January 2006, VOLUME 186
NUMBER 1

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January 2006, Volume 186, Number 1

Chest Imaging

Pictorial Essay

Thoracic Actinomycosis: CT Features with Histopathologic Correlation

+ Affiliations:
1Department of Radiology and Center for Imaging Science, Samsung Medical Center, Sungkyunkwan University School of Medicine, Gangnam-gu, Seoul 135-710, South Korea.

2Department of Diagnostic Pathology, Samsung Medical Center, Sungkyunkwan University School of Medicine, 50, Ilwon-dong, Gangnam-gu, Seoul 135-710, South Korea.

3Department of Medicine, Division of Pulmonary and Critical Care Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Gangnam-gu, Seoul 135-710, South Korea.

Citation: American Journal of Roentgenology. 2006;186: 225-231. 10.2214/AJR.04.1749

ABSTRACT
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OBJECTIVE. Thoracic actinomycosis is a chronic suppurative pulmonary or endobronchial infection caused by Actinomyces israelii, a gram-positive anaerobic organism. We present the CT features of thoracic actinomycosis with histopathologic correlation.

CONCLUSION. The typical CT feature of parenchymal actinomycosis is a chronic segmental air-space consolidation containing necrotic low-attenuation areas with frequent cavity formation. A broncholith can be secondarily infected with Actinomyces organisms, resulting in endobronchial actinomycosis. It usually manifests as a proximal endobronchial calcification associated with distal obstructive pneumonia.

Keywords: broncholithiasis, CT, endobronchial actinomycosis, thoracic actinomycosis

Introduction
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Thoracic actinomycosis is a chronic suppurative pulmonary or endobronchial infection caused by Actinomyces species, most frequently Actinomyces israelii, a gram-positive anaerobic saprophytic organism in the oral cavity that historically was thought to be fungus, but now is considered to be a branching filamentous bacterium [1]. Actinomyces infection typically follows aspiration of endogenous organisms of the oropharynx into the lungs in persons with poor oral hygiene or from extension of cervicofacial infections. The clinical manifestations of pulmonary actinomycosis are productive cough, low-grade fever, and bloodtinged sputum. The prognosis is generally good provided that the infection is recognized and appropriate antibiotic therapy is instituted. A number of articles have described the CT findings of pulmonary actinomycosis [2, 3], and there also have been reports of CT findings of endobronchial actinomycosis associated with a foreign body [4] or broncholithiasis [5, 6]. We present the CT features of thoracic actinomycosis, including endobronchial actinomycosis, with histopathologic correlation.

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Fig. 1A 51-year-old man with early stage parenchymal actinomycosis. High-resolution CT image (1-mm collimation) shows poorly defined peripheral pulmonary nodule in right lower lobe. Note surrounding areas of ground-glass attenuation.

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Fig. 1B 51-year-old man with early stage parenchymal actinomycosis. Photograph of wedge resection specimen shows poorly defined nodule (arrows) formed by dense infiltration of many neutrophils and lymphocytes. Note surrounding areas of intraalveolar fibrinous exudate with some inflammatory cell infiltration (stars), which correspond to areas of ground-glass attenuation on CT. (H and E, ×1)

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Fig. 2 61-year-old man with early-stage parenchymal actinomycosis. Delayed contrast-enhanced CT image obtained with mediastinal window settings shows small subpleural nodule in left upper lobe. Note central low-attenuation area with peripheral enhancing portion of nodule and adjacent pleural thickening (arrows). Actinomycosis was diagnosed using percutaneous transthoracic core biopsy.

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Fig. 3A 61-year-old woman with parenchymal actinomycosis manifesting as subsegmental consolidation. Contrast-enhanced CT image obtained with mediastinal window settings shows subsegmental consolidation in posterior segment of right upper lobe. Consolidation shows central low-attenuation area with peripheral contrast enhancement. Note peripheral bronchiectasis (arrow) and adjacent loculated pleural effusion.

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Fig. 3B 61-year-old woman with parenchymal actinomycosis manifesting as subsegmental consolidation. Image of A obtained with lung window settings shows triangular subsegmental consolidation contains ectatic bronchi (arrow) with surrounding areas of ground-glass attenuation.

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Fig. 4A 42-year-old man with parenchymal actinomycosis manifesting as necrotic peripheral mass. Delayed contrast-enhanced CT image obtained with mediastinal window settings shows poorly defined subpleural mass with low-attenuation area (arrow) in right lower lobe.

