Answer: Malignant teratoma

Histology: The majority of the lesion consists of a dense acellular scar with focal calcification. Focally within the scar is a cystic structure lined by flattened cuboidal epithelium with focal mucinous features. The wall surrounding the structure is fibrous with focal muscle fibers.

Discussion: This patient, upon exploration of the retroperitoneum, was shown to have extensive metastatic embryonal carcinoma from the testis. This case is an example of a primary germ cell tumor of the testis that, despite regression of the primary into a scar with only focal residual teratoma, was associated with massive metastases. The other situation where one sees regression of the primary in the face of metastatic tumor is with choriocarcinoma. In a post-pubertal male, all teratomas in testes should be diagnosed as malignant even though histologically they may appear benign. The only true biologically benign teratoma in males occurs within prepubertal individuals. The way I sign out germ cell tumors in postpubertal males is “Malignant germ cell tumor consisting of …..”, with a list of the various tumor types. If a lesion contains only teratoma, it would be signed out as Malignant germ cell tumor consisting of teratoma”. In this way, there is no confusion that is one is dealing with a malignant neoplasm with the potential to metastasize. Metastatic adenocarcinoma to the testis would be exceedingly rare in a 20-year-old. One would also expect to see a greater degree of cytologic atypia. The most common metastasis to the testis is from adenocarcinoma of the prostate in men with disseminated disease. A rete cyst would be in continuity with the rete and consists of flattened epithelium without mucinous differentiation, and would not arise in the setting of a dense scar.