Atypical Type A Thymoma Identified in Slowly Growing Pulmonary Metastases from Type A Thymoma: A Case Report

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Introduction: Type A thymoma is generally considered a low-risk thymoma subtype with an excellent prognosis and low rates of distant metastasis. Atypical type A thymoma is a recently established entity characterized by more aggressive biological behavior despite retaining the morphology of conventional type A thymoma. Patients with primary conventional type A thymoma who subsequently develop recurrence with atypical histological features are extremely rare. Case Presentation: A 79-year-old man was referred to our hospital due to gradual enlargement of bilateral pulmonary nodules. Seven years earlier, he had undergone complete resection of an anterior mediastinal tumor, which had been diagnosed as conventional type A thymoma. Retrospective review of preoperative computed tomography images revealed that small pulmonary nodules had already been present at the time of the initial surgery but had yet to be identified. The nodules enlarged slowly from 3 mm to 10 mm and from 3 mm to 7 mm over the 7-year period. Positron emission tomography showed no abnormal fluorodeoxyglucose uptake. After thoracoscopic wedge resection of the left lower lobe lesion, histopathological examination demonstrated pulmonary metastasis of type A thymoma with focal hypercellularity and increased mitotic activity, consistent with atypical type A thymoma. No necrosis was identified. The Ki-67 labeling indices of the primary and metastatic lesions were 7% and 20%, respectively. Conclusion: This case demonstrates that even apparently low-risk type A thymoma may exhibit indolent but metastatic behavior and may develop atypical histological features during a prolonged clinical course. Careful long-term surveillance and detailed pathological evaluation, including assessment of the Ki-67 labeling index, may be important in patients with type A thymoma.