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Rare Mesothelioma Cell Types

Rare types of mesothelioma are often subtypes of epithelioid, sarcomatoid and biphasic mesothelioma, though some cannot be classified. Due to a limited number of cases, rare types of mesothelioma can be difficult to diagnose and are associated with varying life expectancies.

Rare mesothelioma types include adenomatoid, cystic, deciduoid, papillary, lymphohistiocytoid, desmoplastic, transitional, heterologous, small cell and local fibrous tumors. Rare types can be malignant or benign. Malignant forms of these cell types typically have a poor prognosis of less than one year, while patients with benign forms often achieve long-term survival. Cell type will help physicians predict tumor progression and determine the best mode of treatment for the mesothelioma patient.

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Rare Types of Epithelioid Mesothelioma

Epithelial cells make up around 70 – 80% of mesothelioma cases and demonstrate a better prognosis than sarcomatoid cells. These cells are found in many different shapes, but often appear round or oblong, sometimes with multiple nuclei. The defining characteristic of epithelioid cells is how they group together, forming clusters with slow metastasis.

Epithelial mesothelioma has four main rare cell types, including adenomatoid, cystic, deciduoid and papillary. With limited data, there is no standard of care for each type, though there are treatments that have been effective in cases that have emerged, including surgery, chemotherapy, radiation or a combination of these treatments (multimodal therapy).

Adenomatoid Mesothelioma

Adenomatoid mesothelioma, also referred to as glandular mesothelioma or microglandular mesothelioma, can be classified as benign or malignant. Adenomatoid cells line small, gland-like structures, and are usually found in the genital glands of both men and women. A limited number of cases have shown a life expectancy of 10 years.

Cystic Mesothelioma

Cystic mesothelioma occurs in the form of cysts and has a better prognosis than most cases of mesothelioma as it largely develops as benign. One study noted more than half of reported patients have experienced recurrence with a progression-free survival rate of 72%. However, other studies have shown potential for malignant transformation and malignant recurrence, which could negatively impact survival. This type of cancer is most common in women of reproductive age.

Deciduoid Mesothelioma

Deciduoid mesothelioma is extremely rare, very unique, and has only been diagnosed in a small number of cases. It typically arises in the peritoneum. Deciduoid mesothelioma in general has been associated with a poor prognosis of only about five or six months. The cause of this cancer is unknown, though some doctors feel that it’s caused by a hormonal imbalance and others suspect a correlation with asbestos exposure.

Papillary Mesothelioma

Papillary mesothelioma, also known as well-differentiated papillary mesothelioma (WDPM), is most common in women of reproductive ages, though men have also been diagnosed. Prognosis is typically favorable, as most cases are benign. One study notes a 10-year survival rate of 30.8%.

Rare Types of Sarcomatoid Mesothelioma

Sarcomatoid mesothelioma is much harder to diagnose than epithelial as sarcomatoid cells strongly resemble healthy tissue. These types of cancer cells are aggressive and grow quickly, limiting treatment options and resulting in poor survival rates for patients affected. Sarcomatoid mesothelioma has three main rare subtypes, including desmoplastic, lymphohistiocytoid and transitional mesothelioma.

Desmoplastic Mesothelioma

Most desmoplastic mesothelioma tumors are composed of sarcomatoid cells, though some epithelial and biphasic cases have emerged. At least 50% of desmoplastic tumors are composed of dense, fibrous tissue. Accumulated cases have shown an average life expectancy of 3.8 months.

Lymphohistiocytoid Mesothelioma

Making up less than 1% of all mesothelioma diagnoses, most cases of lymphohistiocytoid mesothelioma are sarcomatoid, though researchers have noted a few cases of biphasic tumors. These tumors are composed of dense bundles of inflammatory immune cells, closely resembling other conditions. This cancer is commonly misdiagnosed as non-Hodgkin’s lymphoma, lymphoepithelial carcinoma, sarcomatoid carcinoma of the lung and ganglioneuroma.

Transitional Mesothelioma

Transitional mesothelioma is difficult to diagnose due to cell structure. These tumors are extremely rare, with only a few case reports referencing the subtype. Studies have noted life expectancies of less than one year.

Other Rare Types of Mesothelioma

Some mesothelioma cancer subtypes cannot be definitively classified as epithelial, sarcomatoid or biphasic, including heterologous mesothelioma, small cell mesothelioma and localized fibrous tumors. The rarest of all rare types, treatment and prognosis for these cases are largely unknown, leading physicians to rely on standard malignant mesothelioma treatment plans.

Heterologous Mesothelioma

Heterologous tumors are recognized by the presence of bodily tissues from other areas of the body, including bone, cartilage, and soft tissues. One study looking at 27 patients saw an average survival of six months.

Common heterologous elements include osteosarcomatous and chondrosarcomatous. Such cases are labeled as tumors with osteosarcomatous differentiation or chondrosarcomatous differentiation in studies.

Osteosarcomatous Differentiation

Osteosarcomatous differentiation is a type of heterologous mesothelioma. In some cases of malignant pleural mesothelioma, osteosarcomatous differentiation has been recorded. Exceedingly rare, there is not much data for this type of mesothelioma, however, it’s classified by the presence of bone cells within the tumors.

Chondrosarcomatous Differentiation

Chondrosarcomatous differentiation is a type of heterologous mesothelioma, recognized by the presence of spindle-shaped cells from cartilage.

Localized Fibrous Tumors

Also referred to as benign fibrous mesothelioma, localized fibrous tumors (LFT) have been seen in both men and women. Case reports have found that about half of patients experience symptoms, while half are asymptomatic. Typically classified as sarcomatoid, patients usually see a life expectancy of less than one year.

Small Cell Mesothelioma

Small cell mesothelioma is also frequently misdiagnosed. Only seen in a few cases, mesothelioma specialists typically rely on their knowledge of common types of mesothelioma to determine their treatment approach. In a study of eight cases, average survival was 8.2 months.

Diagnosing Rare Forms of Mesothelioma

Diagnosis of rare types of mesothelioma is the same as common types. Examination of symptoms, imaging scans and biopsies are performed to pinpoint the disease, with tissue biopsies being the determining factor of malignancy and cancer type. In some cases, multiple biopsies may be necessary to figure out the exact type, since some tumors are composed of different types of tissues. A pathologist experienced in mesothelioma diagnoses is needed to ensure the correct diagnosis.

Mesothelioma Histology

Histology, the study of cellular anatomy, has been a large component in understanding different mesothelioma cell types and determining the best paths for treatment. A pathologist will first collect a tissue sample, which will be preserved and embedded, making them easy to cut and segment. After sectioning, tissues are mounted and stained for a detailed view into cell structure. In addition to traditional histology methods, immunohistology is frequently used in mesothelioma diagnoses, utilizing antibodies to detect and identify specific proteins that can differentiate mesothelioma from other cancers.

An accurate diagnosis is needed to determine prognosis and treatment for the patient, which can be challenging for rare types of mesothelioma. With more research and emerging cases, mesothelioma specialists can hopefully make advances in treatment options and efforts towards finding a cure.

Author: Linda Molinari

Editor in Chief, Mesothelioma Cancer Alliance

Linda Molinari

Reviewer: Dr. James Stevenson

Medical Reviewer and Thoracic Medical Oncologist

Dr. James Stevenson

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