Abstract
Germ cell tumors, the most common of the testicular neoplasms, originate from primordial germ cells. These tumors are known for their totipotent nature, capable of differentiating into various cell types. This case report presents a rare occurrence of mucinous cystadenoma in a patient who received chemotherapy for metastatic left non-seminomatous germ cell tumor (NSGCT) of the testis. Despite undergoing chemotherapy, a residual retroperitoneal mass persisted. The persisting large mass is usually suggestive of teratoma or malignancy. Upon surgical removal, the histopathology revealed a surprising diagnosis: that of mucinous cystadenoma.