asbestos related lung masses that may be masked on chest nadiognaphs by associated lung and ...... open lung biopsy speci- men (B) reveals early penibron-.
RadloGraphics COMPUTED .
.
Conventional and high resolution computed tomography in the diagnosis of asbestos-related diseases
Pulmonary
PULMONARY .
index terms: TOMOGRAPHY IMAGING
Bronchi and Pleura/Pleural
Cumulative index Lung, diseases Asbestos Asbestosis Lung, CT Pleura, CT Pleura, diseases
Lungs Space terms:
David lt)/TII
THIS EXHIBIT WAS DISPLAYED AT THE 72ND SCIENTIFIC ASSEMBLY AND ANNUAL MEETING OF THE RADIOLOGICAL SOCIETY OF NORTH AMERICA. NOVEMBER 29-DECEMBER 4. 1987. CHICAGO. ILLINOIS. IT WAS RECOMMENDED BY THE CHEST IMAGING AND COMPUTED TOMOGRAPHY PANELS AND WAS ACCEPTED FOR PUBLICATION AFTER PEER REVIEW AND REVISION ON NOVEMBER 11. 1988.
A. Lynch,
M.D.
Gordon
Gamsu,
M.D.
Denise
R. Aberle,
M.D.
Abstract: High resolution CT (HRCT) can image the fine structures of the lung parenchyma and the pleura. Analysis ofsupine and prone HRCT scans in 300 patIents with asbestos exposure shows that asbestosls Is characterized the foliowing findings; (1) parenchymal bands, (2) increased interlobular septa, (3) increased intralobular core structures, (4) subpleural lines, and (5) dependent opacity. Pleural disease Is well displayed on HRCT scans. Correlated studies with whole lung sections confirm the HRCT findings. Masses in asbestosis may be benign or malignant, and CT helps in their differentIation. HRCT correlates well with radiographic and clinical criteria for asbestosls and is more sensitive than either In detecting abnormalities.
Introduction Chest radiographs are widely used to determine the presence and extent of asbestos related lung and pleural disease. Conventional CT scanning is more sensitive than chest radiography for the detection of pleural plaques (8,14,16), and can also identify a variety of benign and malignant asbestos related lung masses that may be masked on chest nadiognaphs by associated lung and pleural disease. Neither conventional chest radiography non conventional CT, however, can reliably detect early panenchymal lung fibrosis. From
the
diology, nia
Medical
cisco. Address
Department
University Center, reprint
of Raof CaliforSan
Fran-
requests
to
D. A. Lynch, M.D. Department of Radiology. University of California San Francisco, San
Francisco,
High
resolution
CT scanning,
performed
with
optimal
technique
(20)
(Table 1, Figure 1), affords much greaten spatial resolution than conventional CT, and is used to detect and characterize a wide range of pulmonary interstitial diseases, including idiopathic pulmonary fibrosis (22), sancoidosis (3), and lymphangitic spread of carcinoma (30). Our experience suggests that high resolution CT may allow us to detect panenchymal asbestosis at an earlier stage than
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do conventional It can
also
CT or chest
identify
Lynch et al.
diseases
small
radiography
pleural
(1,2). that
plaques
are not apparent on conventional images. Asbestos exposed workers are being referred in increasing numbers for CT and high resolution CT, to identify
ed lung and this paper
malities workers
and
characterize
asbestos
disease.
It is the purpose
pleural
to demonstrate
that we have with significant
the
range
nelat-
of
of abnor-
encountered in oven 300 asbestos exposure.
Table 1 Technique for HRCT kVP mA Scan Time Collimation
Matrix Contrast Agent Positioning Scan Area Series #1 Scan 1: Scan 2:
120-140 110-170
2-3 sec 1-1.5 mm 512 None
Supine and prone Scout Supine Through carina Midway
between
carina and right
dome of diaphragm Scan 3: Midway between Scans 2 and 4 Scan 4: At night dome of diaphragm Scan 5: Midway between right dome of
diaphragm and base of night lung Series #2 Scout Prone Repeat Scans 1, 2, 3, 4, 5 at same locations as in Series #1 All scans are reconstructed using a high resolution algorithm
Figure 1 Prone (A) and supine (B) scout views of the thorax, indicating levels for high resolution CT scans in asbestosis
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Background CURRENT RADIOLOGIC CLASSIFICATION SYSTEMS
HISTORY Asbestos is a generic term used to describe a group of silicate minerals which, when crushed, break into fibers rather than dust. Most commercially important asbestos fibers belong to the amphibole group (amosite, crocidolite, anthophyllite, tremolite, and actinolite), but chrysolite is a member of the serpentine group. The great heat resistance and tensile strength of asbestos fibers, and the fact that it is possible to weave the fibers into cloth have led to a vast array of applications in the construction, insulation, textile, packaging, and other industries. Commercial mining of asbestos began in Quebec and South Africa in the middle and late 19th century, and its use spread rapidly. Descriptions of lung fibrosis related to asbestos began to appear soon after the turn of the century. The first report of lung cancer in a patient with asbestosis was published in 1935 (4). Since then, numerous cohort studies have been performed, all showing an excess of lung cancer in workers exposed to asbestos (25). The association of mesothelioma and asbestos exposure was not recognized until 1959. PUBLIC HEALTH CONCERNS Since these early descriptions, the health effects of asbestos have become a cause for increasing public concern. Between 1940 and 1979, an estimated 27 million people were exposed to asbestos at work. In addition, the widespread use of asbestos in construction and insulation has led to considerable concern about the presence of asbestos in homes and schools, and about the health effects of low level exposure (4).
