Pulmonary mediastinal actinomycosis with "pound cake sign" on magnetic resonance imaging mimicking an infiltrative malignant tumor: A case report.

Actinomycosis is a rare chronic suppurative granulomatous disease. Surgical biopsy is often performed in patients with chest actinomycosis because malignancy is suspected in most cases. A 62-year-old man presented to our hospital with fever and exertional dyspnea that had persisted for several months. Contrast-enhanced computed tomography showed an irregularly shaped mass with contrast enhancement in the anterior mediastinum and consolidation in the left upper lung lobe contiguous with this mass, as well as multiple nodules in both lungs. The pulmonary artery trunk was stenotic and surrounded by the mass, and the right heart system was enlarged. Thoracoscopic biopsy was performed but failed to yield a diagnosis. Contrast-enhanced computed tomography after one month revealed an increased mass and worsening right heart strain. 18 F-FDG (fluorodeoxyglucose) positron emission tomography/computed tomography and contrast-enhanced magnetic resonance imaging also suggested a malignant tumor, and an open chest biopsy was performed. No malignant cells were identified and actinomycetes were detected by histopathology and bacterial culture. The patient was treated with antibiotics, following which his contrast-enhanced computed tomography findings and general condition improved.