Mesothelioma in Situ

Mesothelioma in situ is the earliest recognized form of mesothelioma: a single layer of malignant mesothelial cells on the surface of the pleura or peritoneum that has not yet invaded the tissue beneath. It was added to the World Health Organization's tumor classification in 2021.

Origin

"In situ" is Latin for "in its original place" and is used across pathology for cancers that have not invaded. For decades pathologists could not reliably tell a flat layer of malignant mesothelial cells from reactive, benign cells. Tests for loss of the BAP1 protein and of the MTAP protein or CDKN2A gene made it possible, and the 2021 WHO classification of thoracic tumors recognized mesothelioma in situ as a diagnosis.

Significance in Asbestos Context

The diagnosis requires a surface layer of mesothelial cells showing BAP1 or MTAP loss (or CDKN2A deletion), no invasion on biopsy, and no tumor visible on imaging or at thoracoscopy. Patients often come to medical attention with repeated, unexplained pleural effusions. Studies so far suggest many cases progress to invasive mesothelioma, sometimes after several years, so close follow-up is usual. Because it is a mesothelioma diagnosis, it can support a claim, but it is new and uncommon, so a review by an experienced mesothelioma pathologist is especially important.

Example Sentence

After her third pleural effusion in a year, a thoracoscopic biopsy showed a flat layer of mesothelial cells with BAP1 loss and no invasion, and the pathologist diagnosed mesothelioma in situ.

Related Terms

Mesothelioma, Reactive Mesothelial Hyperplasia, BAP1, MTAP, Pleural Effusion, Thoracoscopy, WHO Classification

Notes

• Ask for a second pathology review by a mesothelioma specialist; the diagnosis depends on special stains.
• Keep follow-up appointments and imaging; progression is common.
• Talk to a lawyer promptly. Limitations periods usually run from diagnosis, and this is a mesothelioma diagnosis.