I
Profusion
of small
opacities
is graded
on a scale
of I (fewest)
Reflecting the public health concerns, a system developed by the International Labor Organization (13) (ILO System) for the classification of pneumoconiosis has been extensively used to describe the type and extent of pleural disease and panenchymal opacities in asbestos exposed workers. The system is based on comparison of the patient’s chest radiograph with a set of standard nadiognaphs1. It was designed primarily as an epidemiologic tool for determining the severity of radiographic change in these subjects and is descriptive rather than interpretive in nature. Though some correlation exists between pulmonary function and the ILO classification of the chest radiograph, the ILO grading in any individual is a relatively poor predictor of pulmonary function abnormalities (9). Significant intenobsenven variations occur, especially in the assessment of the type and profusion of panenchymal opacities (23). In our practice, the primary mdication for high resolution CT has been a disagreement between two certified ILO B-readens regarding the presence of asbestos related pleural abnormalities on the presence on profusion of asbestos related parenchymal opacities (2). The majority of these patients have nelatively low (I/I on less) ILO profusion scones (23). In our study (2), 85% of workers with ILO scones between I/O and 1/2 had findings highly suggestive of fibrosis on high resolution CT. Forty-three percent of patients with normal chest radiographs had strong evidence of fibrosis. Other clinical and pathologic studies suggest that an ILO classification of 1/0 or less underestimates early asbestosis by 10 to 20% (23).
to 3 (most)
sion are expressed as a fraction the numerator of which represents nator of which represents the next most seriously considered level.
2. but level
with
0 = no small
the most probable % = unequivocal
opacities.
level of profusion level 2 profusion;
Estimates
of profu-
and the denomi= probably level
I was also considered.
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Asbestos
Related
Asbestos inhalation most commonly causes pleural abnormalities including pleural plaques, pleural thickening, pleural effusion and mesothelioma. The term asbestosis, however, should be reserved for interstitial fibrosis of the pulmonary parenchyma in which asbestos bodies on fibers can be demonstrated. Asbestosis is frequently associated with pleural disease, but asbestos related pleural abnormalities should be considered separately from parenchymal asbestosis, inasmuch as they differ markedly in epidemiology, clinical features and prognosis. BENIGN ASBESTOS RELATED PLEURAL DISEASE
Pleural
Plaques
Pleural plaques are the most common manifestation of asbestos inhalation and are well known as an indication of previous asbestos exposure (4). Though they do not usually cause any change in pulmonary function, mdividuals who develop pleural plaques are more likely to develop panenchymal lung fibrosis than are those who do not, and they may be at increased risk for the development of mesothelioma (7).
Disease
Pleural plaques occur most commonly on the postenolateral aspect of the lower costal, panietal pleura, and on the diaphragm. The visceral pleura, costophnenic angles, and lung apices are characteristically spared. Such plaques appear as discrete, elevated, opaque, shiny rounded lesions on the panietal pleura. They may have a smooth, bosselated, or nodular surface, and they vary in colon from gray to ivory. On conventional and high resolution CT, pleural plaques are seen as discrete linear structures, in the expected position of the panetal pleura. They may be classified as follows: I . Minimal pleural plaques: less than I mm thick, 1-3 cm long, and few in number (Figure 2). 2. Moderate pleural plaques: 1-3 mm thick, 2-5 cm long and multiple (Figure 3). 3. Severe pleural plaques: thicker than 3 mm, cleanly indenting adjacent lung, up to 8 cm in craniocaudal dimension and extensive in width. Conventional and high resolution CT can readily distinguish pleural plaques from extrapleural fat. This distinction is frequently a pnoblem on chest radiography. High resolution CT can also be used to distinguish minimal pleural plaques from intercostal fascia, veins and muscle (Figure 2).
Figure 2 High resolution CT shows minimal pleural plaque (arrow) overlying a night posterior nib. This was not seen on conventional CT. The linear structures (arrowheads) seen in the costovertebral angle probably represent segments of an intercostal vein.
