Malignant mesothelioma is broadly categorized into three histologic subtypes – epithelioid, sarcomatoid, and biphasic (mixed). Although a number of unusual and rare variants have also been detected, the above classification has general diagnostic usefulness because the differential diagnosis of both benign and malignant lesions, and also the subsequent immunohistochemical analysis, is performed using this subclassification.
The most common is the epithelioid variant, which comprises around 60% of all mesotheliomas. The most common histologic appearances associated with this subtype include acinar (glandular), adenomatoid, tubulopapillary, and solid epithelioid patterns. The differential diagnosis also includes metastatic carcinomas and other epithelioid tumors in addition to reactive mesothelial hyperplasia.
Sarcomatoid mesotheliomas comprise of malignant spindle cells that may mimic malignant mesenchymal tumors, for instance, leiomyosarcoma or synovial sarcoma. Desmoplastic mesothelioma, comprising of bland tumor cells lying between dense bands of collagenous stroma, has been classified as a subtype of sarcomatoid mesothelioma. The differential diagnosis also includes sarcomatoid carcinoma and other sarcomas in addition to fibrous pleurisy.
Epithelioid and sarcomatoid characteristics have been observed in both biphasic and mixed mesotheliomas. However, larger samples and multiple tissue sections may be required to demonstrate both the characteristics. The differential diagnosis typically includes synovial sarcoma, and other biphasic or mixed tumors as well.
In case of malignant mesotheliomas that are poorly differentiated and discohesive, it becomes necessary to include non-neoplastic inflammatory disorders and lymphomas within the differential diagnosis.
Well-differentiated papillary mesothelioma is a rare tumor with a unique papillary architecture and bland characteristics which tend to spread superficially without invasion. This specific form of tumor is diagnosed more commonly in the peritoneum.
Based on research, histologic criteria have been defined to differentiate between reactive mesothelial hyperplasia and epithelioid malignant mesothelioma, and also between fibrous pleurisy and the desmoplastic variant of sarcomatoid mesothelioma:
Factors that can lead to reactive mesothelial hyperplasia include connective tissue disease, infections, drug reactions, pulmonary infarct, subpleural lung carcinomas, pneumothorax, traumatic injury, or surgery.
Factors that can induce fibrous pleurisy include infection, exposure to asbestos, traumatic injury, or connective tissue disease.
The detection of stromal invasion is the most reliable criterion for differentiating fibrous pleurisy and reactive mesothelial hyperplasia from malignancy. While detection of stromal invasion can be demonstrated through positive cytokeratin staining, the best way to achieve a definitive diagnosis would involve sufficient biopsies that would include some underlying fibroadipose tissue, or, in specific situations, the pulmonary parenchyma.
Immunohistochemistry
The utilization of immunohistochemical panels plays a vital role in the diagnosis of malignant mesothelioma.
Currently, there is no standalone marker that has adequately high sensitivity and specificity for malignant mesothelioma. As such, it is general practice for pathologists to use a panel of markers (both positive and negative). Different institutions have different opinions when it comes to selecting the markers and panels are usually refined as various studies are published comparing the effectiveness of different markers. The general rule is that the marker should have either sensitivity or specificity more than 80% for the specific tumors being targeted. The utility of individual markers is further categorized into panels, which is more useful when the tumor is suspected to be epithelioid, sarcomatoid, or poorly differentiated.
Pancytokeratin stains have been used successfully in the diagnosis of mesothelioma and these can stain a significant percentage of all mesotheliomas. In case the tumor is pancytokeratin negative, the screening panel should be expanded to include more stains that would help exclude melanoma, lymphoma, angiosarcoma or epithelioid hemangioendothelioma. Some rare sarcomatoid mesotheliomas have been identified, and these are cytokeratin negative, especially those with osteosarcomatous differentiation.
In case of epithelioid mesothelioma, common positive immunohistochemical markers which can be utilized to support a malignant mesothelioma diagnosis include the Wilms’ tumor-I (WT1) antigen (nuclear staining), CK5/6, D2-40 (podoplanin) and calretinin. Effective markers that are positive in case of pulmonary adenocarcinoma include carcinoembryonic antigen, CD15 (Leu-M1), B72.3, Ber-EP4, TTF-1, BG8, and MOC-31. If the differential diagnosis is expanded to include other subtypes of lung tumors, for instance squamous cell carcinoma or metastases from other major sites such as ovary or kidney, then the panel should focus more on those specific requirements. Also, the usefulness of a negative or positive result should be carefully examined.
In case of sarcomatoid or biphasic mesothelioma, cytokeratin reactivity may not necessarily distinguish malignant mesothelioma from other sarcomas, especially if the cytokeratin staining is focal. Since cytokeratin expression can be weak, focal or variable, multiple cytokeratin antibodies, for instance CAM5.2 (or CK 18), AE1/3, and CK7 must be used. Considerable cytokeratin positivity can also been observed in metastatic sarcomatoid renal cell carcinoma or sarcomatoid carcinoma. Calretinin and D2-40 are the most reliable of the positive mesothelioma markers. Although calretinin and D2-40 demonstrate overlap reactivity with other forms of sarcoma, obtaining a positive result may help in confirming a sarcomatoid mesothelioma diagnosis when combined with significant cytokeratin positivity. The potential presence of synovial sarcoma can typically be established via molecular testing for the specific X:18 translocation.
Electron microscopy
With the increase in the number of antibodies appropriate for immunohistochemistry on standard formalin-fixed paraffin embedded tissue as well as the inclusion of additional positive mesothelioma markers, immunohistochemistry has practically replaced electron microscopy in its conventional use as the “gold standard” in the diagnosis of malignant mesothelioma.
If immunohistochemical results are equivocal, electron microscopy is still useful in specific situations for confirming the diagnosis of well-differentiated to moderately-differentiated epithelioid tumors. The electron microscopic characteristics of malignant mesothelioma have been clearly illustrated. The primary epithelioid form comprises of polygonal cells that have several long surface microvilli, abundant tonofilaments, and prominent desmosomes.
