Primary mucinous adenocarcinoma of the thymus

Abstract

A 69-year-old man presented with a 28-mm solid tumor, showing partial calcification in the anterior mediastinum, detected by chest computed tomography. We considered thymoma with cystic degeneration and performed an operation without preoperative biopsy. Since the tumor was in close proximity to the upper mediastinal vessels, the patient underwent thymothymectomy. The tumor was located in the left upper pole of the thymus. There was marked adhesion between the tumor and vein, but no obvious invasion was detected. Microscopically, the tumor showed multi-lobulated cysts. The cyst wall was comprised of both atypical and normal cells and showed papillary areas protruding into the lumen. Immunohistochemically, the tumor cells were positive for cytokeratin (CK) 7, focally positive for CK20 and homeobox protein (CDX2), and negative for thyroid transcription factor-1 and CD5. [¹⁸F]-fluorodeoxyglucose positron emission tomography revealed that there was no tumor in any other part of the body, and the patient had no prior medical history of cancer. The tumor was diagnosed as a primary thymic mucinous adenocarcinoma (pT1N0M0 pStage I and Masaoka stage I).