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a C
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Figure 3 This high resolution CT scan shows moderate, bilateral pleural plaques. These are seen posteriorly and over the domes of both diaphragms (arrows).
This high resolution
CT scan shows extensive pleural plaques indent-
ing the adjacent lung. Diffuse
Pleural
Thickening
Diffuse thickening of the panietal pleura frequently occurs in association with moderate on extensive, panietal pleural plaques. It varies in severity from a thin milky discoloration of the pleura to a thick fibrotic peel encasing the lung, and it may cause restriction of pulmonary function. On the chest radiograph, diffuse pleural thickening is defined as a smooth, uninternupted pleural opacity, extending oven at least one-fourth of the chest wall, with or without costophrenic angle obliteration (18). In about 30% of cases, diffuse pleural thickening can
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be related to a prior, benign, asbestos related pleural effusion. On CT, we have defined diffuse pleural thickening as a continuous sheet of pleural thickening more than 5 cm wide, more than 8 cm in craniocaudal extent, and more than 3 mm thick (Figure 5). It most commonly affects the posterior and postenomedial pleura oven the lower lobes. Probably because of its favoned location, diffuse pleural thickening is more frequently demonstrated on CT than on chest nadiognaphs. It may be associated with rounded atelectasis of the adjacent lung (Figune 5).
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Figure 5 High resolution CT at two levels (A) and (B) reveal sheets of pleunal thickening with calcifications along both antenolateral chest walls and the night postenomedial chest wall. In the lower level (B), an area of rounded atelectasis is seen adjacent to the diffuse pleural thickening on the left (arrow)
Visceral
Pleura!
Thickening
Thickening of the visceral pleura (Figure 6) occurs with moderate on heavy asbestos exposure, and increases with the level of exposure
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(28). In many cases, this represents extension to the visceral pleura of subpleunal fibrosis. Such subpleural fibrosis may be recognized on high resolution CT by its irregular outline and by associated panenchymal lung fibrosis.
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a -
0 V
Figure 6 High resolution CT in a patient with extensive honeycombing of the night lung (A) reveals diffuse thickening and irregularity of the pleura (B). Because of its irregularity and the associated honeycombing, this is likely to represent a combination of visceral pleural fibrosis and subpleural fibrosis.
“
6B
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Pleural
Effusion
Benign, asbestos related pleural effusion tends to occur earlier than other pleural changes, normally within the first 20 years after initial asbestos exposure. In one study, unexplained benign effusions were found in 34 of I 135 asbestos exposed workers (6). The prevalence of effusion increases as the level of asbestos exposure increases. Effusions tend to be small, and two-thirds are asymptomatic. The fluid may be serous on hemorrhagic. The association of a newly recognized effusion with the presence of linear strands that radiate toward the pleura on with rounded atelectasis suggests that the effusion may be asbestos related and benign (19). The diagnosis of benign. asbestos related effusion remains a diagnosis of exclusion, however, and even though culture and cytologic studies of the pleural fluid are initially negative, the patient may return 3-10 years later with mesothelioma. Pleural effusion is frequently followed by the development of diffuse pleural thickening, and rounded atelectasis, if not present with the effusion initially, may develop later. ASBESTOS RELATED PULMONARY
PARENCHYMAL
DISEASE
Clinical
Diagnosis
of Asbestosis
The pathologic diagnosis of asbestosis is based on the presence of lung fibrosis, with histologic evidence of asbestos exposure (31). In almost all cases, however, the diagnosis is made on clinical grounds without resort to lung biopsy. The American Thonacic Society (ATS) statement of 1986 (31) sets out the basis for the clinical diagnosis of asbestosis (Table 2, 1). This statement advises caution in the diagnosis of asbestosis when the chest radiograph is normal. Others have suggested that the presence of three of the clinical criteria with an appropniate history of exposure should also be considered diagnostic, even though the chest nadiognaph is normal, since the chest radiograph may be normal in up to 18% of patients with histologic asbestosis demonstrated by biopsy (15). Other causes of pulmonary fibrosis, especially idiopathic pulmonary fibrosis, could fulfill
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the ATS criteria for asbestosis. Asbestosis is much more prevalent than idiopathic intenstitial pulmonary fibrosis in an asbestos exposed cohort, however. Tabie 2 ATS Criteria for Diagnosis of Asbestosis (3 1) Necessary Criteria: 1 . a reliable history of exposure 2. an appropriate time interval between exposure and detection Clinical Criteria: 1 . chest radiographic evidence of type 5, t, on u,2 small irregular opacities of a profusion of 1/1 or greater 2. a restrictive pattern of lung impairment with forced vital capacity below the lower limit of normal 3. a diffusing capacity below the lower limit of normal 4. bilateral late- on pan-inspiratony crackles at the posterior lung bases, not cleared by cough The specificity of these criteria Increases with increasing numbers of positive criteria, but the findings on the chest radiograph are the most important.
