INTRODUCTION: Carcinoma of unknown primary (CUP) is rarely located in the mediastinum. Most cases are revealed to be metastatic lymph node carcinoma, which carries a poor prognosis. The optimal treatment for CUP confined to the mediastinum is yet to be established, and the long-term outcome of induction therapy in combination with surgery for mediastinal CUP is unclear. CASE PRESENTATION: A 46-year-old man with no history of malignancy was diagnosed with anterior mediastinal adenocarcinoma through biopsy. The patient underwent chemoradiation for the tumor, which was initially suspected as invasive T4 lung cancer. After a favorable response to presurgical therapy, the tumor was deemed more likely a mediastinal tumor, and it was completely resected simultaneously with the thymus, the partial left lung, and the partial left innominate vein. The tumor contained histologic features identifiable as a lymph node tissue and lacked any thymic tissue, which led to the final diagnosis as metastatic lymph node adenocarcinoma
however, its origin was unknown. No signs of recurrence were detected for 13 years after surgery. CONCLUSIONS: Our case suggests that even patients with mediastinal CUP deemed an advanced disease can achieve long-term survival after undergoing induction therapy and definitive surgery.