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Fig. 4B 42-year-old man with parenchymal actinomycosis manifesting as necrotic peripheral mass. Image of A obtained with lung window settings shows irregularly marginated mass with patchy areas of groundglass attenuation and interlobular septal thickening (arrows).

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Fig. 4C 42-year-old man with parenchymal actinomycosis manifesting as necrotic peripheral mass. Photograph of gross specimen from wedge resection of right lower lobe shows peribronchial cavitary mass (arrows). Note mild thickening of proximal bronchus (arrowheads). Scale: cm.

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Fig. 4D 42-year-old man with parenchymal actinomycosis manifesting as necrotic peripheral mass. Photomicrograph of histopathologic specimen shows bronchiectasis with surrounding areas of dense infiltration of inflammatory cells and fibrosis. Note intraluminal granulation tissue (star) and Actinomyces colonies (arrow) within ectatic bronchus. Additional Actinomyces colonies (arrowheads) also are noted within lung parenchyma. (H and E, ×1)

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Fig. 5A 60-year-old man with parenchymal actinomycosis manifesting as lobar consolidation. Contrast-enhanced CT images obtained with mediastinal window settings reveal right upper lobe lobar consolidation. Consolidation shows significant contrast enhancement with multifocal low-attenuation areas (black arrowheads). Note bronchiectasis (arrow, A) within consolidation and enlargement of right paratracheal lymph node (white arrowhead, A).

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Fig. 5B 60-year-old man with parenchymal actinomycosis manifesting as lobar consolidation. Contrast-enhanced CT images obtained with mediastinal window settings reveal right upper lobe lobar consolidation. Consolidation shows significant contrast enhancement with multifocal low-attenuation areas (black arrowheads). Note bronchiectasis (arrow, A) within consolidation and enlargement of right paratracheal lymph node (white arrowhead, A).

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Fig. 6 80-year-old man with parenchymal actinomycosis manifesting as pulmonary mass. Contrast-enhanced CT image obtained with mediastinal window settings shows mass replacing lingular division of left upper lobe. Note multiple large areas of low attenuation within enhancing mass and adjacent pleural thickening and ipsilateral dependent pleural effusion.

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Fig. 7A 67-year-old man with parenchymal actinomycosis that resolved completely after treatment with antibiotic medication. Contrast-enhanced CT image obtained with mediastinal window settings shows segmental consolidation containing central low-attenuation area and several cavities.

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Fig. 7B 67-year-old man with parenchymal actinomycosis that resolved completely after treatment with antibiotic medication. CT image obtained with lung window settings at same level as A after 7 months of antibiotic therapy with amoxicillin and clavulanic acid shows complete resolution of consolidation with only residual parenchymal scarring (arrows) evident.

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Fig. 8 39-year-old man with parenchymal actinomycosis and pleural effusion. Contrast-enhanced CT image obtained with mediastinal window settings shows right middle lobe consolidation and moderate-sized pleural effusion. Note central low-attenuation area (arrow) within consolidation and mucoid impaction within bronchus (arrowhead).

Parenchymal Actinomycosis
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According to Cheon et al. [3], chronic segmental air-space consolidations that contain low-attenuation areas with peripheral enhancement and adjacent pleural thickening are typical CT findings of pulmonary actinomycosis. In an early stage of infection, the disease manifests as a small, poorly defined, peripheral pulmonary nodule with or without interlobular septal thickening (Figs. 1A, 1B, and 2). With the slow progression of infection, the pulmonary nodule gradually increases in extent to manifest as an air-space consolidation or a mass (Figs. 3A, 3B, 4A, 4B, 4C, and 4D). Although the consolidation is usually segmental in distribution at the time of diagnosis, it can replace a whole lobe at a later stage (Figs. 5A, 5B, and 6). Typically, the air-space consolidation contains central areas of low attenuation with frequent cavitation (Figs. 7A and 7B).

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Fig. 9A 57-year-old man with parenchymal actinomycosis manifesting as chronic necrotizing pneumonia with transfissural extension. Contrast-enhanced CT image obtained with mediastinal window settings shows extensive parenchymal consolidation in superior segment of right lower lobe with areas of necrosis and multiple small cavities. Note subcarinal lymphadenopathy (arrow) and hypertrophied intercostal artery (arrowheads) supplying chronic necrotizing pneumonia.