Conventional
CT Features
of Asbestosis
Features of panenchymal fibrosis are often apparent on conventional CT. They are frequently difficult to characterize, however, and are nonspecific with respect to etiology, especially if the scanning is done in the supine position only. Features commonly encountered in conventional CT scans in cases of asbestosis are the following: I . Subpleural lines These are curvilinear opacities within I cm of the pleura and panallel to it (1,32) (Figure 7). 2. Parenchymol bands These are linear opacities 2-5 cm in length running through the lung, usually to contact a pleural surface. They do not course in the direction of pulmonary blood vessels (1,14) (Figure 8). 3. Pulmonary arcades These are branching linear structures that appear on conventional CT as arcades that are most prominent posteriorly, in the dependent lung (Figure 9). It may be impossible to distinguish these arcades from prominent vessels, especially in dependent areas of the lung (1,29). 2
Small
opacities
rounded
of increasing classified as
9, Number
size; small 5, t or u.
3
are classified irregular
opacities
as p. q or r in order are similarly
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related
diseases
0 0 3
a 3 0 3 0
0 -I -‘I
a 0 C
aU)
Figure 7 Conventional CT shows bilateral curvilinear opacities parallel to the posterior pleural surfaces (arrows). These are partially masked by subpleunal dependent opacity. Figure 8 Conventional CT shows bilateral panenchymal bands (arrows). Figure 9 Conventional CT shows prominence of pulmonary arcades in dependent and nondependent areas of the lung (arrows). Note that these arcades are thicken than normal vessels and contact the pleural surface.
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4. Subpleural dependent opacity (density) This is a band, 2-30 mm thick of poorly manginated lung of greaten than normal attenuation that parallels the dependent pleura and obscures the underlying lung morphology (Figure 10). Dependent opacity usually cleans when the region is no longer dependent. It may be found in subjects without lung abnonmality (1). 5. Reticulation This is a network of linear
Figure 10 Conventional rows).
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opacities that is seen on conventional CT, usually at the lung bases, and usually posteriorly (Figure II). 6. Honeycombing This is a pattern of multiple cystic spaces that are less than I cm in diameter and have thickened walls (Figure 12). Most commonly, honeycombing is subpleunal in distribution, and predominates in the postenior lower lobes. The adjacent visceral pleura is frequently thickened (1).
CT shows bilateral subpleural dependent
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0 0 3 IC
a 3 0 3 0
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_ Figure 1 1 Conventional CT shows fine peripheral reticulation, by dependent lung opacity.
Conventional CT shows extensive honeycombing and less marked honeycombing on the left side.
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High
related
Resolullon
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CT Features
of Asbestosis
resolution CT can demonstrate the subpleural lines (Figure 13), panenchymal bands (Figure 14), interstitial thickening (Figure 15), subpleunal dependent opacity (Figure 16), and High
honeycombing (Figure I 7) described above. Because of increased spatial resolution, these features are seen with greaten frequency and more clarity on high resolution CT than on conventional CT. In addition, high resolution CT is capable of recording the following:
3_
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7. Septal thickening This is manifested by linear opacities 1-2 cm in length, which are usually best seen in the periphery of the lung and which extend to the pleura (Figure 18). 8. Thickened centrilobular core structures These are seen as fine branching lines nadiating from a central core within a secondary pulmonary lobule. They are usually about I cm
from the pleura, and they frequently extend to a pleural surface (Figure 19). They most likely represent areas of fibrosis around radiating bronchioles. The conventional and high resolution CT features of asbestosis cannot be considered specific for this disease; they may occur in 0then types of pulmonary fibrosis.
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Figure 13 Supine (A) and prone (B) high resolution CT scans show curvilinear subpleural lines in the posterior right lung (arrows). These lines persist and, because the dependent lung opacity is no longer present, are seen with greaten clarity on the prone image.
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Figure 14 Supine (A) and prone (B) high resolution CT scans show numerous bilateral panenchymal bands (anrows). These do not significantly change in appearance on the prone scan (B).
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Figure 15 A supine high resolution CT scan (A) shows subpleural dependent opacity. The prone high resolution CT scan (B) shows that the subpleural dependent lung opacity has cleaned, to reveal thickening of intenlobulan septa (arrows).
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Figure 16 A supine high resolution CT scan (A) shows a broad band of dependent lung opacity in the posterior lungs, masking fine lung detail. The prone high resolution CT scan (B) shows widespread, fine thickening of intenlobulan septa (arrows).
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h8A Figure 18 A prone high resolution CT scan (A) shows focal thickening of intenlobular septa. An open lung biopsy specimen (B) taken from the area of the abnormality seen on the high resolution CT scans reveals subpleunal fibrosis (arrows), and marked thickening and fibrosis of an intenlobulan septum (arrowhead).
