[Pulmonary actinomycosis presenting as a mass lesion on chest X-ray film].

A 61-year-old man presented with fever, productive cough, and occasional blood-streaked sputum. Chest X-ray films disclosed a poorly defined mass in the right middle lung field. A transbronchial lung biopsy specimen showed epithelial changes indicative of lung cancer, and a right lower lobectomy was performed. Because the resected specimen contained a cavity filled with colonies of actinomycetes, pulmonary actinomycosis was diagnosed. The cavity was surrounded by inflammatory infiltrations and fibrosis with occasional atypical epithelial changes suggestive of benign hyperplasia. Although pulmonary actinomycosis is uncommon today, it deserves attention because of the potential difficulty in differentially diagnosing it from lung cancer.