Imaging in Thoracic Blastomycosis 

Blastomyces dermatitidis is a thermally dimorphic fungus that causes the systemic pyogranulomatous disease termed blastomycosis. Blastomycosis is the least common of the endemic systemic mycoses; the other, more common mycoses include histoplasmosis and coccidioidomycosis. Lungs, and to a lesser extent, skin and bone, are the most common organs involved with this fungus. Hematogenous dissemination can occur. (See the images below.)^{[1, 2, 3, 4, 5]}

The diagnosis of thoracic blastomycosis is made on the basis of a demonstration of organisms in culture or on fungal stains (10% potassium hydroxide) of sputum, bronchoscopy specimens, or secretions obtained from cerebrospinal fluids or dermal, subcutaneous, or other lesions. Cultures are positive in more than 90% of cases. Culture growth may take from 1 to several weeks.

Radiographic findings are nonspecific and variable, with radiographic patterns of thoracic blastomycosis being indistinguishable from those of other mycotic infections.

Chest radiography is the first imaging study performed. The most common pattern observed is acute, nonspecific focal lung opacity, which is found in 25-75% of patients.^{[6, 7, 8, 9, 10]}

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Chest radiographs usually reveal focal lung opacities in the upper lobes (seen in 25-75% of patients); these opacities are often nodular in character. In adults, the upper lobes are affected more frequently than the lower lobes; the ratio is approximately 2:1. In children, opacities most commonly involve the lower lobes. Lung opacities may be patchy or confluent; they may be subsegmental, segmental, or nonsegmental (see the images below). The radiographic appearance is similar to that seen with community acquired pneumonia; slow improvement, lack of change, or even progression of disease over time should raise the possibility of granulomatous infection.

The next most common radiographic presentation (occurring in as many as 30% of patients) is that of a focal discrete mass, either single or multiple. The mass is usually well circumscribed; such masses are variable in size and occasionally contain air-bronchograms. When solitary, a mass may mimic primary carcinoma, especially when associated with unilateral lymph node enlargement or bone destruction (see the images below).

Cavitation occurs less commonly in patients with blastomycosis than in patients with tuberculosis or chronic histoplasmosis; the reported incidence is approximately 15-20% (see the images below).

In a minority of patients, a miliary or diffuse interstitial disease pattern is seen at presentation; patients have respiratory failure and need mechanical ventilation. This pattern may be observed in previously healthy immunocompromised patients. In many patients, the focal lung opacities or mass may be observed in association with the diffuse interstitial pattern, a finding that supports the hypothesis that pulmonary dissemination occurs from a focal pulmonary site (see the image below).

In contrast to histoplasmosis, hilar and mediastinal adenopathy and calcification are uncommon (occurring in 10-20% of cases).

Pleural involvement and significant effusion are uncommon (20%). Rarely, lung or pleural involvement extends into adjacent bones or soft tissues. Pleural thickening without free effusion is a more common radiographic finding.

Osteolytic lesions in the skeleton usually are associated with superficial abscesses. Rarely, mediastinal involvement results in superior vena cava obstruction or brachial plexopathy.

Computed tomography (CT) scan findings of thoracic blastomycosis are variable. As with chest radiography, nonspecific lung parenchymal opacification is most commonly observed, followed by mass lesions (see the images below).

In a review of CT findings in 16 patients with pulmonary blastomycosis, Winer-Muram et al reported the following^[11] :

• A localized mass - 14 patients (88%)
• Consolidation - 9 patients (56%)
• Masses ranging from 3-16 cm in diameter (mean, 8 cm)
• A majority of masses containing air bronchograms - 12 of 14 patients (86%)
• Unilateral abnormalities - 11 patients
• Abnormalities involving both lung - 5 patients

In addition, no lobar predominance was noted. Cavitation was observed in 2 patients; calcified hilar nodes was observed in 7 patients (44%); and enlarged noncalcified nodes was observed in 1 patient.