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Figure 19 A prone high resolution CT scan (A) reveals abnormal, radiating, fine lines (arrows) within several secondary pulmonary lobules. An abnormal subpleunal line is also seen. An open lung biopsy specimen (B) reveals early penibronchiolan fibrosis (arrowhead) in the area of the high resolution CT abnormality. The biopsy was prompted in part by the high resolution CT findings.
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Differences resolution
CT of asbestos
between prone and supine CT The pathologic changes
of asbestosis are usually most marked at the postenon lung bases. In the supine position, this part of the lung is dependent, tends to be compressed, and has a larger blood volume than the nondependent lung. This is the basis for the normal lung density gradient in the postenor lung bases, on supine scans. In some normal individuals, a band of increased attenuation is seen in the immediate subpleural region of the dependent lung (Figure 16). We found this feature to be present in #{243}O% of workers with asbestosis, and in only 30% of those without evidence of lung disease. In normals, this subpleunal dependent opacity always clears when the lung is no longer dependent. In asbestosis, the opacity occasionally persists, but usually cleans in the prone position. For this reason, the features of early lung fibrosis are often better demonstrated in the prone position. Persistent subpleural opacity in nondependent lung is usually abnormal, but its exact significance is often uncertain. In dependent areas of lung, thickened septa and core structures may be due to vascular distention which does not persist when the lung is not dependent. Septal thickening in lung that is not dependent is always abnormal. Curvilinear subpleunal lines are seen in dependent areas of the lung in about 30% of patients with asbestosis, and in an equal percentage of those without evidence of lung fibrosis. In the normal patient, these lines will clean when the lung is no longer dependent (1). Persistence of a subpleunal line in lung that is not dependent, when associated with other high nesolution CT evidence of asbestosis is abnormal and suggests parenchymal fibrosis. Honeycombing and panenchymal bands are usually fixed abnormalities and unaffected by lung position. The prone high resolution CT scan is frequently critical in identifying fine structural abnonmalities in the posterior lung and in confirming the fixed nature of septal thickening and subpleunal lines. high
Pathology
come
of Asbestosis
(4)
When asbestos fibers are inhaled they impacted in the terminal bronchioles,
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be-
related
diseases
respiratory bronchioles, alveolar ducts, and alveoli. They are surrounded by one on more macnophages which initiate an inflammatory response. Some of the fibers become coated with layers of protein and hemosidenin, forming fennugineous bodies that are easily seen on light microscopy in the alveolar lumen, in sputum on in lung lavage fluid. These fenrugineous bodies are indicative of asbestos exposure. The vast majority of fibers remain uncoated and may be seen only with electron microscopy.
As the inflammatory response progresses, neticulin and collagen fibers are laid down in the intenstitium, first around centnilobulan bronchioles, and later in the alveolar wall. These early changes are seen most prominently in the subpleunal regions. Thickening of intenlobuIan septa occurs, and bands of fibrosis may extend deep into the lung parenchyma. The fibrosis is initially very patchy with large areas of intervening nomal lung. Therefore, biopsies must be taken from multiple representative sites to establish the diagnosis of early asbestosis. Later states of asbestosis are characterized by progressive fibrosis with obliteration of alveoli, alveolar ducts, and bronchioles, and by the development of honeycombing: thick walled ainspaces 5-15 mm in diameter. Dilatation and irregularity of the distal bronchi and bronchioles is seen: so called “traction bronchiectasis. Associated visceral pleural on subpleural fibrosis is common. “
Radiologic
Pathologic
Correlation
The findings observed on high resolution CT in pulmonary parenchymal asbestosis can in most cases be explained in terms of the pathologic processes described above: I . Septal thickening The thickening of the intenlobular septa seen on high resolution CT in pulmonary asbestosis is due to fibrosis (Figure 18). 2. Thickened core structures The prominent cone structures seen radiating from the center of the secondary pulmonary lobule probably represent bnonchiolan and penibronchiolan fibrosis (Figure 19).
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3, Parenchymalbands Panenchymal bands seen running through the lung on high resolution CT probably represent fibrosis in connective tissue septa, extending into the lung panenchyma (Figure 20). 4, Honeycombing The honeycombing seen on high resolution CT, in general, comesponds to histologic honeycombing (Figure 21). Some small, thick walled, peripheral cystic spaces may represent dilated bronchioles, however. 5. Subpleural lines Some curvilinear subpleural lines may represent the proximal extent of a row of subpleunal honeycomb cysts. Other
subpleunal lines develop in the dependent lung in normal subjects and disappear when the lung is not dependent. As mentioned, patients with asbestosis may have a similar line which persists in lung that is no longer dependent (Figure 13). The morphologic correlate of this line is unclean. Yoshimura and coworkers have ascribed it to penibronchiolan fibrosis (32). 6. Dependent opacity This is a zone of subpleunal opacity seen in the dependent lung on high resolution CT. In patients with asbestosis, it probably reflects decreased lung compliance and areas of atelectasis. Dependent opacity has no pathologic correlate.
