Silicosis and coal workers’ pneumoconiosis (CWP a.k.a. black lung disease) are two forms of occupational dust disease that give similar imaging appearances, but are caused by different pathologic changes within the lungs. Silicosis is the more fibrotic of the two entities and can be an isolated disease process or overlap with, and be part of CWP. Imaging changes in silicosis and coal workers’ pneumoconiosis begin with a simple type of disease process in which the interstitial changes appear as small rounded opacities (micronodules) occurring in the upper and the middle lung zones (probably due to lesser lymphatic clearance than in the lung bases). With increasing amounts of coal and/or free silica dust exposure, increased profusion of nodules results. Although coal workers’ pneumoconiosis looks like silicosis, the silicotic nodules are more likely to be larger and more sharply defined, with the greater likelihood to develop into progressive massive fibrosis (PMF).
Hilar and mediastinal adenopathy (hi) – enlargement of lymph nodes – can develop, associated in some cases with eggshell calcifications (eg), much more common in silicosis than in CWP. This can precede the development of micronodules. In rare instances, massive lymph node enlargement can compress the superior vena cava and/or the esophagus. Tracheal deviation can occur, if there is only a unilateral progressive massive fibrosis lesion or if one progressive massive fibrosis lesion predominates.
Both silicosis and coal workers’ pneumoconiosis cause micronodules, which are of two types – the centrilobular nodules and the subpleural nodules. Subpleural nodules can coalesce and form pseudo-plaques, which may or may not calcify and go on to develop rounded atelectasis (ra). Centrilobular nodules can rarely centrally calcify (cn) and in addition, they can coalesce (ax) and form masses of less than 1 cm in greatest dimension, usually with persistent, well-defined nodular margins.
The coalescence of micronodules (ax) can eventually evolve into progressive massive fibrosis (PMF) or “Large Opacities”, taking at least five years to develop, measuring 1 cm or more in greatest dimension, where the mass is more homogeneous, losing its nodular margination and even developing a fibrous capsule or rim. PMF in silicosis tends to be matted aggregates of silicotic nodules, whereas in CWP it is often more amorphous and homogeneous with a collagen capsule. Once progressive massive fibrosis develops, this is considered the complicated type of the disease process. These masses are commonly bilateral, usually posterior in location in the upper lung zones, have a lentiform or lens/ovoid shape with well-defined lateral margins that parallel the chest walls (and can also be close to and parallel the major fissure). There is associated volume loss and upward distortion of the intrathoracic contents (di), associated with elevation of the hila and resulting lower lung zone compensatory hyperexpansion with the development of centrilobular emphysema (em) and bullae (bu), unrelated to smoking, which can become a significant etiology of symptoms and disability. Bilateral upper lung zone PMF clearly separated from the pleura by aerated lung has been compared to having an “angel wings” appearance. PMF can develop micro-calcifications within their masses and rarely because they develop a fibrotic margination, can become rim calcified. Over a 10-year period or more, the areas of progressive massive fibrosis can migrate towards the hila. As they migrate, the scarring and contraction causes the airways to become obstructed and distorted leading to paracicatricial emphysema along the lateral margins of the progressive massive fibrosis, associated with fibrous bands/scarring extending to the pleural surfaces. The centrilobular nodules become less apparent or disappear between the lateral margin of the progressive massive fibrosis and the lateral chest wall, which now has intervening emphysema. Silicosis is more likely to cause progressive massive fibrosis. Severe ventilatory failure, right-sided heart failure, cor pulmonale (cp) from secondary pulmonary arterial hypertension (PAH) and refractory spontaneous pneumothorax (px) are more common in silicosis than in CWP. In coal workers’, it often takes 20 or more years in the mines to develop progressive massive fibrosis. PMF can eventually cavitate (cv), especially if it is large, due to ischemic central necrosis. However, tuberculosis (tb)/tuberculomas, atypical mycobacterial infection, primary and metastatic carcinoma (ca), nodular sarcoidosis, and if there is current rheumatoid arthritis, rheumatoid pneumoconiosis (rp) with Caplan’s nodules must be considered in the differential diagnosis of PMF/large opacities. Sarcoidosis is the “great mimicker” and can cause micronodules, progressive massive fibrosis and all of the findings often associated with coal workers’ pneumoconiosis and/or silicosis.
More on Silicosis:
Silicosis is caused by the inhalation of high concentrations of free crystalline silica. These particles are less than 10 microns in diameter, and those reaching the alveolar spaces and deposited there, are usually 1 to 3 microns in diameter. X-ray and CT pathology increases with the number of respirable free silica particles deposited in the lungs. Silicon dioxide is responsible for the development of silicosis and can occur in nature in three crystalline forms: a) Quartz, b) Cristobalite and c) Tridymite.
