Distinguishing Benign Thymic Lesions from Early-Stage Thymic Malignancies on Computed Tomography

Introduction: The increasing use of computed tomography (CT) has led to frequent identification of asymptomatic lesions in the anterior mediastinum. The purpose of this study is to identify CT features that distinguish benign thymic lesions from early-stage malignant thymic neoplasms. Methods: We retrospectively reviewed preoperative CT imaging for 66 patients, who had undergone thymectomy for benign thymic lesions or early-stage malignant thymic neoplasms. All variables with a p value of less than 0.2 on univariate logistic regression analysis were evaluated by multivariate analysis. Stepwise selection was performed, and variables with a p value less than 0.05 were retained in the final model. Results: Thirty-eight malignant (58%) and 28 benign thymic lesions (42%) were included. Patients with benign thymic tumors were significantly younger (median age, 49.5 years) than patients with malignant tumors (60.0 years; p = 0.007). Malignant tumors were larger in short-axis dimension (p = 0.028) and more frequently in a nonmidline location in the anterior mediastinum (p = 0.029). Intralesional fat was seen exclusively in benign masses (p = 0.002). Seven benign tumors (25%) and one malignant tumor (2.6%) had a triangular thymic shape (p = 0.023). In multivariate analysis, lower age, smaller short-axis dimension, and lack of infiltration of the mediastinal fat were significant independent predictors of benign pathologic results. Conclusion: Intralesional fat, midline location, and triangular thymic shape are more frequently found in benign thymic lesions. Lack of infiltration of the mediastinal fat, younger patient age, and smaller size are independent predictors of benign thymic lesions. These features may help characterize thymic masses as benign and avert potentially unnecessary invasive diagnostic procedures.