Learning Objectives
Master the unique characteristics of Mesothelioma. Distinguish it from lung adenocarcinoma based on risk factors, immunohistochemistry, and ultrastructural findings for the USMLE Step 1.
1. Clinical Presentation and Associations
Mesothelioma is a highly aggressive malignancy of the pleura (or, less commonly, the peritoneum). It is strongly linked to chronic asbestos exposure, typically with a long latency period of 25–40 years.
| Finding | Description |
|---|---|
| Risk Factors | Asbestosis is the primary driver. Tobacco smoking is NOT a risk factor for mesothelioma (though it is for bronchogenic carcinoma). |
| Pleural Findings | Hemorrhagic pleural effusion (exudative) and extensive pleural thickening that can encase the lung (“pleural rind”). |
| Symptoms | Dyspnea, chest pain, and weight loss. |
2. Histology and Immunohistochemistry
Differentiating mesothelioma from lung adenocarcinoma is a frequent board exam topic. Immunohistochemistry (IHC) is the gold standard for diagnosis.
| Diagnostic Tool | Mesothelioma Finding | Comparison to Carcinoma |
|---|---|---|
| IHC Markers | Calretinin (+) and Cytokeratin 5/6 (+) | Most carcinomas are negative for these markers. |
| Light Microscopy | May show Psammoma bodies (laminated calcifications). | Also seen in Papillary thyroid cancer, Serous ovarian cancer, and Meningioma. |
3. Electron Microscopy (EM) Findings
Ultrastructural features are highly specific for mesothelial cells.
| EM Feature | Details |
|---|---|
| Microvilli | Typically long, slender, and branching (vs. short/blunt in adenocarcinoma). |
| Cell Structure | Polygonal tumor cells with prominent desmosomes and tonofilaments. |
Activity:
High-Yield Clinical Pearls:
- Smoking Paradox: While smoking increases the risk of lung cancer in asbestos workers by ~60x, it does not increase the risk of mesothelioma specifically.
- Psammoma Bodies: Remember the mnemonic PSaMMoma (Papillary thyroid, Serous cystadenocarcinoma, Meningioma, Mesothelioma).
- Imaging: Look for a unilateral pleural mass or nodular thickening that follows the contours of the diaphragm and mediastinum.