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Fig. 9B 57-year-old man with parenchymal actinomycosis manifesting as chronic necrotizing pneumonia with transfissural extension. Coronal reformation of contrast-enhanced CT image shows transfissural extension (arrow) of necrotic consolidation from superior segment of right lower lobe (large star) into upper lobe (small star). Note thickening of adjacent interlobar fissure (arrowheads).

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Fig. 9C 57-year-old man with parenchymal actinomycosis manifesting as chronic necrotizing pneumonia with transfissural extension. Photograph of right pneumonectomy specimen shows large cavitary lesion in superior segment of right lower lobe (large star), communicating with another cavitary lesion in right upper lobe (small star) through transfissural extension (arrow). Note thickening of adjacent interlobar fissure (arrowheads). Scale: cm.

Other associated CT findings include hilar or mediastinal lymphadenopathy, bronchiectasis within the consolidation, localized pleural thickening, and pleural effusion (Fig. 8). An extensive consolidation sometimes can cross the adjacent interlobar fissure (transfissural extension) and extend into the neighboring pulmonary lobe (Figs. 9A, 9B, and 9C). Further progression of the infection may involve the pleura and chest wall.

At histopathologic correlation, the central low-attenuation area seen on CT represents microabscess or necrotic material contained within ectatic bronchi. The peripheral enhancing portion of the consolidation represents chronic inflammation with a varying degree of fibrosis. Confirmative diagnosis is based on histopathologic findings of Actinomyces colonies or sulfur granules (yellowish aggregates of clustered mycelia) or on filamentous structures within the tissue specimen positive for Grocott-Gomori methenamine-silver staining [7].

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Fig. 10A 48-year-old man with actinomycosis associated with bronchiectasis. High-resolution CT image obtained with 1-mm collimation shows bronchial wall thickening and bronchiectasis in right lower lobe consolidation.

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Fig. 10B 48-year-old man with actinomycosis associated with bronchiectasis. Contrast-enhanced CT image obtained with mediastinal window settings at lower level than A shows air-fluid level (arrow) within bronchiectatic airway.

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Fig. 10C 48-year-old man with actinomycosis associated with bronchiectasis. Photograph of specimen obtained from right lower lobectomy shows bronchiectasis (stars) and bronchial wall thickening with inflammation and fibrosis. Note Actinomyces colonies (arrows) within lumen of ectatic bronchus.

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Fig. 11A 47-year-old man with endobronchial actinomycosis associated with broncholithiasis. Contrast-enhanced CT image obtained with mediastinal window settings shows small broncholith (arrow) obstructing bronchial lumen that supplies superior segment of right lower lobe. More distally, necrotic subsegmental consolidation containing mucoid material and another broncholith (arrowhead) is seen with bronchial dilatation. Also note small calcified hilar and subcarinal lymph nodes, suggesting previous tuberculous infection.

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Fig. 11B 47-year-old man with endobronchial actinomycosis associated with broncholithiasis. Contrast-enhanced CT image obtained at lower level than A shows triangular subsegmental consolidation contains typical low-attenuation area with adjacent pleural thickening (arrowheads).

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Fig. 11C 47-year-old man with endobronchial actinomycosis associated with broncholithiasis. Photomicrograph of histopathologic specimen of endobronchial calcified nodule after decalcification shows numerous filamentous structures (arrow) representing Actinomyces organisms covering broncholith (B). (Grocott-Gomori methenamine-silver stain, ×200)

The differential diagnosis of actinomycosis includes a necrotic lung cancer, tuberculosis, semiinvasive pulmonary aspergillosis, and other subacute necrotizing bacterial pneumonias.

Bronchiectatic Form
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Actinomyces species tend to colonize devitalized tissue [5]. Secondary actinomycotic infection can involve a devitalized lobe or segment that already had been damaged by previous tuberculosis or by other bacterial infections, resulting in parenchymal destruction and bronchiectasis. A bronchial infection by Actinomyces species exacerbates further the preexisting bronchiectasis and bronchial inflammation and peribronchial parenchymal loss. CT features include localized areas of bronchiectasis, irregular bronchial wall thickening, and irregular peribronchial consolidation with or without abscess formation (Figs. 10A, 10B, and 10C).

