The three cell or subtypes of mesothelioma, epithelial, bi-phasic (mixed) and sarcomatoid, are different not only in their aggressiveness and response to treatment but in their appearance under the microscope as well.
Although immunohistochemical staining has improved over the years and it now plays an important role in the diagnosis of mesothelioma, for many scientists, electron microscopy is considered the standard by which mesothelioma is differentiated from other forms of cancer. This is particularly true in the differential diagnosis of epithelial mesothelioma versus adenocarcinoma.
The epithelial cell type is the most common, comprising 50% to 70% of all malignant mesotheliomas diagnosed. Because it is the least aggressive of the cell types, it generally responds the best to treatment, and offers the best prognosis. Under the microscope, this cell type appears relatively uniform, and is described as having a tubular papillary structure. Each individual cell is cube-shaped and has an easily identifiable nucleus.
By contrast, sarcomatoid mesothelioma represents 7% to 20% of cases diagnosed, and is the most aggressive subtype. It generally does not respond well to treatment, therefore, prognosis with this cell type is considered poor. The differential diagnosis of sarcomatoid mesothelioma versus other types of sarcoma is more challenging than in the epithelial variant, and chemical staining can be confusing. Microscopic analysis normally shows a spindle cell or storiform structure with enlongated nucleii not as apparent as in the epithelial type.
The bi-phasic or mixed cell type, as the name implies, is a combination of elements of both the epithelial and sarcomatoid subtypes, with components of each in the same tumor, or found in specific groupings throughout the tumor. Prognosis for biphasic mesothelioma is normally intermediate between the two types. This cell type makes up 20% to 35% of mesotheliomas.
Rare histological variants of malignant mesothelioma include:
- Small cell mesothelioma.
- Lymphohistiocytoid mesothelioma.
- Deciduoid mesothelioma.
- Desmoplastic mesothelioma.
It is important not to confuse malignant mesothelioma with benign tumors that also begin in the mesothelium. Benign or low grade mesotheliomas almost always involve the peritoneal cavity, but are rare in the pleura. This group includes:
- Well-differentiated papillary mesothelioma.
- Benign multicystic mesothelioma.
- Adenomatoid tumor.
When discussing treatment options, cell type should be considered. Information regarding cell type is normally found in the ‘Final Diagnosis’ section of the pathology report.