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Asbestos-Related Disease Imaging

Practice Essentials

Asbestos is the name given to a group of naturally occurring minerals that are resistant to heat and corrosion. Asbestos includes the mineral fibers chrysotile, amosite, crocidolite, tremolite, anthophyllite, actinolite, and any of these materials that have been chemically treated or altered. Chrysotile is by far the most common type of asbestos fiber produced in the world and accounts for virtually all asbestos used commercially in the United States.

The main preventable cause of malignant mesothelioma has been exposure to commercial materials made or contaminated with asbestos. The most common exposure to commercial asbestos is occupational, although workers’ families are also at risk from indirect “take-home” exposures transported by contaminated items such as clothing. Contamination of the living environment from asbestos-containing products is another source of exposure.

Asbestos has been used in products such as insulation for pipes, floor tiles, building materials, and vehicle brakes and clutches. Heavy exposures tend to occur in the construction industry and in ship repair, particularly during the removal of asbestos materials due to renovation, repairs, or demolition. Workers are also likely to be exposed during the manufacture of asbestos products (such as textiles, friction products, insulation, and other building materials) and during automotive brake and clutch repair work. Exposure to asbestos occurs through inhalation of fibers in air in the working environment, ambient air in the vicinity of factories handling asbestos, or indoor air in housing and buildings containing asbestos materials.

Because the development of asbestosis is dose dependent, symptoms appear only after a latent period of 20 years or longer. This latent period may be shorter after intense exposure.

Case studies of asbestos-related disease are illustrated in the images below.

Case 1. Postero-anterior (PA) chest radiograph in

Case 1. Postero-anterior (PA) chest radiograph in a 58-year-old man with a history of occupational exposure to asbestos shows right diaphragmatic pleural plaque calcifications, linear calcification along the left pericardium, and bilateral pleural plaques along upper ribs.

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Case 4. The soft-tissue window setting of this che

Case 4. The soft-tissue window setting of this chest computed tomography (CT) scan shows the envelope-like mass along the pleural surface surrounding the lung. This was a mesothelioma.

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The spectrum of asbestos-related thoracic diseases includes the following
:

Benign pleural effusion

Pleural plaques

Diffuse pleural thickening

Rounded atelectasis

Asbestosis

Mesothelioma

Lung cancer

Helsinki guidelines for the diagnosis of asbestos-related disorders defines asbestosis as diffuse interstitial fibrosis of the lung as a consequence of exposure to asbestos dust.
 Mesothelioma is a malignant pleural or peritoneal tumor that rarely occurs in patients who have not been exposed to asbestos.

The diagnostic approach to asbestos-related intrathoracic disease is different from that of other diffuse lung diseases because of the medicolegal implications.
The likelihood of asbestos-related disease should be determined, and other possible causes should be eliminated. An assessment of the extent of disease is used to calculate compensation. Therefore, imaging plays a pivotal role in the diagnosis, staging, response assessment, and surveillance of asbestos-related disease.

The key clinical goals of imaging in malignant pleural mesothelioma are early detection of disease; optimizing sensitivity and specificity for anatomic involvement of unresectable planes to identify patients who are suitable for surgical resection; improving prognostication; and assessing response to treatment as a surrogate for therapeutic benefit.

Preferred examination

High-resolution computed tomography (HRCT) scanning is playing an increasingly important role in the diagnosis of diffuse interstitial lung disease. However, chest radiography remains the initial modality for the detection and characterization of pleural and parenchymal disease. Ultrasonography has a role in characterizing pleural effusions and guiding pleural aspiration and biopsy.

Nuclear medicine study has a limited role in the investigation of asbestos-related intrathoracic disease. Gallium-67 (67Ga) citrate testing has been used to differentiate benign from malignant, asbestos-related pleural disease and to give a quantitative index of inflammatory activity.

The imaging of mesothelioma typically includes computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) scans; these scans may be acquired during initial tumor diagnosis and staging, treatment response assessment, or patient surveillance.

Optical imaging, using electromagnetic radiation in (or near) the visible light region of the spectrum, is now being applied in the intraoperative setting for mesothelioma. Initial results indicate the potential for optical imaging to aid surgeons in their attempt to achieve a macroscopic complete resection.

The Helsinki guidelines for the diagnosis of asbestos-related disorders recommend the use of CT imaging in diagnosis of asbestos-related diseases under the following circumstances
:

A borderline finding of lung fibrosis (ILO 0/1-1/0) is detected

There is a discrepancy between lung function finding of restriction and radiographs interpreted as normal

Widespread pleural changes severely hamper the radiographic visibility of lung parenchyma

The criteria for diagnosis of asbestosis on CT include the following:

Sum grade of ≥2–3 bilateral irregular opacities in lower zones according to the reference film, or

Bilateral honeycombing (sum grade ≥2) would be sufficient to represent fibrosis according to the ICOERD system

The guidelines further note that subpleural curvilinear lines or dots in HRCT are findings of bronchiolar fibrosis.

According to the  Austrian Mesothelioma Interest Group (AMIG) consensus statement for the diagnosis and staging of malignant pleural mesothelioma, after initial imaging with CT scan and confirmation of disease via video-assisted thoracic surgery (VATS), a potential candidate for surgical treatment should undergo PET-CT scanning to rule out distant metastases and involvement of the abdomen and the mediastinal lymph nodes. In some cases of unclear involvement of adjacent structures (eg, chest wall), MRI can be added in order to judge the resectability.

Limitations of techniques

The limitations of chest radiography in the diagnosis and evaluation of asbestos-related disease are well recognized. The quality of the radiograph and the size, shape, position, and degree of calcification determine whether the radiologist can detect pleural plaques on the image. While the identification of bilateral, scattered, calcified, costal, and diaphragmatic pleural plaques is virtually diagnostic of asbestos exposure, studies have shown an 11% false-positive rate with chest radiographs. In particular, extrapleural fat mimics pleural thickening and is a significant cause of false-positive readings. Conversely, a high false-negative rate has also been reported.

CT scans have long been known to be more sensitive and specific than chest radiographs for the diagnosis of asbestos-related pleural disease.

Radiographic-pathologic studies have shown that chest radiographic findings are normal in as many as 20% of patients with asbestosis. HRCT scanning is more sensitive and specific than other studies, particularly when images are obtained with the patient in the prone position, which allows differentiation of mild parenchymal changes from dependent density (increased attenuation of the posterior [usually basal] lung, which is gravity induced and secondary to nonaeration of dependent alveoli).

In addition, it should be noted that mild asbestosis is difficult to distinguish from interstitial fibrosis in heavy smokers. In one report, of 24 patients judged to have absestosis radiographically, only 6 had asbestosis histopathologically, while the remaining 18 cases (in patients with a mean smoking history of 53 pack-years) had interstitial fibrosis judged to be most consistent with smoking-associated fibrosis.

Tumor contrast enhancement in MRI provides information about tumor vascularity, but the conventional time delay between contrast injection and initiation of image acquisition might be too short for optimal assessment of mesothelioma. A perfusion-based MRI technique, designed to capture early contrast-enhancement features of the pleura characteristic of early-stage mesothelioma, may increase the role of MRI in tumor detection.

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