Imaging findings of a thymoma
One typically sees a well-defined, rounded, or lobulated anterosuperior mediastinal soft tissue mass arising from one of the thymic lobes with asymmetric growth toward one side of the midline, occasionally with necrotic, cystic, hemorrhagic, or calcific changes. About 33% of thymomas invade through the capsule and involve adjacent tissues or structures such as the mediastinal fat, pleura, pericardium, great vessels, heart, or lung and may extend through the diaphragm into the peritoneal cavity or retroperitoneum. Metastatic disease is most commonly to the pleura, often mimicking malignant pleural mesothelioma with unilateral pleural thickening, masses, or diffuse nodular circumferential pleural thickening encasing the ipsilateral lung. Pleural effusions, lymphadenopathy, or distant hematogenous metastases are present less commonly.