Teratomas of the testis are rare in pure form and there is limited new information on these tumors which have one particularly unusual clinical feature namely, the fact that those occurring in prepubertal boys are invariably benign irrespective of their histology whereas those occurring postpuberty have a malignant potential even when histologically mature. The one exception to the latter comment is the rare entity of a mature cystic teratoma (dermoid cyst) of the testis. Although sporadically reported previously there is only one sizable case of dermoid cysts of the testis recently reported (33). The diagnosis should be reserved for lesions that are grossly typical of a dermoid cyst and are unassociated with adjacent intratubular germ cell neoplasia unclassified. The more common mature teratoma that has a malignant potential has a solid and cystic gross sectioned surface contrasting with the predominantly cystic nature of the sectioned surface of a dermoid cyst.
It has recently been appreciated that primitive neuroectodermal components in testicular germ cell tumors may in some instances be conspicuous, and even dominate the microscopic picture, rarely representing most or the entire neoplasm (26). These foci of neoplasia which typically have a significant component of malignant small cell neoplasia with varyingly prominent differentiation of neuroectodermal type, are usually recognized when there is an associated teratomatous component from which they arise but when the latter are inconspicuous problems in differential diagnosis may result. Foci resembling various forms of central nervous system tumors such as medulloepithelioma or ependymal neoplasms, or neuroblastoma may be encountered. These are aggressive neoplasms and should be distinguished from immature teratoma, something that should not be difficult because they usually form a confluent mass lesion.BURNT-OUT (REGRESSED) GERM CELL TUMORSA most remarkable phenomenon in testicular pathology, and which interestingly has no companion in the female gonad, is that seen when germ cell tumors undergo apparent spontaneous regression, resulting in complete or almost complete hyaline scarring. This phenomenon which is variously referred to as spontaneous regression or "burnt-out" germ cell tumor has been the subject of a recent study by the Indiana University group and that paper (3) is recommended. In these cases the testis typically shows a firm white appearance and on microscopic examination hyaline scarring is seen, sometimes with a minor component of associated viable neoplasia, such as seminoma or teratoma or at least intratubular germ cell neoplasia. The latter may be a subtle clue to the diagnosis that what one is observing is something more specific than just an old infarct. Sometimes the scar shows rather prominent vessels, hemosideri n laden macrophages, and particularly in cases that are presumptively "dead" embryonal carcinoma, intratubular calcifications. Although these regressed tumors may be clinically apparent as testicular abnormalities to inspection, sometimes they only show up on ultrasound, the classic situation being when a patient presents with a retroperitoneal germ cell tumor, which is actually a metastasis from the regressed testicular primary.