[A case of pulmonary actinomycosis in which differentiation of primary and metastatic lung cancers was difficult].

The patient was a 72-year-old woman who had undergone low anterior resection for T2N1M0 stage IIIA colorectal cancer. A chest X-ray examination was performed 10 months later for persistent cough, and a solid nodule was found in S5 on the mediastinal side of the middle lobe. There were no malignant findings on bronchoscopy, but FDG-PET was performed because primary or metastatic lung cancers could not be ruled out. High FDG accumulation was detected with an SUV value of 13.7, and thus surgical resection was performed for diagnosis and treatment. The postoperative diagnosis was pulmonary actinomycosis. Bronchoscopic diagnosis of pulmonary actinomycosis has been found to be difficult in many reported cases. False positivity of other inflammatory diseases on FDG-PET is common, but there are few reports of false positive pulmonary FDG-PET findings in actinomycosis. Therefore, pulmonary actinomycosis should be kept in mind for the differential diagnosis of cases that are positive in FDG-PET.