High-Risk Thymoma Hidden Within a Cystic Thymic Lesion: A Diagnostic Clue from Diffusion-Weighted MRI

Cyst-predominant anterior mediastinal lesions are commonly attributed to benign thymic cysts. However, thymomas—the most common thymic epithelial tumor—may rarely present as a predominantly cystic mass with only a small solid component, posing a diagnostic challenge. The World Health Organization (WHO) classifies thymomas according to the morphology of tumor epithelial cells and the degree of lymphocytic infiltration, a schema that correlates with biological behavior and prognosis. Unlike low-risk subtypes, high-risk thymomas frequently require multimodal treatment because of their more aggressive clinical course. MRI has increasingly been utilized as a problem-solving modality for indeterminate lesions identified on CT. In particular, diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) measurement has emerged as a potentially useful imaging biomarker for predicting histologic subtypes of thymic epithelial tumors. We report a case of a WHO type B3 thymoma presenting as a predominantly cystic anterior mediastinal lesion, in which MRI findings suggested the possibility of a high-risk subtype. The imaging and corresponding histopathologic findings are presented in this paper.