A. Seminoma (35%)
Three histologic subtypes of pure seminoma have been described. However, stage for stage, there is no prognostic significance to any of these subtypes. Classic seminoma accounts for 85% of all seminomas and is most common in the fourth decade of life. Grossly, coalescing gray nodules are observed. Microscopically, monotonous sheets of large cells with clear cytoplasm and densely staining nuclei are seen. It is noteworthy that syncytiotrophoblastic elements are seen in approximately 10-15% of cases, an incidence that corresponds approximately to the incidence of hCG production in seminomas.

Anaplastic seminoma accounts for 5-10% of all seminomas. Diagnosis requires the presence of 3 or more mitoses per high-power field, and the cells demonstrate a higher degree of nuclear pleomorphism than the classic types. Anaplastic seminoma tends to present at a higher stage than the classic variety. When stage is taken into consideration, however, this subtype does not convey a worse prognosis.

Spermatocytic seminoma accounts for 5-10% of all seminomas. Microscopically, cells vary in size and are characterized by densely staining cytoplasm and round nuclei that contain condensed chromatin. More than half the patients with spermatocytic seminoma are over the age of 50.

B. Embryonal Cell Carcinoma (20%)
Two variants of embryonal cell carcinoma are common: the adult type and the infantile type, or yolk sac tumor (also called endodermal sinus tumor). Histologic structure of the adult variant demonstrates marked pleomorphism and indistinct cellular borders. Mitotic figures and giant cells are common. Cells may be arranged in sheets, cords, glands, or papillary structures. Extensive hemorrhage and necrosis may be observed grossly.

The infantile variant, or yolk sac tumor, is the most common testicular tumor of infants and children. When seen in adults, it usually occurs in mixed histologic types and possibly is responsible for AFP production in these tumors. Microscopically, cells demonstrate vacuolated cytoplasm secondary to fat and glycogen deposition and are arranged in a loose network with large intervening cystic spaces. Embryoid bodies are commonly seen and resemble 1- to 2-week-old embryos consisting of a cavity surrounded by syncytio- and cytotrophoblasts.

C. Teratoma (5%)
Teratomas may be seen in both children and adults. They contain more than one germ cell layer in various stages of maturation and differentiation. Grossly, the tumor appears lobulated and contains variable-sized cysts filled with gelatinous or mucinous material. Mature teratoma may have elements resembling benign structures derived from ectoderm, mesoderm, and endoderm, while immature teratoma consists of undifferentiated primitive tissue. In contrast to its ovarian counterpart, the mature teratoma of the testis does not attain the same degree of differentiation as teratoma of the ovary. Microscopically, ectoderm may be represented by squamous epithelium or neural tissue; endoderm by intestinal, pancreatic, or respiratory tissue; and mesoderm by smooth or skeletal muscle, cartilage, or bone.

D. Choriocarcinoma (< 1%)
Pure choriocarcinoma is rare. Lesions tend to be small within the testis and usually demonstrate central hemorrhage on gross inspection. Microscopically, syncytio- and cytotrophoblasts must be visualized. The syncytial elements are typically large, multinucleated cells with vacuolated, eosinophilic cytoplasm; the nuclei are large, hyperchromatic, and irregular. Cytotrophoblasts are uniform cells with distinct cell borders, clear cytoplasm, and a single nucleus.

Clinically, choriocarcinomas behave in an aggressive fashion characterized by early hematogenous spread. Paradoxically, small intratesticular lesions can be associated with widespread metastatic disease.

E. Mixed Cell Type (40%)
Within the category of mixed cell types, most (up to 25% of all testicular tumors) are teratocarcinomas, which are a combination of teratoma and embryonal cell carcinoma. Up to 6% of all testicular tumors are of the mixed cell type, with seminoma being one of the components. Treatment for these mixtures of seminoma and NSGCT is similar to that of NSGCT alone.

F. Carcinoma in Situ (CIS)
In a series of 250 patients with unilateral testicular cancer, Berthelsen et al (1982) demonstrated the presence of CIS in 13 (5.2%) of the contralateral testes. This is approximately twice the overall incidence of bilateral testicular cancer. The presence of contralateral atrophy or ultrasonographic microlithiasis in patients with testicular tumors warrants contralateral biopsy. If diagnosed, CIS is usually treated by external beam radiation therapy.