Latest Classification Updates for Mesothelioma In Situ

Classification Updates for Mesothelioma In Situ

There are new guidelines on the diagnosis and classification of mesothelioma in situ. A new article in the Journal of Cancer Metastasis and Treatment talks about why these updates are important.

The latest updates were first published by the World Health Organization (WHO). The previous WHO classification of pleural mesothelioma included three histological subtypes. But WHO recently recognized mesothelioma in situ as a fourth mesothelioma subtype.

Pathologists can tell the difference between the subtypes by looking at the cells under a microscope. Each subtype has a slightly different cell shape, growth pattern, and nuclear characteristics.

The fourth mesothelioma subtype is a very early form of mesothelioma. Previous studies suggest that it may be an early sign of mesothelioma. If patients know they have it, they may have years to try to keep mesothelioma from developing.

Differences Between Mesothelioma Subtypes

Pleural mesothelioma is a very rare type of cancer. Only about 2,500 people in the US receive a mesothelioma diagnosis every year. Asbestos inhalation is the primary cause of pleural mesothelioma.

Most people with pleural mesothelioma have a poor prognosis. But the mesothelioma subtype makes a difference. People with the epithelioid subtype tend to respond better to mesothelioma treatments. These often include chemotherapy, radiation, and surgery.

Epithelioid mesothelioma is the most common subtype. About half of pleural mesothelioma tumors are epithelioid. Studies show that epithelioid patients usually live longer than people with other subtypes.

Ten to 20 percent of pleural mesotheliomas are sarcomatoid. People with sarcomatoid mesothelioma may not respond as well to treatment.

The third mesothelioma subtype is biphasic. Biphasic mesothelioma tumors contain a mix of both kinds of mesothelioma cells. The differences between the subtypes are subtle. It takes an experienced professional to tell the difference between each mesothelioma subtype.

Mesothelioma in Situ: The Fourth Mesothelioma Subtype

The new fourth subtype is mesothelioma in situ. In people with mesothelioma in situ, only a thin layer of mesothelial cells is affected. These cells show a loss of BAP1 protein. People with mesothelioma in situ do not have any pleural tumors. It may be years before tumors develop. In some patients, they never develop.

Mesothelioma in situ, as a concept, is quite old. But it could not be proven definitively as an early warning sign of mesothelioma. This lead to controversy in the scientific field about its definition and diagnosis. The ability to diagnose mesothelioma in situ is also very recent.

The biggest question that doctors are asking now is what should be done if this diagnosis is made. What should the oncologist or surgeon should do when a patient is diagnosed with Mesothelioma in situ?

Unfortunately, this is a question that remains to be answered.

For the moment, patients and oncologists are grateful for the latest classification updates. With the arrival of mesothelioma in situ as a distinct entity. This new classification has the potential to change how the medical team views mesothelioma. And it may provide a means to intervene earlier in the disease.


Farkas, Julia R., Mark Sharobim, and Jefree J. Schulte. “Updates on the pathologic diagnosis and classification of mesothelioma.” Journal of Cancer Metastasis and Treatment 8 (2022): 37.

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