Ancillary Studies in the Diagnosis of Mesothelioma
Last updated on June 17, 2024
Ancillary Studies in the Diagnosis of Mesothelioma
Why Ancillary Studies Matter
Distinguishing malignant mesothelioma from adenocarcinoma is one of the biggest diagnostic challenges in pathology. While histology alone provides important clues, it’s often not enough to make a definitive diagnosis. That’s where ancillary studies—including histochemistry, immunohistochemistry, and electron microscopy (EM)—come into play.
These tools enhance diagnostic accuracy, helping pathologists avoid misdiagnosis, which is critical because treatment options differ drastically between mesothelioma and adenocarcinoma.
Histochemistry: The Role of Mucin and Hyaluronic Acid Stains
Histochemical stains have been used for decades to distinguish mesothelioma from adenocarcinoma. The key difference?
- Adenocarcinomas produce mucin
- Mesotheliomas produce hyaluronic acid
Mucin Staining
- Periodic Acid-Schiff (PAS) with Diastase:
- Positive in adenocarcinomas (because of mucin production).
- Negative in mesotheliomas.
- Mucicarmine and Alcian Blue at pH 2.5:
- Mucicarmine stains mucin-positive adenocarcinomas.
- Alcian Blue (without hyaluronidase) may highlight hyaluronic acid in mesothelioma.
- Colloidal Iron:
- Can also stain hyaluronic acid, but results are variable.
Hyaluronic Acid Staining
- Alcian Blue (after hyaluronidase digestion):
- Loss of staining confirms the presence of hyaluronic acid, which is suggestive of mesothelioma.
Key Limitation:
Hyaluronic acid is water-soluble, meaning it can be lost during processing, leading to false negatives. That’s why histochemistry alone isn’t enough—you need immunohistochemistry for confirmation.
Immunohistochemistry: The Gold Standard for Mesothelioma Diagnosis
Immunohistochemistry (IHC) is the most commonly used ancillary tool in mesothelioma diagnosis. It relies on antibodies that either confirm mesothelioma or exclude adenocarcinoma.
Markers That Are Negative in Mesothelioma (Exclusionary Markers)
Since CEA (carcinoembryonic antigen) was first identified as an adenocarcinoma marker in 1979, many exclusionary markers have been developed.
| Marker | Positive in Mesothelioma? | Positive in Adenocarcinoma? |
|---|---|---|
| CEA | Negative | Positive |
| Ber-EP4 | Negative | Positive |
| B72.3 | Negative | Positive |
| CD15 (Leu-M1) | Negative | Positive |
| MOC-31 | Negative | Positive |
| BG-8 | Negative | Positive |
Markers That Are Positive in Mesothelioma (Inclusionary Markers)
Over the past two decades, newer markers have emerged that reliably identify mesothelioma.
| Marker | Positive in Mesothelioma? | Positive in Adenocarcinoma? |
|---|---|---|
| Calretinin | Positive | Negative |
| WT-1 | Positive | Negative |
| CK5/6 | Positive | Negative |
| D2-40 (Podoplanin) | Positive | Negative |
| Mesothelin | Positive | Negative |
🔬 Key Takeaway: A diagnostic panel should include at least two positive mesothelial markers and two negative adenocarcinoma markers for high accuracy.
New and Emerging Markers
Several newer markers are still undergoing clinical validation:
CK5/6: A Reliable Positive Marker for Mesothelioma
- Expression: Strong in epithelioid mesotheliomas, weaker in sarcomatoid mesotheliomas.
- Benefit: More specific for mesothelioma than calretinin.
- Limitation: Can be focally positive in some lung squamous cell carcinomas.
Calretinin: The Most Sensitive Mesothelial Marker
- Expression: Consistently positive in benign and malignant mesothelial cells.
- Benefit: Highly sensitive and specific for mesothelioma.
- Limitation: Rarely, ovarian and endometrial carcinomas can show weak positivity.
MOC-31: A Cautionary Tale
- Once thought to be a specific adenocarcinoma marker, MOC-31 is now known to stain up to 39% of mesotheliomas, making it less reliable.
Electron Microscopy: The Ultimate Diagnostic Tool
While histochemistry and IHC are the mainstays of mesothelioma diagnosis, electron microscopy (EM) remains a powerful tool in difficult cases.
How EM Helps
- Confirms mesothelial origin in equivocal cases.
- Distinguishes mesothelioma from adenocarcinoma when IHC results are inconclusive.
- Useful for legal and research purposes (e.g., asbestos exposure cases).
Key EM Features of Mesothelioma vs. Adenocarcinoma
| Feature | Mesothelioma | Adenocarcinoma |
|---|---|---|
| Microvilli | Long, slender, numerous | Short, few |
| Glycocalyx | Absent | Present |
| Secretory Products | Hyaluronic acid | Mucin |
| Desmosomes | Large, numerous | Small, fewer |
Key Takeaway: If IHC is inconclusive, EM can confirm mesothelioma with high accuracy.
Recommended Diagnostic Workflow
Given the importance of ancillary studies, a step-by-step diagnostic approach is recommended:
Start with Histology and Cytology:
- Look for pleural thickening, papillary formations, and mulberry-like clusters.
- Check for hyaluronic acid in effusions.
Use Immunohistochemistry (IHC):
- Apply at least two positive mesothelial markers and two exclusionary markers.
- If results are conflicting, proceed to electron microscopy.
Consider Electron Microscopy (EM) in Difficult Cases:
- If IHC is inconclusive, EM can confirm the diagnosis.
- EM is particularly useful for medico-legal cases.
If Still Uncertain, Perform Molecular Testing:
- BAP1 Loss → Strongly suggests mesothelioma.
- CDKN2A (p16) Deletion → Frequently seen in mesothelioma, but not adenocarcinoma.
Final Thoughts
Histochemistry helps but isn’t definitive.
Immunohistochemistry (IHC) is the gold standard for diagnosis.
Electron microscopy (EM) is useful for difficult cases.
Using multiple markers increases diagnostic accuracy.
Molecular tests like BAP1 and p16 deletion may play an increasing role.
Key Takeaway: No single test is enough—using a combination of histochemistry, IHC, and EM ensures a more accurate diagnosis of mesothelioma vs. adenocarcinoma.