**1. Introduction**

Mesothelioma is a rare and malignant tumor arising from the mesothelial or submesothelial cells of the pleura, peritoneum, or pericardium. Until 2021, the term "malignant" had been used as a prefix for mesothelioma in order to distinguish it from the well-differentiated papillary mesothelioma. In the recently updated WHO Classification, this was renamed well-differentiated papillary mesothelial tumor (WDPMT), to highlight its differences from diffuse mesothelioma, the word "malignant" has been dropped [1]. Mesothelial tumor diagnoses according to the 2021 WHO Classification of


#### **Table 1.**

*Mesothelial tumors.*

the tumors of the pleura and pericardium are summarized in **Table 1**. If not otherwise stated, most cases of mesothelioma in the literature refer to diffuse mesothelioma. There are rare benign mesothelial tumors such as adenomatoid tumor and WDPMT, only the latter will be briefly discussed in this review. Mesothelioma in situ refers to a flat noninvasive form of mesothelioma and localized mesothelioma is histologically identical to diffuse, but macroscopically solitary, circumscribed mass. Both of these are very rare, only a very few cases have been described [2, 3].

More than 80% of all diffuse mesotheliomas originate in the pleura and 10−15% are peritoneal [4, 5]. Clinical manifestations of mesothelioma are usually nonspecific and, due to a broad spectrum of differential options, can be difficult to diagnose especially in the early stage. The diagnosis of mesothelioma has to be made in the context of appropriate clinical, radiologic, and surgical findings. Because patients with mesotheliomas frequently present with effusions, sampling of pleural or peritoneal fluid for biochemical and cytological examination is often the first source of material [6–8]. The sampled diagnostic material bears limitations in pathological analysis. As cytological smear alone is insufficient for diagnosing mesothelioma, the utilization of immunochemistry (IC) must be applied to confirm both the mesothelial origin and its malignant nature, and exclude other potential mimickers such as metastatic carcinomas [8–11]. Final confirmation of the diagnosis and establishing the histological type, grade, and invasiveness of mesothelioma can be done in biopsy or operation material. Mesotheliomas are histologically divided into epithelioid, sarcomatoid, and biphasic varieties.

Current review aims to highlight the basic steps of the pleural and peritoneal mesothelioma pathological diagnosis along with most important technical handling details for clinicians and broad life science audiences. The sample figures of cytological and histological findings are from the archives of the North Estonian Medical Centre, the identity of patients remains unrevealed and the ethics committee permission is, therefore, unrequired.
