Incidental Anterior Mediastinal Nodular Lesions on Chest CT in Asymptomatic Subjects

The aim of this study was to investigate the prevalence and characteristics of nodular lesions in the anterior mediastinum that had been found incidentally on screening chest computed tomography (CT) in asymptomatic subjects.

We included 56,358 consecutive participants (mean age 52.4 ± 10.5 years; male-female ratio 35,306:21,052) who underwent a baseline low-dose chest CT scan as part of a health checkup from 2006 through 2013. After the presence of anterior mediastinal nodular lesion had been confirmed, their CT findings, confirmatory diagnosis, and interval CT scan were reviewed. The standardized prevalence ratio for thymic epithelial tumor was calculated on the basis of the Republic of Korea cancer statistics for 2014.

Of the 56,358 participants, 413 (0.73%) had lesions (95% confidence interval: 0.66-0.80%); the prevalence increased with age (p <0.001) and a history of malignancy (p = 0.005). Of the lesions, 85.2% were smaller than 2 cm, 61.3% were round, and 80.2% had CT attenuation higher than 20 Hounsfield units. Among 51 proven cases, 39 lesions (76.9%) were benign and 12 (23.1%) were malignant. The standardized prevalence ratio for thymic epithelial tumor was 2.04 (95% confidence interval: 1.01-3.42). Of 11 resected thymic epithelial tumors, five were carcinomas, 10 were stage I or II, and all were completely resected without recurrence. Of the 237 unconfirmed cases with a follow-up CT scan, 82.2% were stable, 8.9% had increased, and the other 8.9% had decreased.

The prevalence of incidental nodular lesion was 0.73%. Most lesions had CT features that were indistinguishable from thymic epithelial tumors, but a considerable portion of the lesions were suspected to be benign. Incidental thymic epithelial tumors were more prevalent than clinically detected tumors, were early-stage cancer, and showed favorable outcomes.