The industrial exposure can occur in such industries as mining, quarrying, tunneling; stonecutting, polishing, cleaning monumental masonry; sandblasting and glass manufacture; foundry work, pottery, porcelain, lining bricks, boiler scaling and vitreous enameling.
Silicotic nodules result from a process in which the macrophages ingest the silica particle and are killed by intracellular liberation of enzymes. These materials attract macrophages and fibroblasts to form a fibrous nodule with an acellular center and at the periphery of the nodule, a concentric laminated ring-like arrangement of collagen fibers giving it a distinctive “onion skin” appearance on microscopy. These nodules are usually situated near the bronchiolar entrance to the alveoli.
Silicosis has both the simple and complicated types of the disease, but these findings occur at variable time after the exposure to free crystalline silica.
There is an acute form of silicosis, which is not found in coal workers’ pneumoconiosis. This entity is also known as silicoproteinosis, looking similar to alveolar proteinosis, which can occur weeks or months after exposure and progress. This entity is associated with medial upper lung zone consolidated opacities, some with air bronchograms, can have symmetric hilar and mediastinal adenopathy (enlargement), volume loss and progressive fibrosis, bullae (bu) and distortion of the mediastinal structures. Additionally, pneumothorax (px), recurrent pneumonia and centrilobular nodules may develop. It often occurs in those individuals who have experienced a massive inhalation, usually within an enclosed space such as with sandblasting (but, some say it can alternatively occur with heavy free crystalline silica exposure over a period of less than 5 years). Progression leading to death in 1 to 3 years can occur with the acute form of silicosis.
There is an accelerated form that occurs within 4 to 10 years (some say 5 to 15 years) after the exposure and has a more rapid progression of disease changes and a poor prognosis.
There is also a chronic form that usually occurs greater than 10 and often 20 years (some say 20 to 40 years) after the initial sustained exposure to often lower levels of free crystalline silica and has a slower progression of changes compared to the accelerated type, often being asymptomatic or associated with chronic bronchitis, if of the simple type of the disease. Both can progress to the complicated type of disease, the accelerated form more frequently and sooner than the chronic form of the disease, at which time there are usually clinical symptoms.
Free crystalline silica forming silicotic nodules is known to be cytotoxic and thus, decreases resistance to invading pathogens, increasing susceptibility to secondary infections. There is a (3x) increased risk for tuberculosis with chronic silicosis. Other types of atypical mycobacterial infections are also increased. Infection risk is even greater with acute, more than with accelerated forms of silicosis and in those with progressive massive fibrosis. The patients, especially those with the accelerated form of the disease, also have the risk of auto-immune, connective tissue disorders including scleroderma, systemic lupus erythematosus, potentially rheumatoid arthritis and mixed connective tissue diseases. There is a 1 to 3% increased cancer risk from silicosis.
ILO CHEST X-RAY CLASSIFICATION OF SILICOSIS
Interstitial (or Parenchymal) Fibrosis/Silicosis: Diffuse, micronodular scarring of the lung tissue from free crystalline silica exposure in the correct pattern and distribution is called silicosis. On chest x-rays, this occurs as small micronodules called small rounded opacities, involving initially the upper and mid lung zones – classified on the ILO (International Labor Organization) in terms of size and shape, location (upper, middle or lower lung zones) plus profusion (the visual amount of fibrosis per visual unit area). The shape is rounded as opposed to linear/irregular. The size and shape is classified as either “p”, “q” or “r”, depending on the size of the small rounded micronodules. Location is noted – most frequently in the upper and middle lung zones. The system of grading the size and shape of the small rounded opacities uses a numerator and a denominator, with the numerator (top number of the fraction) representing the main impression of the reading physician and the denominator (the bottom number of the fraction) representing the second choice (the in addition to) of the reading physician; i.e., p/q means that the physician interprets that the primary size and shape of the small rounded opacities as “p”, but there are in addition, but of a lesser amount, “q” size and shape small rounded opacities. When recording affected zones, all zones with interstitial changes are recorded, regardless of profusion. The profusion or number of small rounded opacities noted per visual unit area is determined by comparison to a standard reference set of plain radiographs and is rated as either mild, moderate or severe disease by a numbering system put out by the International Labor Organization (ILO) for international epidemiological purposes – 0 for no disease, 1 for mild disease (previously stated to be associated with normal lung markings, that are still visible), 2 for moderate disease (previously stated to be associated with normal lung markings, that are partially obscured) and 3 for severe disease (previously stated to be associated with normal lung markings, that are usually obscured). The system for grading the profusion also uses a numerator and denominator with the numerator (top number of the fraction) representing the main impression of the reading physician and the denominator (the bottom number of the fraction) representing the second choice (tending towards) of the reading physician, i.e., 1/2 means that the physician interprets the primary diagnosis is mild disease, but on second thought it is tending towards moderate disease. The calculation of the overall profusion on chest x-ray involves reviewing all six lung zones for evidence of interstitial fibrosis. It requires a mental averaging of all of the affected lung zones with one caveat – that one removes from the calculation, those zones having 3 sub-categories or more lesser profusion compared to the zone of greatest/highest profusion (or put another way, those zones that have a profusion in which there are two sub-categories or more between them and the zone of greatest/highest profusion are removed from the calculation). As an example, if a patient had small irregular opacities in both mid and lower lung zones of profusion values 1/1, 1/2, 2/1 and 2/2, then the zone having a profusion of 1/1 is removed from the calculation because it is three sub-categories lower than the 2/2 zone of greatest/higher profusion or because there are two sub-categories between 1/1 and 2/2. The overall average profusion would be 2/1. Another example would be 1/1, 1/2, 2/3 and 3/3. The 1/1 and 1/2 zones would be removed and the overall average would be 3/2. The classification of mild, moderate and severe disease is a visual classification unrelated to the patient’s clinical symptoms or physiologic changes.
