Rapid Emergence of Small-cell Carcinoma Due to Malignant Transformation or <i>de novo</i> Formation in the Anterior Mediastinum Following Thymoma Resection: The Importance of a Re-biopsy

A 59-year-old woman presented with multiple mediastinal masses 6 months after post-thymectomy for type B2 thymoma. A diagnosis of small-cell carcinoma (SmCC) via a computed tomography-guided biopsy and fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography revealed no primary lesions outside the anterior mediastinum. The pathologically reevaluated post-thymectomy specimen showed no neuroendocrine differentiation. SmCC is presumed to arise either from malignant transformation of the thymoma or de novo formation. She was treated with carboplatin and paclitaxel to achieve complete response. This case highlights the importance of a re-biopsy to guide appropriate treatment in cases of rapid progression during the course of thymoma.