Testicular germ cell neoplasia, a disease predominantly of young men, is, for unknown reasons, increasing in incidence. Cryptorchidism, a prior testicular germ cell tumor, a family history of testicular germ cell tumors, and somatosexual ambiguity syndromes remain well-established risk factors. Intratubular germ cell neoplasia of the unclassified type represents the common precursor to the great majority of testicular germ cell tumors, and its identification in testicular biopsies reliably identifies those patients who will often progress to an invasive lesion. Seminoma appears to represent the invasive derivative of intra-tubular germ cell of neoplasia of the unclassified type; problematic variants include seminomas with tubular, granulomatous, and edematous patterns. Spermatocytic seminoma is an essentially nonmetastasizing neoplasm unless complicated by the rare development of a sarcomatous component. Embryonal carcinomas usually occur admixed with other germ cell tumor types. The combination of positivity for placental alkaline phosphatase and negativity for epithelial membrane antigen can assist in the distinction of embryonal carcinomas from somatic carcinomas. The treatment of clinical stage I patients with nonseminomatous germ cell tumor with “surveillance only” may be contraindicated depending on features that include the proportion of embryonal carcinoma and the presence of lymphovascular invasion in the orchiectomy specimen. It is important to be aware that pure, mature teratomas in postpubertal patients may be associated with metastases of teratomatous or nonteratomatous type. Yolk sac tumor is characterized by numerous patterns including glandular, myxomatous, sarcomatoid, hepatoid, and parietal variants. Choriocarcinomas classically have a biphasic pattern of syncytiotrophoblast and cytotrophoblast; trophoblastic proliferations lacking a biphasic pattern also occur but are difficult to classify unless this category is broadened. Mixed germ cell tumors, consisting of two or more different elements, are quite common. The polyembryoma is a distinctive, well organized form of mixed germ cell tumor consisting of embryonal carcinoma and yolk sac tumor.
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