Thoracic Imaging 2003 - Society of Thoracic Radiology

Imaging of Occupational and Environmental Lung
Disease
David Lynch, M.D.
Pneumoconioses
The most common pneumoconioses are coal workers’ pneumoconiosis,
silicosis, and asbestosis. Although the prevalence
of coal workers’ pneumoconiosis in the US appears to be declining,
silicosis remains a significant problem worldwide requiring
continued surveillance of multiple occupational groups, particularly
foundry workers and sandblasters, including surveillance
for the known complication of silicotuberculosis. In highly
exposed workers, silicosis may occur after less than 10 years of
exposure. The radiographic and CT features of coal workers'
pneumoconiosis and silicosis are similar (1-5). Both conditions
present with predominantly upper lobe nodules, which may later
coalesce to form mass-like opacities (progressive massive fibrosis).
The CT findings in these diseases vary with the size of the
opacities seen on the chest radiograph. Opacities classified as
type p by the ILO criteria are characterized on HRCT by tiny
branching structures or a cluster of small dots (1). Centrilobular
emphysema is a frequent association. By contrast, opacities of
the q and r type are characterized by sharply demarcated round
nodules or irregular contracted nodules (4). The nodules may
be centrilobular or subpleural in location, and tend to predominate
in the posterior upper lobes. Subpleural micronodules may
become confluent to form a "pseudo-plaque". About 20% of
coal workers develop irregular opacities suggestive of lung
fibrosis, associated with functional impairment (6). In some
such patients, CT scanning and biopsy have shown changes
identical to those of idiopathic pulmonary fibrosis (7). This
type of lung fibrosis appears to predispose to development of
lung cancer (8). It is important for the radiologist to be aware
that silica exposure predisposes to emphysema and lung cancer,
even in nonsmokers.
Progressive massive fibrosis (PMF), sometimes referred to as
complicated pneumoconiosis or conglomerate pneumoconiosis,
is much more common in silicosis than in coal workers’ pneumoconiosis.
On the chest radiograph, PMF presents with oval
opacities, typically seen in the posterior upper lobes, with associated
hilar retraction. Because the masslike fibrosis is usually
lenticular rather than spherical in shape, it is often less dense
than expected on the frontal radiograph. Sequential evaluation
of these masses often shows apparent migration toward the hila,
leaving a peripheral rim of cicatricial emphysema. Although
usually symmetric, masses may be unilateral. Unilateral PMF
may be distinguished from lung cancer by the presence of lobar
volume loss and peripheral emphysema. On CT, PMF typically
appears as an upper lobe mass (often bilateral) with irregular
borders, frequent calcification, and surrounding cicatricial
emphysema. Thickening of the adjacent extrapleural fat is common.
A central area of low density is often seen in masses
which are greater than 4 cm in diameter, and likely represents
necrosis. Cavitation is less frequent. The presence of PMF
should always raise the suspicion of tuberculous or atypical
mycobacterial superinfection. CT may help in the diagnosis of
mycobacterial infection by showing occult cavities.
Asbestos-related diseases
In asbestos-related disease, the radiologist must determine
the presence or absence of pleural disease as a marker of
asbestos exposure, and identify complications of asbestos exposure,
including asbestosis, lung cancer, mesothelioma, pleural
abnormalities, and benign asbestos-related lung masses. The
table provides an illustration of the latency and relative frequen-
cy of the pleural and parenchymal manifestations of asbestos
exposure.
There are four distinct types of asbestos-related pleural disease.
Benign asbestos-related pleural effusion has the shortest
latency (5 to 20 years), but is the least common. Pleural
plaques are the most common manifestaton of asbestos exposure.
Diffuse pleural thickening is less common, and often
complicated by rounded atelectasis. Finally, malignant
mesothelioma is one of the most feared, though least common,
manifestations of asbestos exposure. CT scanning is more sensitive
than the chest radiograph for detection of pleural plaques
(9), particularly non-calcified pleural plaques. Both diffuse
pleural thickening and pleural plaques may be associated with
pulmonary restriction (10, 11).
CT is an excellent modality for early detection of asbestosis.
Minimal criteria for diagnosis of early asbestosis include the
presence of interstitial lines bilaterally at more than one slice
level (12).
Occupational/environmental malignancy
Based on epidemiologic studies, approximately 15% of lung
cancers in men and 5% of lung cancers in women are due to
occupational exposures. Pulmonary carcinogens recognized by
the International Agency for Research on Cancer include arsenic,
asbestos, beryllium, bis-(chloromethyl) ether, cadmium, chromium
(IV), mustard gas, nickel, radon, and silica. The clinical presentation
and pathology of lung cancers due to such exposures
does not differ from that of cancers due to other causes.
Bronchogenic carcinoma is estimated to develop in 20% to
25% of asbestos workers who are heavily exposed (13). There is
typically a latency period of approximately 20 years, and the
greater the exposure to asbestos, the higher the risk of lung cancer.
Asbestos exposure and cigarette smoking appear to be synergistic
in development of cancer (Table 1). There is a strong
association between the presence of histologic asbestosis and
development of lung cancer (14), but this asbestosis is not
always apparent on imaging studies. There is a relative
increased incidence of malignancy in lower lobes, compared
with the upper lobe predominant distribution of lung cancer in
the general population.
Tiitoola et al. screened 602 asbestos exposed workers with
low-dose CT, and identified 111 patients with noncalcified nodules,
of which 5 were malignant (15). However, as with screening
for non-occupational lung cancer, the effectiveness of
screening for asbestos-related lung cancer has not been demonstrated.
The incidence of mesothelioma in asbestos-exposed individuals
is about 10%. Mesothelioma presents radiographically
with focal or diffuse pleural thickening or with a pleural effusion.
MR may help in staging mesothelioma by identifying
transdiaphragmatic spread, or spread into the chest wall. PET
scanning may help identify spread into the chest wall and to
mediastinal or distant nodes (16).
Hypersensitivity pneumonitis
Hypersensitivity pneumonitis (HP) is an inflammatory interstitial
lung disease caused by recurring exposure to a variety of
occupational and environmental antigens. Microbial agents are
the most common inciting antigens, but other important agents
include proteins (particularly from birds) and inorganic agents
such as isocyanates (17). HP features widely variable clinical,
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