A presumed extragonadal germ cell tumor that turned out to be a gastric cancer-a case report.

A solely retroperitoneal mass in males in combination with elevated serum Alpha-Fetoprotein (AFP) and beta-human choriogonadotropin (β-HCG) levels is highly indicative of a metastatic testicular cancer. Although testicular cancers are rare, they represent the most common diagnosed cancer in males between 14 and 40 years. However, in cases without evidence of a primary testicular tumor, the rare diagnosis of a retroperitoneal extragonadal germ cell tumor (EGCT) must be assumed. Here, we describe the first published case of a 66-year-old man presenting with this typical clinical picture and the diagnosis of an AFP and β-HCG producing advanced gastric cancer with retroperitoneal lymph node metastases mimicking a primary retroperitoneal EGCT. The final diagnosis was only made by gastroscopy performed after a CT-guided retroperitoneal lymph node biopsy revealed an adenocarcinoma, suggesting an upper gastrointestinal tract primary origin. However, a specific initial anamnesis and also in the primary staging, including a full-body CT-scan there was no hint for another primary tumor. Only the slightly unusual extension of the retroperitoneal mass up to the ligamentum hepatoduodenale and the pylorus, as well as the atypical age made us question our initial diagnosis. This extraordinary case is of special clinical interest to all practising physicians and once again highlights the importance of keeping rare differential diagnosis such as AFP-producing gastrointestinal tumors in mind.