DOI:10.2214/AJR.07.2402
AJR 2007; 189:1402-1406
© American Roentgen Ray Society
Silicoproteinosis: High-Resolution CT Findings in 13 Patients
Edson Marchiori1,
Carolina Althoff Souza2,
Tatiana Gontijo Barbassa1,
Dante L. Escuissato3,
Emerson L. Gasparetto1 and
Arthur Soares Souza, Jr.4
1 Department of Radiology, Federal University of Rio de Janeiro, Rio de Janeiro,
Brazil.
2 Department of Diagnostic Imaging, The Ottawa Hospital, 501 Smyth Rd., Ottawa,
ON, Canada K1H 8L6.
3 Department of Radiology, Federal University of Parana, Curitiba, Brazil.
4 Department of Radiology, Faculty of Medicine of São José do Rio
Preto, São José do Rio Preto, Brazil.
Received April 11, 2007;
accepted after revision June 25, 2007.
Address correspondence to C. A. Souza
(carolina_althoff{at}yahoo.ca).
Abstract
OBJECTIVE. The purpose of this study was to evaluate the
high-resolution CT findings of silicoproteinosis.
CONCLUSION. Silicoproteinosis usually manifests as bilateral
consolidation in the posterior portions of the lungs and as numerous
centrilobular nodules. Calcification within areas of consolidation is a common
finding.
Keywords: CT • lung • silicoproteinosis • silicosis
Introduction
Silicosis, caused by the inhalation of crystalline free silica, is the most
common occupational disease involving the lungs. Most cases occur after
decades of exposure to relatively small amounts of silica, which causes
chronic pulmonary damage [1,
2]. An acute form of silicosis,
silicoproteinosis, can occur after relatively short exposure to very high
levels of fine particulate silica. The disease manifests within a few years of
the initial exposure and causes rapid deterioration, which invariably leads to
acute respiratory failure. The pathologic features of silicoproteinosis differ
substantially from those of chronic silicosis and resemble those of primary
alveolar proteinosis
[3–6].
High-resolution CT has an important role in the diagnosis of diffuse lung
disease, allowing better characterization of the abnormalities than does chest
radiography. Although the high-resolution CT findings of primary alveolar
proteinosis have been well described
[7], to our knowledge, there
has been no study of high-resolution CT findings in a series of patients with
silicoproteinosis. The purpose of this study was to describe the
high-resolution CT findings in a series of 13 patients with proven
silicoproteinosis.
Materials and Methods
The study included 13 consecutively enrolled patients with the diagnosis of
silicoproteinosis who underwent high-resolution CT at one of the four
institutions involved in the study. Institutional review board approval was
not required at any of the centers. All patients were men with a median age of
29.5 years (range, 18–44 years). They were sandblasters with a history
of exposure to silica varying from 11 months to 8 years (median, 3.2 years).
Two of the patients were previously described in a case report
[4]. Dry cough and dyspnea were
the most common clinical manifestations, present in seven (54%) of the 13
patients. In five (38%) of the patients, dyspnea was the only symptom; in one
case it was associated with low fever.
The diagnosis of silicoproteinosis was based on a clinical history of
rapidly progressive respiratory symptoms and a history of environmental
exposure to silica after other possible causes of lung disease were excluded
on the basis of clinical history and laboratory tests. In 10 patients,
bronchoalveolar lavage at presentation showed a considerable amount of
proteinaceous material with a positive reaction to periodic acid–Schiff
(PAS) stain. Necropsy performed in three cases revealed intraalveolar
accumulation of PAS-positive proteinaceous material, mild peribronchial
granulomatous inflammation, and a few silica particles
(Fig. 1).
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Fig. 1 —Photomicrograph of autopsy specimen from male cadaver (age at
death, 28 years; cause of death, silicoproteinosis) shows abundant
intraalveolar proteinaceous material and positive reaction to stain. (Periodic
acid–Schiff, x100)
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High-resolution CT examinations were performed with different scanners but
similar protocols: 1- to 2-mm collimation at 10-mm intervals, supine position,
acquisition at end inspiration, and reconstruction with a
high-spatial-frequency algorithm. The high-resolution CT images were available
in hard copies photographed at lung (width 1,000 or 1,500 H; level, –650
or –750 H) and mediastinal (width, 350 or 400 H; level, 40 or 60 H)
window settings. The interval between the high-resolution CT examination and
bronchoalveolar lavage varied from 1 to 7 days (median, 3.9 days). In three
cases, autopsy was performed approximately 1, 2, and 4 months after
high-resolution CT.
Two chest radiologists retrospectively reviewed the images for the presence
and distribution of air-space consolidation, ground-glass opacity, and
nodules. Associated findings such as air bronchograms and foci of
calcification within areas of consolidation, interlobular septal thickening,
fibrosis (defined as the presence of traction bronchiectasis and architectural
distortion), pleural effusion or thickening, and mediastinal or hilar
adenopathy also were recorded. The definition of these findings followed the
glossary of terms proposed by the Fleischner Society
[8]. Nodules were classified
according to size (< 15 mm or 
15 mm in diameter), number (< 10 or
> 10), and distribution (centrilobular, peribronchovascular, random). The
overall distribution of the parenchymal findings was classified as unilateral
or bilateral and as upper or lower zone predominance based on the tracheal
carina or random if no zonal predominance was found. The final decision about
the presence and distribution of abnormalities was reached by consensus
between the two radiologists.
