Pericardial ectopic thymoma: Case report of a rare tumor and a brief review of the literature

Thymoma is an uncommon tumor originating from the thymus. It is often associated with an autoimmune disease such as myasthenia gravis (MG). The majority of thymomas occur in the anterior mediastinum. However, ectopic thymomas tumors, which arise from thymic tissue improperly located because of aberrant embryonic migration, are far rarer. Of these, ectopic thymomas originating in the pericardium are particularly rare and only a few cases were reported. Due to the presence of ectopic pericardial thymoma in an uncommon site, it can be mistaken for various benign and malignant tumors, leading possibly to unnecessarily more aggressive treatment.
A 51-year-old man who had no significant medical history was incidentally detected to have a mediastinal mass on chest X-ray at the preoperative evaluation for an elective orthopedic operation. He had no autoimmune disease or paraneoplastic syndromes. Imaging studies revealed a large mass in the pericardial cavity, anterior to the right atrium, without invading surrounding structures or organs. The CT-guided biopsy led to the diagnosis of a type AB thymoma, confirmed with immunohistochemistry. Video-assisted thoracoscopic surgical resections were carried out. The diagnosis was confirmed on final histology, with a preserved capsule and negative margins. Based on the 2025 AJCC/UICC TNM staging system, the tumor was staged as T1b N0 M0. The patient has been free of recurrence or metastasis for four years.