Figure 20 A high resolution CT scan of an isolated lung specimen from a patient with asbestosis (A) shows multiple coarsely thickened parenchymal bands (arrows) extending to the pleural surface. A Gough section of this lung at the same level (B) confirms that these represent thickened tissue septa (arrows).
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Figure 21 A high resolution CT scan of an isolated lung specimen from a patient with asbestosis (A) shows focal honeycombing (arrows). A Gough section of this lung at the same level (B) confirms the presence of peniphenal honeycombing (arrows). ASBESTOS RELATED LUNG MASSES Asbestos exposure is associated with a vaniety of benign and malignant lung masses (17). Bnonchogenic carcinoma, mesothelioma, and rounded atelectasis are known, but smallen benign pleural based masses, intrafissunal pleural plaques, and mass-like fibrotic sheets also occur. Unanticipated masses will be found by CT in about 10% of workers with significant asbestos exposure: in our experience, more than 90% of these will be various benign focal lesions. Benign
Lung Masses
Rounded atelectasis Though most commonly found with asbestos pleural disease, rounded atelectasis on ‘folded lung” may occur in chronic pleural disease of any etiology. On CT, it is seen as a mass-like lesion adjacent ‘
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to a site of pleural abnormality (commonly, diffuse pleural thickening) (Figure 22). Associated findings are loban volume loss, and bronchi and vessels curving gently towards the mass from the hilum (the “comet tail”). Rounded atelectasis is most commonly situated in the postenomedial part of the lung, which is also the most common site of diffuse pleural thickening. The mass of rounded atelectasis may metract inwards, leaving only a tenuous connection with the pleura. It may be rounded, wedge shaped, on lentiform; and it may be well on poorly margmnated. Air bronchognams and calcification may be seen within this lesion. “Crow’s feet” on fibrotic panenchymal bands often radiate from the mass into the sunrounding lung. Multiple masses, sometimes symmetrically distributed, are not uncommon. The nadiologic features are sufficiently chanactenistic to preclude biopsy in classic cases (5,’1142,17,24).
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benign pleural based that do not meet all the rounded atelectasis (17). less than 2 cm in diameter, to an area of pleural abnon-
mality (usually diffuse pleural thickening). They are usually wedge shaped or lentiform. Their configuration and relationship to a pleural abnormality suggest that these small masses are within the spectrum of “folded lung.”
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22B Figure 22 High resolution CT at two levels (A&B) shows a mass-like opacity adjacent to a sheet of pleural thickening surrounding the posteromedial bronchi of the right lung. Vessels curve gently toward the lateral aspect of the mass. Posterior displacement of the major fissure mdicates loban volume loss. These findings represent rounded atelectosis.
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,SSSS Figure 23 High resolution CT shows a small, well defined opacity contiguous with an area of diffuse pleural thickening in the postenomedial left lung. Bronchi and vessels curve toward it. This may represent either a dense band of fibrosis with a cicatnicial crowding of bronchi and yessels, on a small area of rounded atelectasis.
Fissural pleural plaques In addition to panietal pleural plaques, similar plaques may occur on the visceral pleura. Visceral plaques are much less common than panietal plaques, and are recognizable only when they involve intenloban fissures. Fissunal plaques are most common near the lower ends of the major fissures. They may calcify, and they are usually found with moderate to extensive panietal pleural plaques or diffuse thickening.
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On conventional CT, a fissunal plaque usually appears as a band like opacity (Figure 24), but it may be nodular. The fissure is often visible but only as an avasculan zone on ill-defined linear opacity, and the precise relationship of the plaque to the fissure is not apparent. On high resolution CT, the normal fissure is visible as a thin white line in virtually all instances. The plaque can, as a result, be precisely localized within the fissure.
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24B Figure 24 Conventional CT (A) shows a mass-like density in the anterior part of the left lung (arrow). The major fissure is not clearly seen. High resolution CT (B) shows both major fissures as thin white lines, and demonstrates that the band-like density is in fact an intrafissunal pleural plaque.
Fibrotic bands In advanced asbestosis, dense sheets of fibrosis over I cm wide may extend through the lung. In cross section, these may simulate lung masses. They usually extend inward from an area of pleural abnon-
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mality (Figure 25). With less advanced asbestosis, condensation of several smaller fibrotic bands, on cicatnicial crowding of bronchi and vessels may also simulate a mass.
