Differentiating thymoma from thymic cyst in anterior mediastinal abnormalities smaller than 3 cm

Background: Computed tomography (CT) screening for lung cancer has led to frequent findings of small anterior mediastinal masses. It is very hard to distinguish small thymomas from thymic cysts. The objective of this study was to develop a clinical model for predicting small thymomas (<3 cm) in asymptomatic patients. Methods: Patients who underwent thymectomy for anterior mediastinal masses between 2004 and 2016 were included. All preoperative CT scans (pre- and post-enhanced) were retrospectively reviewed. Size, location, contour, shape, presence of calcification, and enhancement [Hounsfield units (HU)] were evaluated. A nomogram was built based on the predictive factors. For external validation, patients undergoing thymectomy in 2017 were enrolled and thymoma prediction was computed using the proposed nomogram. Results: The study population consisted of 43 patients with thymoma and 57 with thymic cyst. The multivariable analysis identified a lobulated contour and a large difference in HU between post- and pre-enhancement as predictive factors of thymoma. These factors were included in the nomogram, which showed 95% (19/20) power for predicting thymoma after external validation. Conclusions: This clinical model can be used to predict thymoma in patients with small, asymptomatic thymic abnormalities on CT screening.