An important tool used in the definitive diagnosis of disease is histology, the microscopic examination of cellular anatomy. Mesothelioma tumors typically present with three distinct histological abnormalities.
Malignant Epithelial Mesothelioma
Epithelial cells make up the tissues that line the surfaces and cavities of the human body. In a malignant epithelial mesothelioma tumor, up to 60% of the cells take on a different shape and set of properties called epithelioid, with a distinctly round or cuboidal shape and elongated microvilli when viewed under a microscope.
Malignant epithelial mesothelioma is by far the most common histologic subtype of mesothelioma; patients who are diagnosed with epithelial mesothelioma typically have the longest survival rates among mesothelioma patients. The histological changes associated with these kinds of tumors, however, are very similar to those found in adenocarcinoma and other tumors of the lungs, breasts and colon, requiring more discriminating diagnostic procedures. A new test called the miRview Meso test has shown great promise in this area.
Malignant Sarcomatoid Mesothelioma
Connective tissues are the fibrous tissues that connect and support other tissues and organs in the human body; they make up muscles, tendons, cartilage, blood and bone. Cancers that arise from connective tissues are called sarcomas, and sarcomatoid mesothelioma is so named because under the microscope, its cells are elongated and spindle-shaped like those of a sarcoma, overlapping in a disorderly fashion. Between 15% and 20% of all malignant mesothelioma diagnoses are classified as sarcomatoid.
Sarcomatoid mesothelioma is more aggressive than other histologic subtypes of mesothelioma and consequently more difficult to treat. Sarcomatoid mesothelioma cells are notably more resistant to chemotherapy than either epithelial or biphasic mesothelioma. Patients who have been diagnosed with sarcomatoid mesothelioma generally have the lowest survival rates among mesothelioma patients.
Malignant Biphasic Mesothelioma
Malignant biphasic mesothelioma is a histological subtype made up of both epithelial and sarcomatoid mesothelioma cells. Between 20% to 35% of mesothelioma patients are diagnosed with this form of the disease. There is no commonly accepted standard for diagnosis among members of the medical community although the World Health Organization recommends that the diagnosis be used when a tumor is comprised of at least 10% epithelial cells and at least 10% sarcomatoid cells. Survival rates for patients diagnosed with biphasic mesothelioma vary according to the ratio of epithelial to sarcomatoid cells in the tumor: The higher the number of sarcomatoid cells, the poorer the patient’s prognosis will be.
In rare cases, tumors of other histological types have been reported. Deciduoid, desmoplastic, lymphohistiocytoid, and small cell mesotheliomas have all been identified.