Despite numerous histochemical, ultrastructural, and immuno-histochemical studies, differentiation between malignant epithelial pleural mesothelioma and adenocarcinoma of the lung remains extremely difficult.

Diagnosis of Mesothelioma is a problem for the following reasons:

1. Histopathological diversity in Mesothelioma

2. Pleural cavity is a common site for metastatic disease, florid reactive fibrosis and hyperplasia and less commonly primary sarcoma.

Accurate diagnosis of mesothelioma is necessary to assess the prognosis of the patient. It is also essential in relation to occupational-related compensation claims following exposure to asbestos.

Histochemical and immunohistochemical panel commonly used by pathologists:

MUCIN HISTOCHEMISTRY:

Before the advent of immunohistochemistry , histochemical studies played an important role in the diagnosis of mesothelioma.

Mesothelioma produce the acid mucopolysaccharide, hyaluronic acid which can be identified by staining with Alcian blue (pH 2.5) with and without hyaluronidase.

Identification of neutral mucin droplets within tumour cells or tubular lumina by periodic acid - Schiff reaction with diastase pretreatment strongly indicates that the tumour is an adenocarcinoma and not mesothelioma.

IMMUNOHISTOCHEMISTRY:

 Immunodiagnosis of mesothelioma is one based on exclusion. There is no single immunohistochemical marker entirely specific for mesothelioma hence a panel of antibodies are used to establish the diagnosis.

Immunohistochemistry is useful in the following areas:

1. To differentiate between malignant epithelial mesothelioma and adenocarcinoma:

A combination of monoclonal antibodies to carcinoembryonic antigen (CEA) , Leu-M1 , Ber-EP4,  MOC31 or B72.3 can distinguish 90% pulmonary adenocarcinomas from pleural mesothelioma . In a recent publication, it was suggested that immunostaining for cytokeratin 5/6 could assist in distinguishing epithelial mesothelioma from lung adenocarcinoma.

Antimesothelioma antibodies are useful in identification of epithelial mesothelioma.  Thrombomodulin is extremely useful and is applicable to formalin fixed, paraffin embedded tissue.

2. To differentiate between malignant epithelial mesothelioma versus reactive mesothelial hyperplasia:

Epithelial membrane antigen (EMA)- strong and diffuse positive membrane staining and/or Human Milk Fat Globulin-2(HMG2) favours mesothelioma over hyperplasia.  EMA may rarely show patchy weak membrane staining of reactive mesothelium.  p53 protein is also highly specific for mesothelioma over reactive mesothelial hyperplasia (this is less specific than EMA). Staining for cytokeratin may be useful as it may help in the identification of invasion into deeper structures.

3. To differentiate between malignant sarcomatous mesothelioma  from sarcoma,  localized fibrous tumour and reactive serosal fibrosis :

The pancytokeratin marker AE1/AE3 and low molecular weight cytokeratin CAM5.2 are useful in the diagnosis of mesothelioma.

 Most sarcomas, reactive serosal fibrosis and localized fibrous tumours are cytokeratin negative. 

(Note:  Epithelioid variant of sarcoma ,  synovial sarcoma and leiomyosarcoma may occasionally show cytokeratin positivity ).

Vascular tumours:  Mesotheliomas do not express vascular markers (CD31, FactorVII ) .

Epithelioid leiomyosarcoma:  It may not be possible to differentiate epithelioid leiomyosarcoma from sarcomatoid variant of mesothelioma as smooth muscle differentiation may be present in the latter ( according to some authors these should be classified as leiomyoid mesothelioma ).

Synovial sarcoma:  These may be confused with mixed and sarcomatoid variants of mesothelioma. D-PAS mucin or positive immunostaining for adenocarcinoma markers in the epithelial elements are particularly useful. bcl2 protein is present in almost all cases of synovial sarcoma. It is rarely present in mesothelioma.

4. To differentiate malignant mesothelioma from lymphoma and thymoma:

Primary pleural lymphoma are high grade and are differentiated from mesothelioma by using cytokeratin and lymphoid markers.

Thymoma arising in pleura may be confused with solid and lymphohistiocytoid pattern of mesothelioma.  Immunohistochemistry is of little help in this area.