Short Guide On the radiologic features of silicosis and silicotuberculosis (combined disease)

This guide briefly describes the radiologic features of silicosis and silicotuberculosis. Silicotuberculosis here means silicosis with changes consistent with either active tuberculosis (requiring treatment) or inactive sequelae of past tuberculosis. Tuberculosis in this Guide is not necessarily disease caused by Mycobacterium tuberculosis.  Nontuberculous mycobacterial (NTM) disease occurs in silica exposed populations. The radiologic features may overlap.

The guide does not cover the computed tomography (CT) features of silicotuberculosis except in passing in the subsection on the difficulties in distinguishing between silicosis, tuberculosis and silicotuberculosis.

The radiologic features of combined disease may be complex, resulting in difficulties in distinguishing between silicosis and active and inactive tuberculosis even for experts.  It is not, therefore, expected that readers inexperienced in reading radiographs of the separate diseases of silicosis and tuberculosis will be able to become competent at reading chest radiographs of silicotuberculosis after consulting this guide and accompanying materials.

This guide should be used together with the chest radiographs in the Training and learning repository of materials on the radiology of silicotuberculosis.

“Opacities” is used in the guide for the parenchymal abnormalities of silicosis to be consistent with the ILO International Classification of Radiographs of Pneumoconioses, but “nodules” is also widely used.

Comments on this guide are welcome. Please email them to info@nioh.ac.za

1. The radiologic features of silicosis

The parenchymal abnormalities of silicosis include both small and large opacities. The small opacities are typically 2-5 mm but can range from 1- 10 mm. Large opacities have their longest dimension exceeding 10 mm.

The small opacities appear first in the upper zones of the lungs but at diagnosis are commonly in the upper half of the lungs. In advanced disease all zones can be affected.  The opacities are usually bilateral and uniformly distributed in the affected zones, without clustering (clumping) of opacities in small regions of a zone. They are described as having an angel wing distribution. The supra-clavicular areas are usually spared unless there is advanced disease. Generally, the opacities are well-defined and regular in shape, size and density.  In the minority of cases the opacities may be calcified.

Hilar and mediastinal lymph nodes may be enlarged, although some studies have reported that hilar adenopathy is uncommonly seen on chest radiographs of silicosis (Ehrlich et al., 1988).   Egg-shell calcification (i.e. at the periphery) of lymph nodes is often seen when the nodes are calcified.

Large opacities, named progressive massive fibrosis (PMF) if silicosis, are oval or rounded, bilateral (unilateral large opacities occur, however) with upper zone predominance. The large opacities are usually accompanied by small opacities but not always. They may first be seen as coalescence of small opacities – a cloudy shadowing encompassing an area of small opacities but with the margins of the small opacities visible – and typically occur in the periphery of the lungs before migrating medially leaving traction bullae in their wake.  Sequential radiographs showing these features are useful because PMF is then suggested, rather than the common differentials of lung cancer and a large tuberculosis mass.

The ILO International Classification of Radiographs of Pneumoconioses (ILO Classification) is accompanied by standard radiographs showing the radiologic features of the pneumoconioses. Small rounded opacities are shown on the p, q and r standard radiographs and large opacities on the A, B and C radiographs. p, q and r radiographs are consistent with uncomplicated silicosis and A, B and C with progressive massive fibrosis of silicosis. They are, therefore, useful for readers to familiarize themselves with the usual features of silicosis.

The standard digital radiographs, along with a guide to their use and radiologic features, are free. The 2022 revised edition of the ILO International Classification of Radiographs of Pneumoconioses is available on the ILO website free of charge: www.ilo.org/radiographs or www.ilo.org/pneumoconioses

The National Institute for Occupational Safety and Health (NIOSH) have developed a valuable self-study syllabus for learning the ILO Classification. (See Appendix 1 page 7.)

2. The value of chest radiographs in silicotuberculosis

The World Health Organization recommends chest radiography for pulmonary tuberculosis triaging, diagnosis and screening (WHO, 2016). Chest x-rays are also valuable in silicotuberculosis. This may be particularly so in settings where paucibacillary disease is to be expected, for example in high prevalence HIV infection settings.

Some aspects of using chest radiographs in silica exposed populations -some of whom have silicosis -should be borne in mind, however. Chest radiographs have been shown to increase sensitivity of active tuberculosis detection when added to sputum smear for silica exposed gold miners where HIV infection prevalence is high [Day et al., 2006].  Importantly, though, the specificity of chest x-rays for active tuberculosis has been shown to be low, presumably because of the complicating features introduced by silica exposure and/or silicosis¹. In the Day et al. study the specificity of combinations of symptoms, signs (weight loss) and chest x-ray was 55% or 59%. A more recent study (Maboso & Ehrlich R, in press) found that the CXR was very sensitive in detecting active tuberculosis, but that specificity was poor at 41%. High sensitivity with low specificity means that there will be many individuals who screen positive but who do not have active tuberculosis (false positives). Where confirmatory bacteriological tests (e.g. Xpert MTB/RIF and sputum culture) are not readily accessible and empiric tuberculosis treatment is common, reliance on chest radiographs may result in unnecessary treatment and lack of consideration of alternative diagnoses.

Footnote¹. Prof. Rodney Ehrlich, UCT, highlighted the importance of the low specificity of chest radiographs for active tuberculosis in silica exposed populations with high prevalences of silicosis. Maboso & Ehrlich (In press) cover this issue.

The radiologic features of silicotuberculosis

The difficulties in distinguishing active and inactive tuberculosis and silicosis are described with illustrative radiographs in Maboso et al. (2023). The article is available in the Training and learning repository of materials on the radiology of silicotuberculosis.  The radiographs in the Repository show examples of silicotuberculosis and tuberculosis with features that could be read as silicosis.

3. Radiologic features suggestive of active or inactive tuberculosis in individuals in whom silicosis is a consideration

This section focuses on selected radiologic changes suggestive of tuberculosis, active or inactive, assisting in the distinction from silicosis². 

Second version

Appendix 1: National Institute for Occupational Safety and Health (NIOSH) Study Syllabus for Classification of Radiographs of Pneumoconioses

https://www.cdc.gov/niosh/learning/b-reader/default.html