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Figure 25 At the left base posteriorly, high resolution CT shows a band of masslike opacity adjacent to an area of diffuse pleural thickening, with curving strands of fibrosis radiating into the surrounding lung. This was stable on followup and, because of its bandlike configuration and evidence of fibrosis on other scans, most likely represents a dense sheet of fibrosis. Malignant
Lung Masses
Occupational exposure to asbestos predisposes to bnonchogenic carcinoma, pleural and penitoneal mesothelioma, and other neoplasms (25), including cancer of the esophagus and stomach, and perhaps of the head and neck, breast and colon. Bronchogenic carcinoma For bronchogenic carcinoma (Figure 26), the increased risk appears linearly related to cumulative asbestos exposure, and begins approximately ten years after first exposure (25). The type of asbestos fiber does not appear to influence the risk of lung cancer, but its physical state does have an effect. Long, thin asbestos fibers are more carcinogenic in animals. Asbestos exposed textile workers, who are more likely to be exposed to long, thin fibers, have a higher relative risk of dying of lung cancer than do asbestos cement workers, insulation workers, on miners. Smoking is an important cofacton for
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the development of bronchogenic carcinoma. Asbestos exposed smokers have a death rate from lung cancer that has been estimated at 50-90 times that of nonsmokers who are not asbestos workers (26). The risk for asbestos workers who do not smoke is between one and five times that of nonsmokers who do not work with asbestos. All histologic types of lung carcinoma are increased in incidence, but adenocancinoma is increased more than others. Asbestos related bronchogenic carcinoma has no distinguishing CT or pathologic features. Concomitant pulmonary fibrosis can hamper nadiologic recognition of asbestos related bronchogenic carcinoma. Mesothelioma The risk of mesothelioma in asbestos workers is strongly related to the type of asbestos fiber (4). Crocidolite poses a much higher risk than amosite on chrysotile. The incidence rises exponentially from the time of first exposure. For this reason, it increases sharply after 20 to 30 years. The risk of meso-
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26B Figure 26 These are conventional (A) and high resolution (B) CT scans of a patient with bilateral asbestos related bnonchogenic carcinoma. The conventional scan (A) shows bilateral lung nodules (arrows). The high
resolution scan (B) cleanly shows the left major fissure (small arrows), and demonstrates the left lung nodule to be in the superior segment of the lower lobe. The night lung nodule was in the posterior segment of the upper lobe.
thelioma is not linearly related to the level of exposure, though penitoneal mesotheliomas are more common in heavily exposed workers. The CT features of mesothelioma include focal pleural thickening that may be nodular on plaque-like. The plaques of mesothelioma tend to be more irregular and bulkier than asbestos related hyaline pleural plaques. Pleural effusions of varying sizes occur in 80 to 100% of cases. Later circumferential pleural encasement occurs (Figure 27). Ipsilatenal intrapulmonary lung nodules may occur in up to 60% of patients. Spread to hilan and medlastinal nodes, through the chest wall, and across the diaphragm, can readily be detected by CT
Distant metastases, though common at autopsy, are namely apparent clinically prior to death. The CT findings cannot distinguish mesothelioma from pleural involvement by carcinoma. Although this distinction may also be difficult on histologic study, a number of immunochemical, monoclonal, and ultnastnuctunal features can now be used to differentiate between mesothelioma and metastatic cardnoma. CT is of value in determining the extent of the mesothelioma when surgery (pleurectomy on radical pneumonectomy) is being considered. CT can also help to follow the nesponse of the neoplasm to chemotherapy on radiotherapy.
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Conventional CT at two levels (A&B) in a case of mesothelioma, shows nodular, irregular pleural thickening encasing the night lung, with loculated areas of pleural fluid. The pleural process is seen to invade both the anterior and the posterior mediastinum. It also dissects beneath a calcified pleural plaque (arrow) and appears to infiltrate the ribs (arrowheads) indicating chest wall invasion.
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Radiologic stability of a nodule for two years or more is the most important criterion of benignity: prior chest nadiognaphs on CT are invaluable for establishing this criterion. Masses with classic features of rounded atelectasis are best followed by repeated chest radiographs or CT. Any atypical features, on evidences of recent change, make biopsy mandatony. Smaller pleural based masses present more of a problem than classic rounded atel-
ectasis. Features that favor the benignity of these lesions include continuity with a site of diffuse pleural thickening, and a wedge on lentiform shape. In the absence of prior imaging studies, however, it may be advisable to biopsy these smaller masses. Intrapulmonary nodules are best evaluated by high resolution CT with CT densitometny (27). High resolution CT can also confirm the presence of intrafissunal pleural plaques. Nodules that are not definitely calcified or intrafissunal on high resolution CT may warrant biopsy or removal.
