For most pathologists, diffuse malignant mesothelioma (DMM) is a disease for which its rarity renders it unfamiliar, its histologic diversity diagnostically challenging, and its medical–legal implications overly stressful. Because DMM has a dismal prognosis and very limited treatment options compared to its much more common mimics, accurate diagnosis is paramount.

The World Health Organization’s classifications of tumors of the pleura [1] and peritoneum [2] include DMM; however, as the vast majority of DMM is pleural, it is pleural tumor upon which this book focuses. DMM is the most common primary malignant neoplasm arising within the pleura. The WHO’s classification also recognizes four DMM histologic subtypes: epithelial, sarcomatous , biphasic, and desmoplastic ; however, the designation of desmoplastic DMM—generally considered a variant of sarcomatous DMM—as a separate histologic subtype is controversial. Although desmoplastic DMM is strongly mimicked by chronic fibrous pleuritic, and has an especially bad prognosis, neither of these features warrants the stature of independent subtyping.