Rare Cell Types of Mesothelioma

Mesothelioma is a complex disease, with some subtypes being particularly rare and challenging to diagnose and treat. These subtypes often fall under the broader categories of epithelioid, sarcomatoid, and biphasic mesothelioma but can also exist outside these classifications.

Distinct Subtypes of Epithelioid Mesothelioma

Epithelioid mesothelioma is the most common form, accounting for 70-80% of cases, and generally offers a more favorable prognosis than sarcomatoid types. This subtype is characterized by cells that are typically round or oblong and may contain multiple nuclei. They are known for forming clusters and tend to metastasize more slowly. Within this category, there are several rare subtypes:

• Adenomatoid Mesothelioma: This subtype may present as benign or malignant, often found in gland-like structures, particularly in the genital glands. Prognosis can extend up to a decade, depending on the nature of the tumor.
• Cystic Mesothelioma: Commonly benign and most prevalent in women of reproductive age, cystic mesothelioma forms smooth cysts and has a risk of recurrence and malignant transformation.
• Deciduoid Mesothelioma: A highly rare and aggressive form, deciduoid mesothelioma typically occurs in the peritoneum and has a relatively short median survival time.
• Papillary Mesothelioma: This subtype is often benign and more common in women, characterized by slow growth and a generally favorable prognosis.

According to Modern Pathology, In a study, 21 cases of deciduoid mesothelioma were examined using a broad set of immunohistochemical markers and electron microscopy for 9 cases. The group consisted of 15 men and 6 women, average age 60, with most cases (17) originating in the pleura and 4 in the peritoneum. Histological examination showed all cases had large, polygonal or ovoid cells with distinct borders and dense cytoplasm. Some cases showed cells with varying sizes, reduced cohesion, significant nuclear atypia, and high mitotic activity (>5 per 10 HPF), while others had more uniform cells with lower mitotic rates. Patients with the former characteristics had a mean survival of 7 months, compared to 23 months for the latter group, indicating a high-grade, aggressive subset within deciduoid mesothelioma affects prognosis.

Sarcomatoid Mesothelioma’s Rare Subtypes

Sarcomatoid mesothelioma is known for its aggressive nature and poor prognosis. It includes rare subtypes like desmoplastic, lymphohistiocytoid, and transitional mesothelioma:

• Desmoplastic Mesothelioma: Predominantly composed of dense, fibrous tissue, this subtype has a short average survival.
• Lymphohistiocytoid Mesothelioma: Accounting for less than 1% of cases, it can be misdiagnosed due to its resemblance to other conditions.
• Transitional Mesothelioma: Extremely rare and difficult to diagnose, it often results in a prognosis of less than a year.

Other Rare Mesothelioma Categories

Beyond these subtypes, there are mesothelioma forms that do not fit neatly into the typical classifications. These include heterologous mesothelioma, small cell mesothelioma, and localized fibrous tumors. Given their rarity, treatment approaches and prognoses are less defined and often follow general mesothelioma treatment protocols.

Diagnostic Approach to Rare Mesothelioma Types

Diagnosis of these rare mesothelioma forms mirrors the approach used for more common types, involving symptom assessment, imaging, and biopsy analysis. Histological examination, studying the microscopic anatomy of cells, is crucial. Immunohistology, which uses antibodies to detect specific cellular proteins, is also employed to differentiate mesothelioma from other cancers.