**4. Histological sampling and typing of mesothelial tumors**

#### **4.1 General considerations of histological diagnostic material**

Tissue sampling is currently achieved either by image-guided/thoracoscopicguided or surgical biopsy, both of which are recommended by major guideline committees. Surgical biopsies in principle generate more tissue materials, occasionally as much as pleural decortication and extrapleural pneumonectomy.

Biopsies comprise too little tissue and are known to suffer from sampling bias. Microscopically, tissue fields from pleural and peritoneal cavities are often obscured by inflammation and fibrinous debris. Subpleural or intraperitoneal fat sampling, important in the assessment of invasion, may be absent in cases of significantly thickened pleura or peritoneum. False-positive immunostaining may be seen in tiny needle biopsy specimens with crushed artifacts and at the edges of biopsy samples [21].

Larger materials give better overview, especially of intra-tumoral heterogeneity and invasion, but to get these results, the materials should be sampled extensively. The histologic diagnosis should be based on both the appropriate morphology and on IC findings.

#### **4.2 Well-differentiated papillary mesothelial tumor (WDPMT)**

WDPMT is a relatively uncommon subtype of mesothelial neoplasm with a distinct molecular profile [63] and histological appearance [25, 64]. It arises most commonly in the peritoneal cavity, but can also be found in the pleural cavity, pericardium, and tunica vaginalis [25, 64, 65]. WDPMT typically exhibits indolent behavior and is generally considered of low malignant potential [64].

*Mesothelioma: Overview of Technical, Immunochemical and Pathomorphological Diagnosing… DOI: http://dx.doi.org/10.5772/intechopen.106570*

#### **Figure 3.**

*Peritoneal well-differentiated papillary mesothelial tumor histology. A, H&E stain shows fibrovascular papillae lined by a simple uniform cuboidal epithelium, without nuclear atypia or mitoses (original magnification ×400). B, Calretinin expression both in nuclei and cytoplasm of lining epithelium confirms its mesothelial origin. The lining epithelial cell has enlarged appearance due to very intense staining (Ventana, RTU, ×400). C, BAP-1 expression is retained and shows uniform nuclear expression confirming benign nature of lining mesothelial cells (BioSB, RTU, ×400). D, PAX8 negativity helps to differentiate the serous neoplasms of ovaries and peritoneum (Abcam, 1:200, ×400).*

Histologically, WDPMT usually has an architecture of fibrovascular papillae, lined by a simple uniform cuboidal epithelium, with little to no nuclear atypia or mitoses (**Figure 3A**). Areas of invasion are typically not seen [64, 66]. The lining epithelium bears immunochemical profile of mesothelium, showing nuclear and cytoplasmic positive expression of calretinin (**Figure 3B**). BAP-1 staining is particularly helpful as retained nuclear expression shows benign nature of lining epithelial cells (**Figure 3C**). Great care should be taken to differentiate WDPMP from serous neoplasms of the ovaries and peritoneum, where IC markers, for example PAX8, are highly useful (**Figure 3D**) [23].

#### **4.3 Diffuse mesothelioma histological diagnosis**

Examples of diffuse mesothelioma histological types are illustrated in **Figure 4**. Epithelioid mesothelioma comprises approximately 80% of all pleural mesotheliomas and is defined as being composed of epithelioid, rounded, or polygonal cells [1, 62, 67]. Epithelioid mesothelioma can have various architectural patterns depending if the cells are located in solid sheets or form tubular, papillary, adenomatoid, and trabecular patterns [62, 67]. Sarcomatoid mesothelioma is the second most common subtype, composed of elongated spindle cells arranged in solid sheets or within fibrous stroma [62, 67]. Biphasic mesotheliomas are composed of both epithelioid and sarcomatoid components and at least 10% of each component is required for definite diagnosis in resection specimen. Regardless if a diagnosis is made in biopsy or extended operation material, sarcomatoid components should be reported and quantified in the pathology report, because it influences the treatment and prognosis.

