Malignant Mesothelioma

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Malignant pleural mesothelioma is a complication most commonly occurring due to asbestos exposure. Tobacco is not considered an enhancing factor/synergistic in the etiology for mesothelioma, as it is for lung carcinoma. This malignancy most often involves the lung lining (pleura, but can also involve the heart lining (pericardium), the abdominal lining (peritoneum) and the testicle lining (tunica vaginalis testis).

Imaging Findings:

In the classic case, which is not always the circumstance, there is:

  • Pleural thickening that is lobulated.

  • Pleural thickening that is often greater than 1 cm in thickness.

  • Pleural thickening that is encasing the entire lung.

  • Pleural thickening that most importantly involves the mediastinal pleura.

  • An associated ipsilateral pleural effusion (exudative).

Other Imaging Findings:

  • Discrete Pleural Nodules.

  • Nodular Fissural Thickening.

  • Seeding Along Various Paths – be they fissures or along chest tube, trocar and biopsy tracks as well as incision sites. Seeding can often occur after video thorascopic surgery or image-guided biopsies.

  • Volume Loss to the Lung (due to encasement, which prevents the effusion from shifting the contents towards the contralateral side, but the hemi-thorax can be normally expanded and/or even displaced away, but less likely).

  • Pneumothorax.

  • Invasion into the Intercostal Muscles.

  • Invasion Beyond the Chest Wall into the adjacent other muscles with a very useful finding of Obliteration of Extra-Pleural Fat Planes.

  • Rib Displacement or Destruction, as well as vertebral body invasion and destruction.

  • Extension Below the Diaphragm, which is very important in determining prognosis.

  • Mediastinal Invasion including the pericardium.

  • Invasion of Veins and encasement of them.

  • Spread to the Contralateral Hemi-thorax.

  • Rare Osteosarcomatous Component with extensive focal calcification.

  • Late Liver and Lung Metastases, which can be lymphatic and/or hematogenous. Lymph nodes can be involved as can the adrenals and kidneys.

  • Clubbing and Hypertrophic Pulmonary Osteoarthropathy can occur.

Types of Mesotheliomas:

  • Epithelial – usually with a better prognosis.

  • Mesenchymal (Fibers and Sarcomatous).

  • Mixed Types.

Asbestos and Asbestiform Fibers as the Primary Risk Factor:

Asbestos Fibers can be serpentines and amphiboles. The most common serpentine type of asbestos is called Chrysotile, otherwise, known as “White Asbestos.” The Amphiboles include Crocidolite, otherwise, known as “Blue Asbestos”, Amosite, Tremolite, Actinolite and Anthophyllite. Crocidolite is the most carcinogenic followed by Chrysotile because of its greatest use and frequent contamination with Tremolite and lastly Amosite in occupational settings. Anthophyllite is not known to cause mesotheliomas. In non-occupational settings, however, Tremolite, which is often found in the soil and used by villagers to whitewash their homes in Greece, Turkey and New Caledonia can also cause mesotheliomas. Asbestiform Fibers not due to asbestos, but due to fibers minerals such as Zeolite and more importantly Erionite, primarily found in central Turkey can also be associated with mesothelioma, asbestos-type pleural plaquing and interstitial fibrosis.

Other Risk Factors:

  • Irradiation to the chest.

  • Tuberculosis or other chronic inflammatory diseases.

  • Perhaps AIDS.

  • The Simian Virus SV40, which was a contaminant in polio vaccine in the late 50s and early 60s, may account for a number of mesotheliomas. Perhaps this combination with asbestos exposure as a co-carcinogen results in the fact that even minimal exposure to asbestos, may result in a mesothelioma.

  • Familial Mesotheliomas with asbestos and erionite exposures.

Differential Diagnosis of a malignant mesothelioma includes:

1. Primary Pleural Tumors:

  • Mesothelioma.

  • Pleuropulmonary Synovial Sarcoma, only if focal looking

  • Liposarcoma.

  • Localized Fibrous Tumor of the Pleura

  • Lipoma (benign).

2. Secondary Tumor Spread to Pleura:

  • Metastases primarily from the lung and/or breast.

  • Lymphoma.

  • Thymoma.

  • Plasmocytoma and rarely other tumors.

3. Non-Tumor Mimickers:

  • Empyema – pus within the chest cavity between the parietal and visceral pleura, which is usually more localized – look for the split pleura sign and small air bubbles.

  • Tuberculous pleuritis.

  • Benign asbestos-caused pleural plaquing.

  • Diffuse pleural thickening.

  • Parapneumonic effusions.

A solitary mass can rarely present as a localized mesothelioma; however, it is most often a localized fibrous tumor of the pleura and/or a pleuropulmonary synovial sarcoma. These are large masses that are sharply marginated. They can occur within the lung and/or chest wall, pericardium or mediastinum.

Comparison of Mesothelioma to Adenocarcinoma:

The origin of mesothelioma (chest wall-based) is in part from the parietal pleura as opposed to adenocarcinoma (lung parenchymal based), which originates purely in the visceral pleura.

  • Plaques help in pointing to an asbestos cause from mesothelioma, but are often not identified.

  • Chest Wall Invasion: Especially if involving more than one site, is more likely a finding of mesothelioma. Look for loss of the extra-pleural fat plane.

  • Volume Loss is greater with mesotheliomas than with metastases from adenocarcinoma.

  • Malignant pleural thickening is more often caused by metastases than mesotheliomas, only because statistically mesothelioma is the rare disease. Unilateral pleural effusions caused by metastatic disease are most commonly from the lung/adenocarcinoma, breast, ovary and gastric carcinoma plus lymphomas.

  • Systemic metastases are often due to malignancies other than mesothelioma, since statistically metastases in mesothelioma are late in the process. This does not rule out a mesothelioma, however.

PET Scanning:

It is helpful in separating out malignant versus benign pleural disease such as pleural mesothelioma versus benign asbestos-caused diffuse pleural thickening or plaquing. The differentiation is determined by increased standardized uptake values (SUVs). SUV values are often significantly higher in mesothelioma or other pleural metastases compared to the infectious or inflammatory conditions, which also have elevated SUVs compared to the benign conditions. There can be overlap however. False-positives can occur with infection, uremic pleuritis and talc pleurodesis. PET scanning is very useful for extra-thoracic metastases, which can further grade the probability and the usefulness of treatment. PET scanning is less helpful for mediastinal nodular metastases. Often, the higher the SUV values, the less the survival time. Skipped areas can occur to the thickened pleura that are PET-positive and thus, PET scanning can also be useful in identifying the best biopsy sites.

Laboratory Values:

Mesotheliomas have decreased carcinogenic embryonic antigen (CEA), whereas lung metastases have increased CEA. Mesotheliomas also have increased fluid hyaluronate.

The above article is presented as a public service by Daniel Powers, M.D., B-Reader and Diagnostic Radiologist, certified by the American Board of Radiology. It represents a general review of some literature. There may be other data and opinions on such.

If you detect any errors, have additional information to point me too, use other useful terms or have comments, please do email them to me at powersmd@gmail.com.