[A case of pulmonary actinomycosis with a unique finding in the chest MR image].

A 57-year old man, who was complaining of a productive cough and right shoulder pain, was admitted to our hospital because of an irregularly shaped mass located at rt. S1 on a chest radiograph. Bronchoscopy revealed no evidence suggesting lung cancer or any specific infection, either pathologically or microbiologically. CT-guided biopsy revealed changes resembling lymphocytic or plasmocytic interstitial pneumonitis with thickening of the alveolar septum and with accumulations of mononuclear cells and plasma cells, indicating the proliferation of bronchus-associated lymphoid tissue (BALT system). Since no definitive diagnosis was considered possible, a right upper lobectomy was performed. Histopathologic examination of tissue from the right upper lobe revealed sulfur granules and branching Gram-positive filamentous bacteria, and the condition was pathologically diagnosed as pulmonary actinomycosis. In the center of the mass lesion, the patient's chest MRI showed a very small area with a low signal intensity in T1- and a high signal in T2-weighted images, which suggested an accumulation of fluid in the actinomycotic abscess. As detailed MR findings in this condition have not been well described in the literature, the MRI evidence seen in this case may be useful for the diagnosis of actinomycosis.