Progressive massive fibrosis is classified by the International Labor Organization (ILO) as large opacities. Remember that coalescence of nodules in either silicosis or coal workers’ pneumoconiosis totaling 1 cm or less, is not considered progressive massive fibrosis. Progressive massive fibrosis occurs when the large opacities become more ill-defined in terms of their nodular components. The ILO defines coalescence of small opacities (ax) as small opacities with the “margins of the small opacities remaining visible, whereas a large opacity demonstrates a homogeneous opaque appearance.” Coalescence of small opacities may be recorded in the presence or absence of large opacities (progressive massive fibrosis). To identify the complicated type of coal workers’ pneumoconiosis or silicosis, one needs progressive massive fibrosis/large opacities. The ILO defines the large opacity of progressive massive fibrosis as “an opacity having the longest dimension exceeding 1 cm.”
Three categories of large opacities are identified and include:
Category A: A large opacity having its longest dimension greater than 1 cm, but less than 5 cm or several large opacities, each one of which is greater than 1 cm in longest dimension, but the sum of which, does not exceed 5 cm.
Category B: One large opacity having its longest dimension exceeding 5 cm, but not exceeding the equivalent area of the right upper lung zone or several large opacities, each of which exceeds 1 cm in longest dimension, with the sum of their longest dimensions exceeding 5 cm, but not exceeding the equivalent area of the right upper lung zone.
Category C: One large opacity whose longest dimension exceeds the equivalent area of the right upper lung zone or several large opacities, each of, which exceeds 1 cm in longest dimension, whose combined longest dimensions exceed the equivalent area of the right upper lung zone.
Silicotuberculosis may be difficult, if not impossible to differentiate radiographically from the changes of complicated silicosis. Silicosis predisposes to tuberculosis and/or atypical mycobacterial infections. It is almost always present in those individuals with acute silicoproteinosis. In accelerated silicosis, mycobacterial infections are found in approximately one-quarter (25%) of the cases, but either typical or atypical organisms may be responsible for the disease. Atypical organisms often include Mycobacterium Kansasii in the Gulf Coast region and Mycobacterium Avium-Intracellulare (battery bacillus) in the upper Midwest. Cavitation may occur. These large opacities often heal, when given anti-tuberculosis medication therapy. Although tuberculosis is the most likely cause of cavitating large opacities, it must be remembered that progressive massive fibrosis without tuberculosis or infection by atypical mycobacteria as well as malignancies can also cavitate. In accelerated silicosis, the large opacities are often caused by tuberculosis (tuberculomas) and often found in atypical locations, predominantly with lower lobe involvement. Progressive massive fibrosis versus tuberculomas are difficult to distinguish by plain radiographs, although tuberculosis-caused large opacities can on occasion abut upon the pleura and cause pleural thickening, unlike progressive massive fibrosis. Often, the tuberculomas resolve with tuberculosis medication therapy with only the silicotic progressive massive fibrosis or larger silicotuberculosis lesions remaining.
My Training and Acknowledgments:
The above diagrams and articles are based upon my education, training and experience as well as from multiple peer-reviewed journal articles, textbooks and lectures.
Medicine is an “art” based upon “science.” We as educators and clinicians are constantly learning and updating our teachings and knowledge base. Certainly, by my presentation I have tried to add additional “art” by the visual diagrams (no pun intended) to the “science” with the hope that it will help further clarify the learner’s understanding of what Radiologists look for when observing silicosis.
Unfortunately, many authors use individualized coined terms, many of which overlap and some terms with the same name, having different meanings to different authors. There is no easy fix. I have tried to standardize the language.
This article is provided as public service by Daniel Powers, M.D.: B-Reader, Board-Certified Diagnostic Radiologist, Certified by the American Board of Radiology.
If you detect any errors, have additional information to point me to, use other useful terms or have comments, please do e-mail them to me at firstname.lastname@example.org.