Invasive thymoma with rapid response to cisplatin, doxorubicin and cyclophosphamide (PAC) chemotherapy, after an early postoperative recurrence-a case report.

Background: Thymomas generally exhibit indolent growth and local progression, with extrathoracic metastases being rare. Recurrence typically occurs months or years after surgical resection. We herein present a rare case of thymoma exhibiting rapid postoperative recurrence and abdominal lymph node metastasis, which showed a dramatic response to chemotherapy. Case Description: A 66-year-old female was referred to our hospital for breast cancer treatment. A preoperative computed tomography (CT) scan coincidentally revealed a 7.5 cm anterior mediastinal tumor, suspected to be invading the pericardium and left lung. Extended thymothymectomy with partial resection of the left lung and pericardium was performed. Intraoperatively, no pleural dissemination was observed, and pleural effusion cytology was negative for malignant cells. The pathology was Type AB thymoma, Masaoka stage III [tumor, node, metastasis (TNM) 8th edition: pT3N0M0, Stage IIIA; TNM 9th edition: pT2N0M0, Stage II], with microscopically positive (R1) margins; thus, postoperative radiotherapy was planned. On postoperative day (POD) 18, she presented with dyspnea and massive left pleural effusion; after repeated thoracocentesis, the effusion recurred, and the patient was readmitted for continuous drainage. On POD 43, a CT scan showed a massive lesion along the left thoracic wall adjacent to the pericardium, with multiple small nodes in the left lung and two enlarged left gastric lymph nodes. A CT-guided needle biopsy of the thoracic wall performed on POD 47 confirmed recurrent thymoma. The patient received chemotherapy with cisplatin, doxorubicin, and cyclophosphamide (PAC), with biochemical evidence of rapid tumor lysis that did not meet formal tumor lysis syndrome criteria. Chest X-ray on day 3 of chemotherapy showed near-complete resolution of the thoracic lesions, and a complete response, including normalization of the abdominal lymph nodes, was achieved after four courses of PAC. Serial follow-up imaging has shown no evidence of recurrence for at least 18 months. Conclusions: This case illustrates the diagnostic complexity of thymoma, including atypical radiological and pathological features together with an unusually aggressive early recurrence. Incomplete resection can lead to early recurrence and progression, and clinicians must remain vigilant for such atypical courses. The tumor nonetheless proved highly sensitive to chemotherapy, underscoring the importance of prompt systemic treatment intervention when recurrence occurs.