Diffusion-weighted MR Imaging in Thymic Epithelial Tumors: Correlation with World Health Organization Classification and Clinical Staging

Purpose: To assess thymic epithelial tumors with diffusion-weighted magnetic resonance (MR) imaging. Materials and Methods: Informed consent from patients and institutional review board approval were obtained. Prospective study was conducted on 30 consecutive patients (21 men and nine women; age range, 35-71 years) with thymic epithelial tumors. They underwent true fast imaging with steady-state precession and single-shot echo-planar diffusion-weighted MR imaging of the mediastinum with b values of 0, 400, and 800 sec/mm(2). Apparent diffusion coefficient (ADC) of the thymic epithelial tumors was calculated by the same observer at two settings and was correlated with World Health Organization classification and clinical staging. Results: There was significant difference in longest diameter (P = .001) and necrotic part of the tumor (P = .014) between low-risk thymoma, high-risk thymoma, and thymic carcinoma. Mean ADC value of both readings of thymic epithelial tumors (n = 30) was 1.24 x 10(-3) mm(2)/sec and 1.22 x 10(-3) mm(2)/sec, with good intraobserver agreement (k = 0.732). There was significant difference in both readings (P = .01 and .20) of low-risk thymoma (1.30 x 10(-3) mm(2)/sec and 1.29 x 10(-3) mm(2)/sec), high-risk thymoma (1.16 x 10(-3) mm(2)/sec and 1.14 x 10(-3) mm(2)/sec), and thymic carcinoma (1.18 x 10(-3) mm(2)/sec and 1.06 x 10(-3) mm(2)/sec). Cutoff ADC values of both readings used to differentiate low-risk thymoma from high-risk thymoma and thymic carcinoma were 1.25 and 1.22 x 10(-3) mm(2)/ sec with area under the curve of 0.804 and 0.851, respectively. There was significant difference in both readings of ADC value of early (stage I, II) and advanced stages (stage III, IV) of thymic epithelial tumors (P = .006 and .005, respectively). Conclusion: ADC value is a noninvasive, reliable, and reproducible imaging parameter that may help to assess and characterize thymic epithelial tumors. (C) RSNA, 2014