Thoracic Imaging 2003 - Society of Thoracic Radiology

SUNDAY
82
Asbestos Related Pleural Disease and Mesothelioma
Francine L. Jacobson, M.D., MPH
Learning Objectives:
Understand sources of exposure to asbestos.
Recognize asbestos exposure on radiographs and CT.
Review current practice for diagnosis, staging and treatment
of malignant pleural mesothelioma (MPM).
Exposure to Asbestos
Asbestos is a fibrous silicate that had been widely used in
commercial applications for its unique combination of attributes.
It is strong, flexible, and will not burn. It resists corrosion
and is an effective insulator. The three most common varieties
are chrysotile, amosite, and crocidolite. Chrysotile fibers, widely
used in commerce, are pliable and most often arranged in
bundles of cylinders. Asbestos fibers can be combined with
binding materials and used in a variety of construction applications.
It is estimated that 3000 commercial products, ranging
from older plastics and paper products to brake linings, floor
tiles, cement pipe, and insulation, have contained asbestos.
Asbestos containing materials were used most extensively for
fireproofing, insulation, soundproofing and decorating in the
United States for thirty years following World War II.
Amosite and crocidolite fibers (collectively called amphibole
fibers) are like tiny needles. When crushed, asbestos fibers are
too small to be seen by the human eye and do not become dust
particles. Small and light, the fibers can remain in the air, available
to be inhaled, for a long time. The EPA estimates asbestos
was included in most of the approximately 107,000 primary and
secondary schools and 733,000 public and commercial buildings
in the United States. It is estimated that between 1940 and
1980, 27 million Americans had occupational exposure to
asbestos. Occupational exposure is greatest among those who
work in asbestos mines, mills, factories, shipyards that use
asbestos, and those who manufacture and install asbestos insulation.
Proximity to such facilities and exposure to the laundry of
those occupationally exposed to asbestos also provide significant
sources of exposure. Two to six million people in the
United States are estimated to currently have significant occupational
levels of exposure.
Evidence of Exposure to Asbestos
Pleural effusion, the earliest radiographic finding, may not
be attributed to asbestos exposure, if a chest radiograph is even
obtained when it is present. It generally occurs within ten years
of exposure and may or may not be present years later when
pleural plaques have developed. Upon testing, the effusion is
exudative with non-specific findings. It can be hemorrhagic and
therefore resolves with diffuse pleural thickening. Further complications
include calcification and development of rounded
atelectasis.
Pleural plaques occur after a latent period of 20-40 years. In
approximately 80% of cases, asbestos exposure is known. The
plaques consist of acellular collagen bundles primarily involving
the parietal pleura and may contain asbestos fibers. For many
years it was felt that the fibers reached the parietal pleural by
penetrating the visceral pleura, however, they may also reach it
via the lymphatics where they incite an inflammatory response.
Plaques grow slowly and continue to grow in the absence of
continued exposure and without malignant potential.
Calcification may occur after 10-20 years. Pleural plaques tend
to occur without other stigma of asbestos exposure, such as
asbestosis.
Pleural plaques may be calcified or not, but chest radiographic
criterion of bilateral hemidiaphragmatic calcification
remains the radiographic criterion for the diagnosis of asbestos
exposure. Pleural plaques may also be seen following middle
ribs, and along the spine. These finding are more apparent on
CT scans.
Malignant Pleural Mesothelioma (MPM)
Due to the long latency period, the incidence of MPM has
continued to increase in the United States despite the removal of
asbestos from commercial use. Two to three thousand cases are
now seen per year; MPM is not limited to those with occupational
exposure but 6-10% of asbestos workers will develop the
disease based on tumorogesesis of amphibole fibers after 35-40
years.
The peak years in which patients present are the 6 th to 8 th
decades of life, particularly when due to occupational exposure.
The occupations involved have been male dominated resulting
in 3-6:1 ratio of men to women patients. The onset of symptoms
is usually insidious characterized by dyspnea, chest pain,
cough and weight loss. Right-sided disease is more common
and SVC syndrome and Horner syndrome may also be seen.
Hypertrophic osteoarthropathy, clubbing and hypoglycemia may
complicate the differentiation of this disease from metastatic
adenocarcinoma. Pathologically, this differentiation remains
difficult. VATS is recommended for diagnosis (98% sensitivity)
with special histochemical stains or ultrastructural analysis.
Due to the tendency of the tumor to seed the incision, incisions
are planned with an eye to be resected at the time of more definitive
treatment.
Early in its course, the tumor burden may be relatively small
with pleural effusion dominant. While the pleural effusion can
be large enough to cause shift of the mediastinum to the contralateral
side, decreased volume of the affected hemithorax is
far more common. Asbestos pleural plaques are present in
approximately 20% of patients. Over time, tumor involving
both the parietal and visceral pleural surfaces will progress to
encase the lung with reduction in lung volume and fixation of
the mediastinum. It will eventually lead to lobulation of pleural
masses and invade the contiguous structures including the lung,
mediastinum, and chest wall. Distant metastases are a very late
phenomenon and very uncommon at initial presentation. The
prognosis has improved, particularly for patients with epithelial
cell type disease who undergo aggressive multimodality therapy,
including extra-pleural pneumonectomy with intra-operative
chemotherapy.
MPM presenting as a pleural effusion may present for imaging
that is routine but not specific to MPM. Once the diagnosis
is established, the primary role of imaging is to help select the
course of treatment. The decision to perform a procedure such
as an extra-pleural pneumonectomy requires confidence that the
entire burden of disease can be removed. If the disease cannot
be entirely resected, only palliative therapy, such as pleurectomy,
will be offered.
Sagittal and coronal image planes are most helpful to assess
extent of disease, particularly regarding potential invasion of
chest wall, mediastinum, diaphragm and abdomen. The modality
of the imaging may actually be less important although MRI
has been used to provide this multi-planar imaging for several
years. MR easily differentiates the separate solid components
of disease from pleural effusion. MR does remain limited