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Fig. 12A 56-year-old man with endobronchial actinomycosis associated with foreign body (chicken bone). Contrast-enhanced CT image obtained with mediastinal window settings shows small endobronchial calcification (arrow) obstructing lumen of bronchus intermedius. Note surrounding areas of low attenuation around endobronchial calcification, suggestive of necrotic material.

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Fig. 12B 56-year-old man with endobronchial actinomycosis associated with foreign body (chicken bone). CT image obtained with lung window settings at lower level than A shows air-space consolidations and areas of ground-glass attenuation in right middle and lower lobes, suggestive of obstructive pneumonia.

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Fig. 12C 56-year-old man with endobronchial actinomycosis associated with foreign body (chicken bone). Photomicrograph of specimen obtained from bronchoscopic removal of endobronchial calcification reveals chicken bone, which was surrounded by acute suppurative inflammation containing multiple Actinomyces colonies in background of numerous neutrophils. (H and E, ×200)

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Fig. 13 47-year-old woman with both parenchymal and endobronchial actinomycosis associated with broncholithiasis. Contrast-enhanced CT image obtained with mediastinal window settings shows small endobronchial calcification (white arrow) obstructing lumen of posterobasal segmental bronchus of left lower lobe. Masslike consolidation (arrowhead) containing several cavities is also seen in anterobasal segment of left lower lobe. Notice adjacent pleural thickening (black arrow). Endobronchial actinomycosis was diagnosed by bronchoscopic biopsy, and parenchymal actinomycosis was diagnosed by wedge resection.

Endobronchial Actinomycosis Associated with Broncholithiasis
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Rare cases of predominantly endobronchial actinomycosis have been reported [4, 6, 8], including an article on endobronchial actinomycosis associated with broncholithiasis [6] and a case report on endobronchial actinomycosis associated with a foreign body [4].

Endobronchial actinomycosis associated with broncholithiasis likely results from secondary colonization of a preexisting endobronchial broncholith by aspirated Actinomyces organisms [5]. The subsequent inflammatory process results in progressive airway obstruction with distal postobstructive pneumonia of the involved pulmonary lobe or segment. Progressive shedding of Actinomyces colonies from the nidus aggravates distal pneumonic consolidation further with subsequent abscess formation.

The CT feature of endobronchial actinomycosis associated with broncholithiasis is a proximal endobronchial calcified nodule (broncholith) associated with distal postobstructive pneumonic consolidation of the involved lobe or segment [6] (Figs. 11A, 11B, and 11C). Distal postobstructive pneumonic consolidation shows a central low-attenuation area with or without cavities. Mediastinal or hilar lymph nodes are enlarged frequently. Most cases show evidence of an earlier tuberculous infection (e.g., calcified hilar and mediastinal lymph nodes or calcified granulomata). In areas where tuberculosis is endemic, the possibility of endobronchial actinomycosis should be suggested when findings of broncholithiasis are present on CT.

Histopathologically, suppurative zones with multiple sulfur granules comprising numerous branching filamentous organisms are identified around the broncholith. Distal pneumonic consolidation seen on CT consists of acute suppurative inflammation with abscess formation or organizing pneumonia with a varying degree of fibrosis.

Endobronchial Actinomycosis Associated with a Foreign Body
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Endobronchial actinomycosis can be associated with aspirated foreign material, such as a chicken bone, a tooth, a fish bone, a grape seed, or a bean [4]. The bronchial infection results from direct aspiration of a foreign body contaminated with Actinomyces organisms. CT may show a radiopaque endobronchial nodule, in cases of an aspirated bone fragment, with distal obstructive pneumonia (Figs. 12A, 12B, and 12C), which is similar to the CT appearance of endobronchial actinomycosis associated with broncholithiasis. Rarely, both the parenchymal form and the endobronchial form can be encountered in one patient (Fig. 13).

In conclusion, the typical CT feature of parenchymal actinomycosis is a chronic segmental air-space consolidation containing necrotic low-attenuation areas with frequent cavity formation and peripheral enhancement. An endobronchial broncholith can be secondarily infected with Actinomyces organisms and a foreign body contaminated with Actinomyces organisms can be aspirated, both resulting in endobronchial actinomycosis. Endobronchial actinomycosis usually manifests as a proximal obstructive calcified endobronchial nodule associated with distal postobstructive pneumonia on CT.

Supported by grant R11-2002-103 from the Korea Science & Engineering Foundation.

Address correspondence to J. Han.

References
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