Mucinous Adenocarcinoma Arising From Teratoma Years After Testicular Nonseminomatous Germ Cell Tumor.

Growing teratoma syndrome (GTS) is a rare complication of nonseminomatous germ cell tumors (NSGCTs). We present a case of a patient with a remote history of NSGCT, previously treated with orchiectomy and chemotherapy, who developed a large retroperitoneal mass over two decades later. The patient had normal tumor markers and multiple inconclusive biopsies. Surgical excision was performed, and the pathology returned as mucinous adenocarcinoma arising within a teratoma. This case highlights both the critical importance of long-term surveillance following NSGCT and the need for complete surgical resection of residual teratomatous disease even years after initial treatment due to the risk of delayed malignant transformation.