What is mesothelioma?

Mesothelioma is a type of cancer that starts from specializing epithelial cells. These cells are usually found on the outside of the lungs and organs inside the abdomen such as the colon and small intestine. 

Another name for mesothelioma is malignant mesothelioma.


What causes mesothelioma?

Mesothelioma is associated with asbestos exposure in most, but not all, cases. Many patients exposed to asbestos will also have areas where the pleura has become abnormally thickened. 

These areas are called pleural plaques and can usually be seen on imaging of the lungs (X-ray or CT scan). 

When examined under a microscope, pleural plaques consist of dense collagen layers that can have the appearance of basket weave.

How do pathologists diagnose mesothelioma?

Mesothelioma is usually diagnosed after a small sample of tissue is removed in a procedure called a biopsy. In some cases, additional surgery may be performed. 

Diagnosing mesothelioma can be difficult because noncancerous conditions such as infection or pleural effusion can lead to changes in epithelial cells that can look similar to cancer under a microscope. 

To diagnose mesothelioma, your pathologist needs to see the epithelial cells that make up a tumor, or that have spread beyond the pleura to surrounding tissues or the lungs. 

The spread of epithelial cells into the surrounding tissues is called invasion.

What other tests can be done to confirm the diagnosis?

When examined under a microscope, other types of cancer can look very similar to mesothelioma. 

Most pathologists will perform a test called immunohistochemistry to help them determine whether the abnormal cells are phenotypes or cells from another part of the body.

When immunohistochemistry is performed, epithelial cells will typically show the following results:

WT-1 - Positive

Calretinin - Positive

D2-40 - Positive

Cytokeratin 5/6 - Positive

MOC-31 - Negation

BerEP4 - Negation

Polygonal CEA - Negative

TTF-1 - Negation

Pathologists usually need a combination of immunohistochemical tests to help them determine whether the abnormal cells are mesothelial or whether they are from another part of the body.

To differentiate mesothelioma from other conditions, some pathologists may use other immunohistochemical tests including BAP1 and mTAP. 

Both of these proteins are found naturally in tissues throughout the body. 

However, it does not usually appear in mesothelioma. For this reason, a negative result for BAP1 or mTAP supports the diagnosis of mesothelioma.

What are the types of mesothelioma?

Three types of mesothelioma depend on the shape of the cells when examined under a microscope. 

Pathologists decide the type by looking at the size and shape of the cells and the way the cells stick together.

Mesothelioma - this type consists of small cells of oval to cuboid shapes that join together, often forming small tubular or papillary structures.

Sarcomatoid mesothelioma - this type consists of cells that are longer than they are wide. Pathologists describe these cells as spindle cells. 

Unlike cells in epithelial-type mesothelioma, spindle cells also do not stick together to form structures and spread widely into surrounding tissues.

Biphasic mesothelioma - this type consists of a mixture of epithelioid and sarcomatoid-like cells. 

To be called biphasic, your pathologist needs to see at least 10% of each cell type in tissue samples examined under a microscope.

Types of Mesothelioma

Desmoplastic mesothelioma is another type of mesothelioma that is related to the sarcomatoid type. 

Diagnosing mesothelioma can be difficult for a pathologist. The tumor consists of dense fibrous tissue with abnormal spindle cells.

The histological type is important because patients with the epithelial type will usually have the best references followed by biphasic and then sarcomatoid. 

The histological subtype may affect the surgical procedures or therapies used to treat the specific type of mesothelioma.

What is the margin?

A margin is the normal tissue that surrounds the tumor and is removed along with the tumor at the time of surgery. 

If you have had a complete resection with extrapleural pneumonectomy, the pathologist will look for cancer cells at the cut edge of the tissue.

If no neoplastic cells appear at the cut edge of the tissue, the margin is called negative. If cancer cells are seen on the edge of the cut tissue, the edge is called positive. 

A positive margin is important because it is associated with a higher risk that the tumor will return to the same site (local recurrence) after treatment.


What does treatment effect mean?

Some patients will receive chemotherapy or radiotherapy before the tumor is surgically removed. 

If you received chemotherapy or radiotherapy before surgery, your pathologist will examine the tumor under a microscope to see how much of the tumor is still alive (viable). This is called the treatment effect.

To determine the effect of treatment, the pathologist will measure the amount of the tumor that is life (viable) and divide this number by the amount of the total tumor. 

The effect of treatment is usually described as greater or less than 50% of the remaining viable tumor.

How well the tumor responds to treatment may help your oncologist understand how well the tumor is being treated with chemotherapy and can be used to help guide further treatment.

What are the lymph nodes?

Lymph nodes are small immune systems scattered throughout the body for body. Cancer cells can travel from the tumor to the lymph node through the lymph channels in and around the tumor. 

The movement of cancer cells from a tumor to a lymph node is called lymph node metastasis.

Lymph nodes from the neck, chest, and lungs can be removed at the same time as the tumor. Your pathology report will describe how many lymph nodes were examined. 

A pathologist will carefully examine each lymph node for cancer cells. Lymph nodes containing cancerous cells are called positive while lymph nodes that do not contain any tumor cells are called negative.

The finding of cancer cells in the lymph node increases the nodal stage (see pathological stage below) and correlates with worse-case references. 

The nodal stage depends on where the lymph node is located with the cancer cells.

lymph node

How do pathologists determine the pathological stage of mesothelioma?

Pathological staging of mesothelioma is based on the TNM staging system, an internationally recognized system originally established by the US Joint Commission on Cancer. 

This system uses information about the primary tumor (T), lymph nodes (N), and distant metastatic disease (M) to determine the complete pathological stage (pTNM). 

Your pathologist will examine the tissue provided and give each part a number. In general, a higher number means more advanced diseases and worse references.

Tumor stage (pT) of mesothelioma

Mesothelioma is given a tumor stage between 1 and 4 based on invasion by the tumor into other structures and excision of the tumor (ability to surgically remove).

staging mesothelioma

Nodal stage (PN) of mesothelioma

Mesothelioma is given a nodal stage between 0 and 2 based on the presence or absence of cancer cells in a lymph node and the location of the lymph nodes that contain the tumor cells.

NX - lymph nodes were not sent for pathological examination.

N0 - No cancer cells were found in any of the lymph nodes examined

N1 - Cancer cells are found in at least one lymph node in the hilum or mediastinum of the same side as the tumor.

N2 - tumor cells are found in at least one lymph node from the opposite side of the mediastinum or outside the chest.

Metastatic stage (pM) of mesothelioma

Mesothelioma is given a metastatic stage of 0 or 1 based on the presence of the tumor at a distant site of the body (for example, the brain). 

The metastatic stage can only be determined if tissue is sent from a remote site for pathological examination. 

As this tissue is rare, the metastatic stage cannot be determined and is listed as BMX.

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