The International Labor Organization (ILO) has a grading system for the amount of interstitial lung disease scarring on chest x-rays in individuals that have had occupational dust and fiber exposures (aka pneumoconioses). The grading is 0 = no lung abnormalities, 1 = mild interstitial lung disease, having the appearance of a pneumoconiosis, 2 = moderate interstitial lung disease having the appearance of a pneumoconiosis, 3 = severe interstitial lung disease having the appearance of a pneumoconiosis. The short form used is only 0, 1, 2, and 3. The long form, which is the ILO form you are most likely to see, allows one to add subcategories such as 1/1 = first thought is mild disease, second thought is mild disease or for example, 1/0 = mild disease tending towards no disease or 1/2 = mild disease tending towards moderate disease. The ILO form also describes the size and shape of the interstitial disease – either small rounded opacities or small irregular opacities, their locations by lung zones – upper, middle or lower and right or left side; the presence of pleural effusions, pleural plaquing or diffuse pleural thickening and the presence or absence of suspect or apparent carcinoma or mesothelioma and other miscellaneous findings.
The CT/HRCT scan technology has been available since the late 1970s. This is far more accurate and detailed than chest x-rays because chest x-rays represent a composite of the body tissues – the skin, fat, ribs, pleura, heart, mediastinum and lung tissue, all superimposed upon each other, whereas the CT and HRCT allow for cross-sectional pictures, that separate out these various structures and, therefore, give more accurate information. In the area of pleural plaquing, chest x-rays are the least accurate followed by CT, followed by HRCT and followed by autopsy, being the most accurate (providing that the same area of interest is scanned or reviewed at autopsy).
Supine spiral CT and/or supine HRCT are best utilized for the identification of pleural plaquing, nodules and moderate-to-severe interstitial lung disease, having the appearance and distribution of asbestosis. Mild asbestosis cannot be determined because when one lies on their back, gravitational blood pooling and gravitational partial lung collapse (atelectasis) caused by pressure from the top and middle of the lungs onto the back of the lungs, results in a haziness in the back of the lungs known as dependent density. Mild interstitial/parenchymal lung disease in asbestosis occurs at the back bottom of the lungs (their posterior aspects) and looks like a hazy density. Therefore, the supine (lying on your back) spiral CT or supine HRCT cannot differentiate when one only has haziness in the back bottoms of their lungs, whether this is due to gravitational dependent density and/or mild interstitial lung disease, having the appearance and distribution of asbestosis or a combination of both. However, moderate and severe disease can be easily determined, since it involves other non-dependent portions of the lungs and has additional imaging findings.
Prone HRCT is the study of choice for identifying mild interstitial lung disease of asbestosis. The HRCT utilizes thinner slices, giving more detailed information about the lung tissue findings, plus the prone position (lying on one’s stomach) forces the gravitational dependent density to move to the anterior aspects of the lungs, which we are not interested in, because asbestosis occurs in its mild form, in most individuals, at the back bottoms of the lungs. Thus, in the prone position, the backs of the lungs are sticking up in the air and if the haziness at the posterior aspects of the backs of the lungs clear, that means that it was all gravity-caused dependent density, whereas if the haziness remains, that means that there are abnormal changes within the lung tissue, which with the correct pattern and imaging findings, has the appearance and distribution of asbestosis. Prone HRCT can, therefore, identify mild, moderate and severe disease. The peer-reviewed literature, including journals and textbooks, have many articles that show what mild, moderate and severe disease looks like in asbestosis or diseases looking similar to it. Thus, although the ILO has yet, in over 30 years of CT/HRCT availability, come up with a specific set of standard images or grading system for CT and HRCT, peer reviewed literature clearly shows mild, moderate and severe disease. Therefore, one can easily grade 0, 1, 2 and 3 – nothing, mild, moderate and severe disease profusion of interstitial lung disease for prone HRCT and 0, 2 and 3 – nothing, moderate and severe disease profusion of interstitial lung disease for supine spiral CT and supine HRCT.
CT and HRCT can be Graded into No, Mild, Moderate and Severe Interstitial Lung Disease, Similar to the Chest X-Ray ILO Grading of 0/0, 1/1, 2/2 and 3/3:
CT and HRCT scans of the chest, based upon peer-reviewed literature, are excellent for identifying the presence or absence of interstitial lung disease, having the appearance and distribution of asbestosis, whose profusion or quantity can be graded as negative or no lung disease = 0, 1 = mild, 2 = moderate and 3 = severe disease. This would be the equivalent on a subcategory ILO x-ray scale of 0/0, 1/1, 2/2, 3/3. Although it becomes a subjective call as to the presence of subcategories, such as moderate tending towards mild or moderate tending towards severe, the imaging presence of such disease findings is indisputable.
Because CT and HRCT scanning are more accurate in the identification of pleural plaquing, the literature has clearly explored this issue over time and various concepts need to be considered:
Only 10% to 40% of Pleural Plaques are Detected by Chest X-Rays. CT and HRCT are More Sensitive to Their Detection:
“Plain radiographs…are only about 10% to 40% sensitive in detecting pleural plaques”, Thoracic Imaging, Pulmonary and Cardiovascular Radiology, Second Edition, page 509, W. Richard Webb and Charles B. Wiggins, Lippincott Williams and Wilkins. This means that chest x-rays miss between 60% and 90% of pleural plaques, which are better shown on CT and HRCT.
One-Third of Pleural Plaques Occurring from Asbestos Exposure are Unilateral. Bilaterality is Not Required:
“Plaques are usually bilateral, although they appear unilateral in up to one third of cases”, Thoracic Imaging, Pulmonary and Cardiovascular Radiology, Second Edition, page 510, W. Richard Webb and Charles B. Higgins, Lippincott Williams and Wilkins.
Extensive Bilateral Pleural Plaquing, Especially When Calcified, Often Results in Abnormal Restricted Pulmonary Function, Even in the Absence of Interstitial Lung Disease/Asbestosis:
Although pleural plaquing is a marker for prior asbestos exposure and the plaques often do not in and of themselves cause disability when limited in number and scattered, when they are extensive, this causes a rigid chest cavity that cannot expand or contract as much as in a normal person and, therefore, in cases of extensive bilateral pleural plaquing, especially if calcified, the individual ends up with a rigid girdle-like or knight-in-armor restriction to the chest wall expansion and contraction and very well may have abnormal pulmonary function test indicating restriction, even in the absence of interstitial lung disease/asbestosis.
Diffuse Pleural Thickening Often Causes Restricted Pulmonary Function, Even in the Absence of Interstitial Lung Disease/Asbestosis:
Diffuse pleural thickening in asbestos-related disease is caused by benign asbestos pleural effusion(s) where the sticky fluid results in sticking together and scarring of the parietal and visceral pleura – the lining of the chest cavity and the lining of the lungs. This causes the lung to stick to the chest cavity and, therefore, does not allow it to expand or contract with each breath as it normally should. The literature states that individuals with diffuse pleural thickening often have restricted pulmonary function. Therefore, an individual with diffuse pleural thickening can have abnormal pulmonary function tests and may have breathing difficulties even without evidence for interstitial lung disease/asbestosis.
This article is provided as a public service by Daniel Powers, M.D.: B-Reader and Board-Certified Diagnostic Radiologist, Certified by the American Board of Radiology.
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