Imaging characteristics of nontuberculous mycobacterial pulmonary nodules

Introduction. Nontuberculous mycobacteriosis (NTM) of the lungs can develop nodules. In order to clarify some of the characteristics of lung NTM nodules, we examined volume doubling time (VDT) and maximum standardized uptake value (SUVmax) in positron emission tomography (PET) of pathologically diagnosed NTM nodules. Methods. From November 2012 to August 2018, clinical and radiological information were retrospectively investigated in eight patients who were surgically resected and diagnosed as NTM. These eight patients were followed up until November 2020 and were confirmed to have no appearance of lung cancer or reappearance of lung NTM nodules. The VDT was calculated using the Schwartz formula. Results. The median maximum diameter of the nodule at the time of the first CT scan was 16.0 (range: 9.9-20.0) mm. The median maximum diameter of the nodule on CT performed before the surgical biopsy was 18.8 (range: 10.4-32.8) mm. The median doubling time calculated from these results was 203 (range: 20-568) days. Caseous granulomas and acid-fast bacilli were histologically confirmed in all eight patients. Culture of excised nodules revealed Mycobacterium intracellulare in five patients and Mycobacterium avium in three patients. Six patients received PET, and median SUVmax was: 7.0 (range: 3.3-21.0). Median VDT was around 200 days. Some patients had irregular-shaped nodules. Conclusions. CT/PET-CT characteristics of lung nodules are not reliable in differentiating lung NTM nodules from malignant ones. To avoid unnecessary resection, it may be better to collect various information on imaging findings in the nodule itself and in opacities other than the nodule.