An 82-year-old Japanese man who previously presented with descending colon cancer was treated via partial resection. He was referred to our hospital because of an abnormal shadow on chest radiography during a general medical checkup. He had no smoking history or risk of diseases associated with immunosuppression, including diabetes mellitus and human immunodeficiency virus infection. Moreover, he did not have low serum immunoglobulin levels. Chest computed tomography revealed a nodular shadow in the middle lobe of the right lung (Fig. 1).
A radiological examination was performed after 3 months. Because the nodule size increased, lung cancer was suspected. Bronchoscopy was proposed; however, the patient requested surgery. The patient was subsequently admitted to our hospital for lung surgery, which was a diagnostic procedure. Thoracoscopic partial lobectomy was performed on the second day of hospitalization, and rapid intraoperative pathological examination revealed a granulomatous lesion with caseous necrosis (Fig. 2).
Acid-fast bacillus staining showed negative results. We performed M. tuberculosis and M. avium complex (MAC) polymerase chain reaction using resected samples in addition to the T-SPOT.TB test. However, the results were negative. The patient was discharged from the hospital, and we decided to wait for the culture examination result. The acid-fast bacillus liquid culture of the specimen showed negative results. However, culture performed using sputum samples collected after surgery showed positive results after 5 weeks. Nevertheless, the DNA-DNA hybridization method could not identify the mycobacterial strain. Hence, mass spectrometry was performed, and results showed the presence of M. lentiflavum. Based on the histological findings and the sputum culture results that met the American Thoracic Society and the Infectious Diseases Society of America criteria, the patient was diagnosed with pulmonary M. lentiflavum infection. During the postoperative follow-up, the patient did not present with symptoms and chest radiography showed no specific findings. Therefore, antibiotics were not prescribed following surgery, and the patient did not present with recurrence 2 years since the surgery.
Chest computed tomography image showing a 2-cm solitary nodule along the major fissure in the middle lobe of the right lung (S4).