What Is Mesothelioma Pathology?

The Role of Pathology in Mesothelioma Diagnosis

Pathology is the branch of medicine that examines tissue, cells, and body fluids to diagnose disease. In the context of mesothelioma, the pathologist's analysis is the final and definitive step in confirming the diagnosis. While imaging studies can suggest the presence of a tumor and biopsies provide the tissue, it is the pathological examination that determines whether the cells are malignant, what type of cancer they represent, and how they should be classified.

Mesothelioma pathology is particularly challenging because the disease can closely mimic other cancers under the microscope. Specialized techniques, expert pathologists, and standardized diagnostic criteria are essential for accurate diagnosis. The pathology report becomes a cornerstone document for both treatment planning and legal proceedings.

The Three Cell Types of Mesothelioma

Epithelioid mesothelioma is the most common cell type, accounting for approximately 60 to 70 percent of all diagnoses. Epithelioid cells tend to form recognizable patterns — tubular, papillary, or solid — and generally respond better to treatment than other cell types. Patients with pure epithelioid tumors typically have the best prognosis, with longer median survival times and better response to chemotherapy and surgery.

Sarcomatoid mesothelioma accounts for approximately 10 to 20 percent of cases and is the most aggressive cell type. Under the microscope, sarcomatoid cells appear spindle-shaped and are arranged in a disorganized pattern that can resemble sarcomas (connective tissue cancers). This cell type is the most difficult to treat and carries the least favorable prognosis. Sarcomatoid mesothelioma is also the hardest to diagnose because it can closely mimic other spindle cell tumors.

Biphasic mesothelioma contains both epithelioid and sarcomatoid cells and represents approximately 20 to 30 percent of cases. The prognosis depends largely on the ratio of the two cell types — tumors with a higher proportion of epithelioid cells tend to behave more favorably. The pathologist documents the percentage of each cell type in the report, which directly influences treatment recommendations.

Immunohistochemistry: The Diagnostic Standard

Immunohistochemistry (IHC) is the primary laboratory technique used to confirm a mesothelioma diagnosis. IHC involves applying antibodies that bind to specific proteins on cell surfaces, producing a visible staining pattern that pathologists can interpret under the microscope. For mesothelioma diagnosis, a panel of multiple markers is used rather than any single test.

Markers that are typically positive in mesothelioma include calretinin, WT-1 (Wilms' tumor 1 protein), cytokeratin 5/6, and D2-40 (podoplanin). Markers that are typically negative in mesothelioma but positive in lung cancer include CEA (carcinoembryonic antigen), TTF-1 (thyroid transcription factor 1), MOC-31, and BerEP4. By using a panel of both positive and negative markers, pathologists can distinguish mesothelioma from adenocarcinoma, squamous cell carcinoma, and other cancers with a high degree of accuracy.

Understanding Your Pathology Report

The pathology report is a detailed medical document that summarizes the laboratory findings. Key elements include the confirmed diagnosis, the cell type and any subtypes, the immunohistochemistry results (which markers were positive and negative), and observations about the tissue architecture and cellular characteristics. The report may also include molecular testing results, such as BAP1 loss or CDKN2A deletion, which can provide additional prognostic information.

Patients and families should not hesitate to ask their oncologist to explain the pathology report in detail. Understanding the cell type, in particular, is important because it directly affects treatment options and prognosis. If there is any uncertainty about the diagnosis, requesting a second pathology opinion from a mesothelioma specialist center is a reasonable and recommended step.

Pathology in Legal Proceedings

The pathology report is among the most important documents in any mesothelioma legal case. It provides the definitive medical evidence that the patient has mesothelioma — a cancer caused almost exclusively by asbestos exposure. The report, along with the preserved tissue blocks and slides, may be reviewed by expert medical witnesses during litigation. Patients should ensure that their medical team preserves all pathology materials, as these can be critical evidence in pursuing full compensation.