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Fig. 2A —21-year-old man with silicoproteinosis. High-resolution CT
scan shows air-space consolidation (arrowhead) within superior
segment of right lower lobe and innumerable ill-defined centrilobular nodules.
Confluent nodules (arrows) are present in upper lobes.
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Fig. 2B —21-year-old man with silicoproteinosis. Unenhanced CT scan
obtained at mediastinal window settings shows consolidation involving
predominantly dependent portions of lungs. Foci of calcification
(arrows) are present within area of consolidation. Small bilateral
pleural effusions and calcified mediastinal lymph node (arrowhead)
are evident.
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Results
High-resolution CT depicted parenchymal abnormalities in all 13 patients.
Air-space consolidation was the most common finding, present in 12 (92%) of
the 13 patients; it was always bilateral and associated with air bronchograms.
In four (33%) of the 12 patients, the areas of consolidation had a
predominantly lower-zone distribution; in the other eight patients, no zonal
predominance was detected. Interestingly, in all 12 patients consolidation
involved mainly the posterior portions of the lungs. Foci of calcification
within areas of consolidation were seen in 10 (83%) of the patients (Figs.
2A,
2B,
3A, and
3B). In the only patient
without consolidation, high-resolution CT showed ground-glass opacities and
small centrilobular nodules throughout the lungs
(Fig. 4).
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Fig. 3A —29-year-old man with silicoproteinosis. High-resolution CT
scan shows innumerable bilateral centrilobular nodules, some of them
confluent. Punctate calcification is present within area of consolidation
(arrow) in right upper lobe. Mild dilatation of trachea is
evident.
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Fig. 3B —29-year-old man with silicoproteinosis. High-resolution CT
scan obtained at lower level than A shows multiple air-space nodules.
Some of these nodules are poorly defined and have ground-glass attenuation
(thin arrows). Calcified mediastinal and hilar nodes (thick
arrows) are evident.
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Nodules were identified in 11 (85%) of 13 patients. In all cases the
lesions were numerous (> 10), were predominantly smaller than 15 mm, and
were centrilobular in distribution. Most of the nodules had homogeneous
soft-tissue attenuation. Nodules with ground-glass attenuation were present in
five (45%) of the 11 cases (Figs.
2A,
2B,
3A,
3B,
4,
5A, and
5B). In nine (82%) of the 11
cases confluent nodules were also seen (Figs.
5A and
5B). Patchy ground-glass
opacities were identified in eight (62%) of the 13 patients, always in
association with consolidation or nodules.
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Fig. 5A —27-year-old man with silicoproteinosis. High-resolution CT
scan shows air-space consolidation with air bronchograms in right upper lobe
and multiple bilateral centrilobular nodules (arrows). Mild
dilatation of trachea is evident.
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A few septal lines were seen in seven (54%) of the 13 patients, always
associated with other abnormalities. In none of the cases was superimposition
of reticular and ground-glass opacities, the so-called crazy-paving pattern,
present. None of the patients had findings of marked pulmonary fibrosis.
Minimal architectural distortion and traction bronchiectasis, however, were
found in four (31%) of the patients.
Calcified lymph nodes were present in 11 (85%) of the 13 patients,
predominantly in the hilar regions (Figs.
2B,
3A, and
3B). The pattern of
calcification varied. Most of the patients presented with diffuse nodal
calcification, although punctate calcification and predominant peripheral
calcification also were present. Noncalcified mildly enlarged mediastinal
nodes, defined as lymph nodes larger than 1 cm in short-axis diameter, were
identified in two (15%) of the patients. Minimal pleural thickening or pleural
effusion was common, present in 11 (85%) of 13 patients
(Fig. 2B). Tracheal dilatation,
defined as transverse tracheal diameter greater than 2.5 cm on axial images,
was an unexpectedly common finding, seen in nine (69%) of the 13 patients
(Figs. 3A and
5A).
Discussion
Silicosis is the most common and oldest recognized occupational lung
disease. Silica-induced lung disease is classified as chronic, accelerated, or
acute depending on the intensity and duration of the exposure to silica dust
[1,
2]. The chronic or classic form
of silicosis typically manifests after 10–20 years of exposure to low
concentrations of silica dust. An accelerated form of silicosis occurs within
4–10 years of heavier exposure. Apart from the early onset and rapid
progression, the radiologic and pathologic manifestations of accelerated
silicosis are identical to those of classic silicosis, consisting of pulmonary
fibrosis and multiple small nodules containing mature collagen and
birefringent silicate crystals
[2]. Acute silicosis is a
fulminating respiratory illness resulting from relatively short exposure to
high concentrations of fine silica dust
[2–6,
9]. The term
"silicoproteinosis" was introduced by Buechner and Ansari
[3] in 1969, when the
histologic similarity between acute silicosis and primary alveolar proteinosis
was first described.
