New Mesothelioma Subtype Called Mesothelioma in Situ

A New Mesothelioma Subtype Called Mesothelioma in SituThe 2015 WHO classification of pleural mesothelioma included three histological subtypes. The new fourth mesothelioma subtype is called mesothelioma in situ. It is a very early form of mesothelioma.

Previous studies suggest that it may be a precursor to invasive mesothelioma. If patients know they have it, they may have years to try to keep mesothelioma from developing.

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.

Four Subtypes of Mesothelioma

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 mesothelioma subtype makes a difference. People with the epithelioid subtype tend to respond better to mesothelioma treatments like chemotherapy, radiation, and surgery.

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

Mesothelioma in Situ: The New Fourth Mesothelioma Subtype

Mesothelioma in situ was first recognized as a concept in 1992. In 2017, the first case of mesothelioma in situ was diagnosed using the now accepted diagnostic criteria.

It has only now been incorporated as an official subtype into the current World Health Organization classification. There has been renewed interest in the concept of mesothelioma in situ late.

The new fourth subtype is mesothelioma in situ. In people with mesothelioma in situ, only a thin layer of mesothelial cells are affected. These cells have a loss of BAP1 protein. 

Patients with mesothelioma in situ do not have any pleural tumors. It may be years before tumors develop. In some patients, they never develop.

This mesothelioma subtype just came up again in a recent article in Pathology. Flinders University anatomical pathologist Sonja Klebe authored the article.

Dr. Klebe gave an update on the first case of mesothelioma in situ in 2017. Four years and 10 months later, the patient had pleural thickening. Later epithelioid mesothelioma was diagnosed. Mesothelioma in situ is a precursor to mesothelioma with an epithelioid subtype.

Clinicians recommend including mesothelioma in situ as the first stage of mesothelioma. And consider more therapy options at the time of diagnosis to improve prognosis.


Klebe, Sonja. “Progression of mesothelioma in situ to invasive disease 4 years and 10 months after initial diagnosis.” Pathology 54, no. 3 (2022): 384-386. https://doi.org/10.1016/j.pathol.2021.06.124



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