[A case of thoracic actinomycosis in the left lung coexisting with pulmonary squamous cell carcinoma in the right lung].

An 80-year-old man was admitted to our hospital with chest pain. Chest computed tomography (CT) showed masses in both lung fields. The findings from a biopsy specimen of the left chest wall showed radially filamentous microorganisms, therefore a diagnosis of thoracic actinomycosis was made. The specimen from the mass in the right upper lobe obtained by transbronchial lung biopsy (TBLB) did not yield a definitive, and we started administration of penicillin. After 10 weeks the mass and consolidation in left upper lobe improved, but the size of the mass in the right upper lobe had increased. FDG-PET showed high accumulation only in the right lung mass, and TBLB was performed again, with pathological findings of squamous cell carcinoma. In most cases it is very difficult to differentiate pulmonary actinomycosis from lung cancer. Pulmonary actinomycosis with coexisting lung cancer is very rare. If penicillin is ineffective in a patient with actinomycosis, pulmonary actinomycosis with coexisting with lung cancer should be considered.