Summary Computed tomography is more sensitive and probably more specific than conventional chest radiography for the diagnosis of asbestos related pleural disease. High resolution CT can identify pleural plaques earlier than conventional imaging. The high resolution CT featunes of panenchymal asbestosis are: thickening of intenlobulan septa and of interlobulan lines, panenchymal fibrotic bands, subpleunal
curvilinear lines, subpleural dependent lung opacity, and honeycombing. Of these featunes, all but the subpleunal curvilinear lines and subpleunal dependent lung opacity have pathologic correlates. Conventional and high resolution CT can identify and characterize asbestos related lung masses, the majority of which are not malignant.
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Aberle DR. Gamsu G. Ray CS. Feuerstein IM. Asbestosrelated pleural and parenchymal fibrosis: Detection with high-resolution CT. Radiology 1988; 166:729-734. Aberle DR. Gamsu G. Ray CS. High resolution CT of benign asbestos-related diseases: Clinical and radiographic correlation. AJR 1988; 151:883-891. Bergin CJ, Mueller NL. CT in the diagnosis of interstitial lung disease. AJR 1985; 145:505-5 10. Craighead JE. Abraham JLC, Churg A, et al. The pathology of asbestos-associated diseases of the lungs and pleural cavities: Diagnostic criteria and proposed grading schema. Arch Pathol Lab Med 1982; 106:544596. Doyle TC. Lawler GA. CT features of rounded atelectasis of the lung. AJR 1984; 143:225-228. Epler GR. McLoud TC. Gaensler EA. Prevalence and incidence of benign asbestos related pleural effusion in a working population. JAMA 1982; 247 :617-622. Fletcher DR. A mortality study of shipyard workers with pleural plaques. BrJ Ind Med 1972; 29:142-145. Friedman AC, Fiel SB, Fisher MS. Radecki PD, Lev-Toaff AS, Caroline DF. Asbestos-related pleural disease and asbestosis: A comparison of CT and chest radiography. AJR 1988; 150:269-275. Gefter WB, Conaut EF. Issues and controversies in the plain film diagnosis of asbestos-related disorders in the chest. J Thorac Imaging 1988; 3:11-28.
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10. Grant DC. Seltzer SE, Antman KH. Finberg HJ, Koster K. Computed tomography of malignant pleural mesothelioma. J Comput Assist Tomogr 1983; 7:626-632. I I . Hanke R, Kretzschmar R. Round atelectasis. Semin Roentgenol 1980; 15:174-182. 12. Hillerdal G. Hemmingsson A. Pulmonary pseudotumours and asbestos. Acta Radiol (Diagn) (Stockh) 1980; 21:615-620. 13. International Labor Office. Guidelines for the use of ILO International Classification of Radiographs of Pneumoconioses. Geneva ILO 1980. ILO Occupatonal Safety and Health Series no. 22 (rev. 80). 14. Katz D. Kreel L. Computed tomography in pulmonary asbestosis. Clin Radiol 1979; 30:207-213. 15. Kipen HM. Lilis R, Suzuki Y. Valciukas JA. Selikoff lJ. Pulmonary fibrosis in asbestos insulation workers with lung cancer: A radiological and histopathological evaluation. Br J Ind Med 1987; 44:96-100. 16. Kreel L. Computed tomography in the evaluation of pulmonary asbestosis: Preliminary experiences with the EMI general purpose scanner. Acta Radiol (Diagn) (Stockh) 1976; 17:405-412. 17. Lynch DA, Gamsu G. Ray CS, Aberle DR. Asbestos-related focal lung masses: Manifestations on conventional and high-resolution CT scans. Radiology 1988; 169:603-607.
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Selikoff IJ, Hammond EC. Asbestos and smoking. JAMA 1979; 242:458-459. Siegelman 55. Khouri NF. Leo FP. Fishman EK. Braverman RM, Zerhouni EA. Solitary pulmonary nodules: CT assessment. Radiology 1986; 160:307-312. Solomon A. Irwig LM. Sluis-Cremer GK. Thomas RG. Du Toit RS. Thickening of pulmonary interlobar fissures: Exposure-response relationship in crocidolite and amosite miners. Br J Ind Med 1979; 36: 195-198. Sperber M, Mohan KK. Computed tomography: A reliable diagnostic modality in pulmonary asbestosis. Comput Radiol 1984; 8:125-132. Stein MG, Mayo J. Mueller, N, Aberle DR. Webb WR. Gamsu G. Pulmonary lymphangitic spread of carcinoma: Appearance on CT scans. Radiology 1987; 162:371-375. American Thoracic Society. The diagnosis of nonmalignant diseases related to asbestos. Am Rev Respir Dis 1986; 134:363-368. Yoshimura H, Hatakeyama M, Otsuji H. et al. Pulmonary asbestosis: CT study of subpleural curvilinear shadow. Work in progress. Radiology 1986; 158:653-658.
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