Spontaneous regression (SR) of thymoma is rare. A 44‐year‐old man with right chest pain underwent computed tomography (CT), which showed an 11.0 cm mass in the anterior mediastinum and right pleural effusion. He refused surgery and was sent home without medication and additional treatment. One year later, the mass had regressed to 5.5 cm, and the right pleural effusion had disappeared. He was then lost to follow‐up. Four years after the initial visit, he presented with diplopia and fatigue. A significant increase in his anti‐acetylcholine receptor antibody levels led to myasthenia gravis (MG) diagnosis. CT revealed a regressed mediastinal mass (3.0 cm). After extended thymectomy, histologic analysis confirmed a thymoma type B2, Masaoka stage IIa. The SR was due to intratumoral infarction. This report is the first to describe MG developing during SR. Anterior mediastinal tumors undergoing SR should be differentiated from thymomas and MG perioperative development should be considered.

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Background A mediastinal aberrant goiter is extremely rare, characterized by independence from the thyroid gland. This tumor most commonly develops in the anterior mediastinum and should be differentiated from a thymoma. Moreover, as mediastinal aberrant goiters frequently have a blood supply arising from the thoracic great vessels, preoperative diagnosis and evaluation of the feeding vessel are vital for safe surgery. Herein, we report a rare case of mediastinal aberrant goiter characterized by high radiodensity on non-enhanced computed tomography (CT). Case presentation A 77-year-old woman underwent non-enhanced CT, which showed a 2.2-cm, well-circumscribed, homogeneous anterior mediastinal nodule with high radiodensity. The CT attenuation value of the nodule was as high as 91.9 Hounsfield units. A thymoma or mediastinal goiter was suspected owing to the tumor location and the lack of a cystic component. We performed anterior mediastinal tumor resection using video-assisted thoracoscopic surgery. Pathologically, variably sized follicles, indicative of thyroid tissue, were observed with hemorrhage and hemosiderin deposition. Moreover, papillary projections of small follicles (Sanderson’s polsters) were scattered without atypia of the follicular epithelium, indicative of an adenomatous goiter. The diagnosis of a mediastinal aberrant goiter was supported by the absence of an anatomical connection with the thyroid gland. The postoperative course was uneventful, and the patient was discharged on postoperative day 5. Conclusion The thyroid gland has a high radiodensity on non-enhanced CT, which correlates with iodine concentration. This radiological feature may be useful for the preoperative differentiation of mediastinal aberrant goiters containing thyroid tissue.