Background. Thymoma is a low-grade malignant epithelial tumor arising in the thymus and rarely metastasizes to distant sites after complete resection. We herein report a rare case of solitary pulmonary metastasis of type Bl thymoma. Case. A woman in her 70s underwent thymectomy with partial resection of the pericardium and right upper lobe for type Bl thymoma 19 years ago. The pathological stage was T3N0M0 Stage III with invasion to the right lung. She received postoperative radiotherapy of 50 Gy with 25 Fr and had no evidence of recurrence until the detection of a 0.8-cm solitary pulmonary nodule in the right lower lobe on follow-up computed tomography (CT) performed 13 years after surgery. The pulmonary nodule gradually grew over the next seven-year follow-up, so we performed wedge resection of the right lower lobe for the diagnosis and treatment. Pathologically, the nodule consisted of epithelial cells with slightly large nuclei and inconspicuous atypia surrounded by a lot of lymphocytes. Immunohistochemically, the epithelial cells were positive for AE1/AE3 and CK19, while the lymphocytes were positive for CD3 and TdT. These findings were similar to those shown in the initial thymoma specimen, and the diagnosis of pulmonary metastasis of type Bl thymoma was made. The patient is doing well 23 months after pulmonary metastasectomy with no evidence of re-recurrence of thymoma. Conclusion. We herein report a resected case of slow-growing pulmonary metastasis from type Bl thymoma with a review of the literature. In cases of type A-Bl thymoma, we should consider the possibility of late recurrence and perform long-term follow-up.
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