Mesothelioma In Situ

**Semantic type:** Neoplastic Process

**Definition:** A preinvasive mesothelial neoplasm that arises on the surfaces that contain mesothelial lining (pleura, peritoneum, pericardium, and tunica vaginalis) and is characterized by the proliferation of mesothelial cells in a single layer without invasion into underlying tissues. It requires demonstration of nuclear BAP1 loss or cytoplasmic MTAP loss by immunohistochemistry, or CDKN2A homozygous deletion by FISH. It is a precursor lesion to malignant mesothelioma, it most often affects the pleura, and usually occurs in older adults.

**Synonyms:** - MIS