A massive retroperitoneal mature teratoma from a "burned-out" testicular teratoma and seminoma.

Germ cell tumors are the most common nonhematologic malignancy of young men which often present with metastasis to the retroperitoneum, however a primary retroperitoneal mass should also be considered. The case presented herein reports a 42-year-old male presenting with a massive heterogenous retroperitoneal mass determined to be a mature cystic teratoma. Further investigation revealed a multifocal right testicular mass containing both a viable pure seminoma and a fibrous scar demonstrating germ cell neoplasia in situ thus representing the rare phenomenon of a "burned-out" mixed germ cell testicular tumor. When the radiologist is faced with a large retroperitoneal fat-containing mass, the differential includes a renal angiomyolipoma, liposarcoma, or germ cell tumor (whether primary or secondary). If pathology reveals a germ cell tumor, it is imperative to perform a thorough evaluation of the gonads, as it is much more common for a retroperitoneal germ cell tumor to be a metastasis from the gonads, rather than primary in origin.