Silicoproteinosis usually manifests within 3 years of the initial exposure
as rapidly progressive shortness of breath often associated with
constitutional symptoms. The course of the disease is relentlessly
progressive. Most of the reported cases have been fatal within months
[3–5,
9]. Acute silicosis most
commonly affects sandblasters, although it has been described in quartzite
millers, tunnel workers, silica flour workers, and workers in the scouring
powder industry
[4–6,
10]. Isolated cases associated
with domestic exposure also have been reported
[11]. The diagnosis of acute
silicosis, as of the occupational diseases in general, is based on clinical
and radiologic manifestations and appropriate history of environmental
exposure. There is no need for pathologic confirmation
[2]. The 13 patients included
in this study were sandblasters with a history of occupational exposure
varying from approximately 1 to 8 years with a median of 3.2 years. The most
common initial manifestation was dyspnea and dry cough, accompanied by low
fever in one case. Nine patients presented with rapidly progressive
respiratory impairment, and eight of them died within 2 years of the initial
diagnosis. The other four patients did not return for clinical follow-up.
The histologic findings of silicoproteinosis resemble those of primary
alveolar proteinosis, that is, PAS-positive lipoproteinaceous material filling
the air spaces [2,
3,
7]. Unlike classic silicosis,
silicoproteinosis manifests as minimal collagen deposition and fibrosis.
Silicotic nodules, the hallmark of classic silicosis, when present, are
smaller than in classic silicosis and may or may not contain weakly
birefringent silicate crystals
[2,
3]. The radiographic
manifestation of silicoproteinosis, like that of primary alveolar proteinosis,
is bilateral parenchymal consolidation
[3,
5,
6,
9,
10]. The high-resolution CT
findings of primary alveolar proteinosis have been described in several
studies and consist of bilateral ground-glass opacities and smooth septal
thickening. The crazy-paving pattern has been considered characteristic of
primary alveolar proteinosis and is highly suggestive of the diagnosis in the
appropriate clinical setting
[7].
The predominant (92%, 12/13) high-resolution CT finding in the 13 cases of
silicoproteinosis described in this study consisted of bilateral air-space
consolidation mainly involving the posterior aspects of the lungs and often
(75%, 9/12) associated with ground-glass opacities. Multiple small nodules, a
feature not described in alveolar proteinosis, were identified in 85% of the
patients. The nodules were centrilobular in distribution and usually
ill-defined, having either soft-tissue or ground-glass attenuation, features
that characterize air-space nodules and reflect the inhalational and a
bronchiolocentric cause of silicoproteinosis
[2,
12]. Confluent nodules were
present in a large percentage (82%) of patients. The predominance of air-space
disease on high-resolution CT presumably reflects the deposition of
proteinaceous material in the alveolar spaces, the pathologic hallmark of
silicoproteinosis. Foci of calcification within areas of consolidation were
identified in 83% of the cases. This finding is clearly depicted on
high-resolution CT and can be considered characteristic of silicoproteinosis.
Calcification is usually not seen in other diseases presenting with air-space
consolidation, such as bacterial and fungal pneumonia, bronchoalveolar
carcinoma, and lymphoma.
None of the patients had marked pulmonary fibrosis. This finding is not
surprising given the acute nature of silicoproteinosis and the absence of
marked fibrosis at pathologic examination
[2]. Although a few septal
lines were identified in 54% of the cases, the crazy-paving pattern was not
seen.
Hilar lymphadenopathy is a common finding in patients exposed to silica
dust and occurs with or without parenchymal disease
[1]. Calcified lymph nodes were
identified in 85% of the patients in this study and noncalcified enlarged
lymph nodes in 15%. The pattern of calcification varied, most commonly being
punctate. Eggshell calcification, a finding commonly seen in chronic
silicosis, was present in a few patients. In the cases of acute silicosis
reported in the literature [3,
9,
10], mediastinal or hilar
adenopathy was a prominent finding on the chest radiographs of a considerable
number of patients and was strongly suspected in others.
Our study had several limitations. It was retrospective and included a
relatively small number of patients. Although the diagnosis was based on
well-defined criteria [2], the
lack of pathologic data limited high-resolution CT–pathologic
correlation. However, we believe the goal of our study, to describe the
high-resolution CT findings of silicoproteinosis in a series of patients, was
met.
The high-resolution CT findings of silicoproteinosis consist of bilateral
air-space disease with consolidation, typically involving the posterior
portions of the lungs, and numerous centrilobular nodules. Punctate
calcification superimposed on areas of consolidation and calcified lymph nodes
are commonly seen. Unlike pulmonary alveolar proteinosis, silicoproteinosis
does not present with the crazy-paving pattern.
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Abstract
Introduction
