The role of immunohistochemistry varies depending on the histologic type of DMM (Epithelioid versus sarcomatous) and the type of the tumor being considered in the differential diagnosis. While some recommendations exist, the number of antibodies necessary for definitive diagnosis or exclusion of DMM is individual to each case.

Epithelioid DMM has a broad range of morphologic features and should be distinguished from primary pulmonary adenocarcinoma involving the pleura and metastatic carcinoma arising from other organs.

Because no absolutely sensitive and specific marker for mesothelioma has yet been identified, a panel of immunohistochemical stains composed of mesothelioma markers (i.e., those that are frequently expressed in DMM, but not in carcinomas) and carcinoma markers (those that are frequently expressed in carcinomas, but not DMM) should be used to establish the diagnosis (Table 5.2). The combination of these markers is chosen based on gross and microscopic appearance, radiology , and clinical history. Because none of the markers are 100 % specific, the International Mesothelioma Panel recommends that at a minimum 2 mesothelial and 2 carcinoma markers, in addition to a broad-spectrum cytokeratin antibody, be included in any panel [8]. Based on their sensitivity and specificity, calretinin , CK5/6, WT1, and D2-40 are currently considered by many to be the best positive mesothelioma markers; and MOC-31, BER-EP4, carcinoembryonic antigen (CEA), and Lewis (y) antigen blood group 8 to be the best carcinoma markers [1].

In addition to these broad-spectrum carcinoma markers, a number of carcinoma markers are available that their expression is highly restricted to certain type of carcinomas. These markers will be helpful to determine the origin of metastatic carcinomas (Table 5.3).

Thyroid transcription factor-1 (TTF-1) and napsin A are helpful in distinguishing lung adenocarcinoma from DMM. TTF-1 is positive in approximately 75 % of lung adenocarcinomas and napsin A in between 58 and 91 %; both are negative in mesotheliomas [9–11].

Markers that are helpful in differentiating squamous cell carcinoma from DMM include Ber-EP4, MOC-31, CEA, BG8, and P63 [12]. P63 can also assist in distinguishing squamous cell carcinoma from adenocarcinoma. Of the mesothelioma markers, WT1 is considered by many to be the best marker in this situation, as CK5/6 and calretinin often are positive in squamous cell carcinomas as well as DMM [13].

Other carcinomas that metastasize to the pleura and should be distinguished from DMM include carcinomas of breast, renal, gastrointestinal, ovarian, and fallopian tube origin. Markers frequently expressed by breast carcinomas that assist in differentiating metastatic breast carcinoma from DMM include estrogen receptor (ER), mammaglobin, and gross cystic disease fluid protein-15 (GCDFP-15) [13]. However, basal-like breast carcinomas are negative for these markers and frequently express calretinin and CK5/6, making their diagnosis as pleural metastases problematic [14, 15].

PAX8 and PAX2 are useful markers for distinguishing metastatic renal cell carcinoma from DMM because they are expressed in most renal cell carcinomas and generally absent in DMM. Renal cell carcinoma marker (RCC) also might be useful; however, its sensitivity and specificity are significantly lower than those of PAX8 and PAX2 [13].

CDX2 is a sensitive and relatively specific marker for intestinal differentiation. Most of the colonic adenocarcinomas , and adenocarcinomas of small intestine, stomach, and esophagus express CDX2 [16]. In addition, gastrointestinal neuroendocrine carcinomas and carcinomas of the pancreas and biliary tree are typically immunopositive with CDX2; while epithelioid DMM are characteristically CDX2 immunonegative [16].

The role of immunohistochemistry is more limited in sarcomatous DMM than epithelioid DMM. Staining for mesothelial markers is less often positive. D2-40 and calretinin are two mesothelioma markers that are most consistently expressed in sarcomatous DMM in a variable percentage of cases [17, 18]. In a study by Roggli [7], only 31 % of sarcomatous DMM were positive with calretinin, and the staining was usually focal with predominantly cytoplasmic and occasionally nuclear reactivity. Expression of CK5/6 or thrombomodulin was also uncommon. Cytokeratin expression is more frequently positive; though. 97 % of the cases were immunopositive with CAM 5.2 and CK AE1/AE3.

Pleural sarcomatous DMM must be differentiated from primary pulmonary sarcomatoid carcinoma involving the pleura, and from metastatic sarcomas (Table 5.4). A malignant sarcomatoid tumor that is positive with cytokeratin must include within its differential diagnosis sarcomatoid carcinoma, sarcomatous DMM, synovial sarcoma,vascular neoplasm such as angiosarcoma and metastatic sarcomatoid renal cell carcinoma.Sarcomatoid DMM can be cytokeratin negative, but it is rare

The combination of cytokeratin and calretinin immunopositivity is highly characteristic of sarcomatoid DMM; however, it cannot always help to distinguish sarcomatous DMM from sarcomatoid carcinoma and synovial sarcoma. When an epithelial component can not be identified in primary pulmonary sarcomatoid carcinoma distinguishing it from sarcomatous DMM is extremely difficult even with the use of immunohistochemical stains. In this situation, clinical and radiologic findings must be carefully considered in an attempt to establish an accurate diagnosis.

In the setting of a keratin-negative sarcomatous and/or epithelial tumor in the lung, an epithelioid vascular tumor (epithelioid hemangioendothelioma or angiosarcoma), malignant melanoma, and lymphoma must be considered in the differential diagnosis , and a panel of antibodies should be selected to assist in accurate diagnosis. Epithelioid hemangioendothelioma and angiosarcoma are often positive for cytokeratin. CD31 and CD34 will help to distinguish them from DMM. With melanoma, S100 and HMB45 are useful markers; and in lymphoma, CD45, CD3, CD20, and CD30 are useful.