Primary Testicular Neuroendocrine Tumor Coexisting With Seminoma Sharing Germ Cell Origin

A 40-year-old, male, Japanese patient presented with the complaint of painless, right testicular swelling. Tumor markers for testicular cancer were normal. He underwent radical orchiectomy with the clinical diagnosis of stage I seminoma. Pathological examination revealed seminoma and coexisting neuroendocrine tumor (NET). Germ cell neoplasia in situ (GCNIS) was present in the vicinity of seminoma, but there was no continuity between NET and seminoma. Tumor cells of both lesions displayed amplification of 12p and isochromosome 12p on fluorescence in situ hybridization, suggesting that both tumors originated from GCNIS. The present report is the first to describe a case of primary testicular NET coexisting with seminoma in an ipsilateral testis.