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Anterior mediastinal masses are occasionally encountered on thoracic imaging studies. Among these lesions, thymoma is the most common etiology. Thymomas are tumors of thymic epithelial cell origin that are characterized by variable histologic and biologic behavior. Although thymomas are often detected asymptomatically, they may present with chest pain, cough, dyspnea, the superior vena cava syndrome, or paraneoplastic syndromes. Thoracic imaging studies typically show a round or lobulated mass in the anterior mediastinum. Calcification in thymomas is usually seen in higher grade lesions and is typically punctuate or amorphous, positioned within the center of the lesion. Peripheral calcification is rare in thymoma and is more commonly seen with cysts, treated lymphoma, or germ cell neoplasms. Thoracic computed tomography features suggesting higher grade thymoma include tumor heterogeneity, vascular encasement, indistinct margins with the lung, lobulation, pulmonary nodules, lymphadenopathy, and pleural disease, although imaging findings have a limited ability to predict stage and prognosis for individual patients. Thymomas may be staged using a number of published systems, the most common of which include the classification proposed by the World Health Organization and the Masaoka system. Thymomas are treated surgically whenever possible as surgery offers the best opportunity for long-term cure. Radiotherapy and chemotherapy are also used, typically for higher stage or incompletely resected lesions. The stage of thymoma according to the Masaoka classification system, the World Health Organization histologic tumor type, tumor size, and completeness of surgical resection are independent features predicting the prognosis of patients with thymoma.