#### **Figure 4.**

*Diffuse pleural mesothelioma histological subtypes. A, epithelioid mesothelioma is composed of rounded cells with eosinophilic cytoplasm and round nuclei with small nucleoli. In this tumor, the cells are located mostly in solid sheets with few gland-like structures (H&E stain, original magnification ×200). B, epithelioid mesothelioma architectural patterns may comprise trabecular, tubulopapillary, and gland-like structures (H&E, ×200). C, Sarcomatoid mesothelioma pattern is characterized by malignant elongated spindle-shaped cells (H&E, × 400). D, diffuse biphasic mesothelioma, which shows both epithelioid and sarcomatoid malignant areas (H&E, ×200).*

IC is essential in establishing a diagnosis, and the choice of antibodies, particularly carcinoma markers, depends on histological architecture, and also whether the tumor has a pleural or peritoneal location. In pleural location, lung adenocarcinoma, SCC, and breast carcinomas are the most frequent differential diagnoses, but metastases from a variety of other organs could be confused with epithelioid mesothelioma. The case of pleural epithelioid mesothelioma presented in **Figure 5**, presence of psammoma bodies along with few papillary areas required an extended panel for testing ovarian serous carcinoma and gastrointestinal carcinomas (not shown), all of which were negative. Peritoneal mesotheliomas most often need to be distinguished from gastrointestinal, renal, and ovarian malignancies.

Epithelioid mesotheliomas are graded using a two-tiered system (low and high grade), combining nuclear grade (mitotic count and nuclear atypia) and presence of necrosis, because these features have been demonstrated to be strongly predictive of survival in patients with epithelioid mesothelioma [1, 62].

Sarcomatoid mesothelioma should be distinguished from metastatic sarcomatoid carcinomas from lung and other sites, particularly renal carcinomas [62]. Differential diagnosis can be challenging because markers can overlap, and will not be fully reviewed here. Immunochemical profile of sarcomatoid mesothelioma is different from the epithelioid. Sarcomatoid mesotheliomas are at least focally positive for cytokeratins AE1/AE, pan-cytokeratin (OSCAR), and anti-cytokeratin clone 1(KL1), as well as cytokeratin CAM5.2 [62, 68]. But sarcomatoid mesotheliomas can be cytokeratin-negative. Sarcomatoid mesotheliomas are positive for mesothelial

*Mesothelioma: Overview of Technical, Immunochemical and Pathomorphological Diagnosing… DOI: http://dx.doi.org/10.5772/intechopen.106570*

#### **Figure 5.**

*Pleural epithelioid mesothelioma histology. A, H&E stain shows tubulopapillary mesothelioma structures. Tumor cells display moderate eosinophilic cytoplasm, mostly round nuclei with vesicular chromatin and small nucleoli. Psammoma body is seen in upper left corner (original magnification ×400). If concentrations are not indicated, antibodies are applied as ready-to-use (RTU) solutions. B, Calretinin diffuse expression both in nuclei and cytoplasm of malignant cells (Ventana, RTU, ×400). C, WT1 positive expression in all mesothelioma cell nuclei, but negative in fibrous stroma (Ventana, RTU, ×400). D, D2–40 strong membranous expression in most of the mesothelioma cells (Dako, RTU, ×400). E, TTF1 negativity in mesothelioma cells differentiates it from adenocarcinoma of the lung (Ventana, RTU, ×400). F, GATA3 negativity in mesothelioma cells differentiates it from breast carcinoma. Weak positivity is seen in the nuclei of lymphocytes (Ventana, RTU, ×400). G, PAX8 negativity in mesothelioma cells to differentiate from serous ovarian carcinoma (Abcam, 1:200, ×400).*

markers such as calretinin, WT1, and D2–40 in limited cases [62, 68]. Sarcomatoid mesotheliomas are often vimentin-positive, whereas epithelioid mesotheliomas are often negative to vimentin. Occasionally, sarcomatoid mesotheliomas express actin, desmin, or S100 [62].
