
Source: Nephron

Causes of Mesothelioma

The most common cause of mesothelioma is the long-term, direct exposure to asbestos (dust and fibers) and possibly fiberglass. Secondary exposure to asbestos is also dangerous. Even brief, low-level exposure to asbestos may potentially cause mesothelioma. Secondary exposure means that a person does not directly work with asbestos. Most commonly, it affects the families (spouses, children) of the people who are exposed to asbestos directly. Typically, when a person working with asbestos would finish work and return home, they would also carry the asbestos dust and fibers on their clothes or in their hair. Those would later be inhaled by other members of the family at home.

Blue asbestos (crocidolite)
Blue asbestos (crocidolite) from the now closed mine at Wittenoom, Western Australia.
Source: John Hayman

One study, conducted in 1966, established that over 50% of all mesothelioma cases in women were the result of a secondary exposure to asbestos, caused by someone else working with asbestos. For example, wives and children of World War II shipyard workers suffered secondary exposure from their husbands and fathers handling asbestos. Wives washing their husbands’ asbestos polluted clothes (and “shaking them out” before washing) were inhaling the substance. Secondary exposure may also mean living in the proximity of asbestos mines, asbestos-related facilities, refineries, power plants, steel mills, rail yards and shipyards.

Asbestos (tremolite) silky fibers
Asbestos (tremolite) silky fibers on muscovite from Bernera, Inverness-shire.
Source: Aram Dulyan

One of the most famous cases of large scale secondary exposures to asbestos involves the town of Libby, Montana. Libby was a home to a vermiculite mine (owned by W. R. Grace and Company), providing over 70% of all vermiculite sold in the U.S. between 1919 and 1990. Zonolite was a brand of vermiculite insulation. Vermiculite is a natural mineral that expands when heated to a high temperature. It is used for many different purposes, like insulation, fireproofing, soil conditioning, absorbing hazardous liquids, hand warming and many others. Pure vermiculite does not contain asbestos and is non-toxic. Today, vermiculite mines around the world are regularly being tested for the presence of asbestos, and they are supposed to sell clean, safe vermiculite. The vermiculite deposits in Libby, however, were polluted with tremolite asbestos, winchite and richterite (the last two being fibrous inosilicate minerals).

Anthophyllite asbestos
Anthophyllite asbestos, image from Scanning Electron Microscope.
Source: United States Geological Survey

The asbestos caused the deaths and illnesses of many of the miners and workers, as well as those of the members of their families. 274 deaths in the area are suspected to be the result of asbestos-related diseases. Recent studies have also found a link between mesothelioma and residential proximity to ultramafic rock – a natural source of asbestos. Some studies indicate that there is a possible link between the simian virus 40 (SV40 – potentially hundreds of millions of people around the world could have been exposed to it through the contaminated polio vaccine) and mesothelioma. In rare cases, mesothelioma could be associated with irradiation.

Source: US Geological Survey

Mesothelioma symptoms and treatment

Mesothelioma is not the same as lung cancer. There is also no direct association between mesothelioma and smoking. Smoking, however, significantly increases the risk of other asbestos-related cancers.

Symptoms of pleural mesothelioma typically appear very late (20 to 50 years after exposure to asbestos), and include: fatigue, anemia, wheezing, cough, blood in the sputum, shortness of breath, pleural effusion (accumulation of fluid surrounding the lung). Above symptoms are similar to many other conditions, and diagnosing mesothelioma is therefore difficult. History of exposure to asbestos hints at possible mesothelioma.

Coronal reformat of a CT of the chest in a patient with right sided medial mesothelioma.
Coronal reformat of a CT of the chest in a patient with right sided medial mesothelioma.
Source: Frank Gaillard

Chest X-ray and MRI or CT scans are usually performed to diagnose mesothelioma. Cytopathology may detect abnormal cells. A biopsy is commonly needed to confirm the diagnosis. There is no universal protocol of screening of people at risk of mesothelioma due to exposure to asbestos. The prognosis for malignant mesothelioma is not favorable, if not grim. Mesothelioma is often only detected at an advanced stage. Survival rate following diagnosis is usually 1 to 2 years. 5-year survival rate is just 10%. When we take these numbers into consideration, it’s easy to see why mesothelioma is one of the most devastating diagnoses.

Micrographs showing mesothelioma in a core biopsy.
Micrographs showing mesothelioma in a core biopsy.
Source: Robertolyra

Treatments include: surgery (with a median survival as low as 11.7 months), radiotherapy (often used as a consolidative treatment post-surgery), chemotherapy (median survival of up to 13.3 months), immunotherapy, Heated Intraoperative Intraperitoneal Chemotherapy, Multimodality Therapy (median survival of up to 14.5 months). More than 2000 cases of mesothelioma are diagnosed every year, although this number may not reflect the true incidence rate. The reason for it is that mesothelioma is often misdiagnosed as adenocarcinoma of the lung.

Micrograph of a pleural fluid cytopathology specimen showing mesothelioma.
Micrograph of a pleural fluid cytopathology specimen showing mesothelioma.
Source: Nephron

Many people suffering mesothelioma, or their families, are entitled to compensation. The mesothelioma settlements have run into millions of dollars – but there were also cases when a plaintiff lost, and got nothing. When going to court to fight for compensation for mesothelioma, it is important to have an expert diagnosis, which is critical for a favorable outcome.

Further reading

National Health Service page on Mesothelioma
Mesothelioma Cancer Alliance
National Cancer Institute page on Mesothelioma
American Cancer Society page on Mesothelioma
WebMD page on Mesothelioma