Carcinoma in situ (CIS) cells develop from primordial germ cells and early gonocytes. Placenta-like alkaline phosphatase (PLAP) was the first marker described in CIS and seminoma. Of greater importance for an understanding germ cell oncogenesis was the discovery of factors essential for maintaining pluri-potency and self-renewal capability of embryonic stem cells CD117 (c-KIT) and OCT-3/4 (OCT-4).

The best support for the theory of the common origin of the germ cell tumors (GCTs) is the presence of the iso-chromosome i(12p), a chromosome with four short arms present in about 80 % of GCTs regardless of their morphology. Non-seminomatous GCTs (NSGCTs) usually have more i(12p) copies of than Seminomas.

GCTs are rare tumors and account for only 1–2 % of malignant tumors in males. White men living in Western industrialized countries show the highest incidence rates, whereas black men in Africa show the lowest. The incidence in Asiatic men lies somewhere between both extremes.

Well-established epidemiologic causes of GCTs are listed in Table 4.2. The average age of patients at presentation significantly correlates with the pathologic type of GCTs (Table 4.3).

Intratubular germ cell neoplasia (IGCNU) is the official name of the lesion, better known as carcinoma in situ of the testis or testicular intratubular neoplasia. The adjective “unclassified” is utilized to emphasize that the morphology of the tumor cells does not permit an assignment to a definite type of GCT.

If the intratubular tumor cells can be definitely recognized, the adjective intratubular is used in connection with the respective name of the GCT (i.e. intratubular embryonal carcinoma or intratubular seminoma). The prevalence of IGCNU in healthy men is estimated 0.4–0.8 % and about 1 % in infertile men. In cryptorchid testes of adults the frequency is 2–4 %. With the exception of spermatocytic seminoma, atypical germ cells are constantly found (in 98 % of the healthy tissue surrounding invasive GCTs of adults (Table 4.4).

The IGCNU cells are attached to the usually thickened basement membrane and push the Sertoli cells towards the lumen of the seminiferous tubules. The cytoplasm is clear because of the large amount of glycogen (PAS+). The nuclei are bigger than those of spermatogonia and contain one or more nucleoli (Fig. 4.1).

Mitoses are frequent but may be overlooked. IGCNU can also spread in a ‘pagetoid’ pattern into the rete testis or present as microinvasive lesion. Lymphocytic infiltration of the interstitial tissue surrounding the affected, mostly clearly narrowed tubules is frequent.

PLAP, c-KIT (CD117), and OCT 3/4 antibodies are positive, and can thus easily be distinguished from non-reacting normal spermatogonia, which are PAS-. IGCNU are aneuploidy and gain i(12p) copies only shortly before they become microinvasive.

The affected testis is usually enlarged, but can be normal or even atrophic. The tumor is uniform, cream-coloured or pink, and always well-circumscribed. The cut surface is lobulated and the consistency varies from soft to firm, depending on the amount of tumor stroma. Nowadays in the majority of cases the tumors are confined to the testis, but extension to the epididymis and spermatic cord can occur and were very frequent (43 %) in old series. In large tumors necrosis may occur (Fig. 4.2).

Hemorrhage is not frequent and can indicate the presence of a non-seminomatous tumor component. Small hemorrhagic spots are very characteristic of seminoma with syncytiotrophoblastic giant cells.

Seminoma cells are rather uniform with a rounded shape and a mostly clear, glycogen-(PAS-positive) and lipid-rich cytoplasm. In ‘classical’ seminomas tumor cells typically grow in cords and/or nests separated by more or less delicate trabeculae. In some cases, however, small nests of tumor cells are surrounded by abundant fibrotic tissue (Figs. 4.3, 4.4, 4.5, 4.6 and 4.7; Table 4.5).

Lymphocytic infiltration of tumor stroma is a typical feature of about 90 % of seminoma and can be used as a diagnostic criterion in poorly preserved, improperly processed surgical specimens.

The amount of lymphatic infiltration varies from some few lymphocytes to heavy infiltration and formation of lymphoid follicles. Furthermore, in about one-third of seminomas, epithelioid granulomas also develop.

Lymphocytic infiltrates probably represent a true immunologic reaction directed against the tumor. In fact, in old series, observed before the introduction of new therapy protocoling, lymphocyte rich seminomas did significantly better than those without lymphocytes.

Seminoma with syncytiotrophoblastic cells: using H&E staining multinucleated giant cells are detected in about 7 % of seminomas, mostly in the surroundings of vessels. With immunohistochemistry against hCG, however, positive cells are present in about 25 % of all cases. The presence of hCG cells should not be confused with true choriocarcinomas.

Accumulation of edematous fluid in interstitial tissue gives the tumor a microcystic or cribriform (seminoma) appearance.

In pseudoglandular and tubular seminoma, tumor cells form small gland-like clefts.

The name intratubular or interstitial seminoma derives from the predominant way the tumor cells spread.

The formerly anaplastic or atypical seminomas have been renamed as seminomas with high mitotic rate, which is indeed more appropriate because the degree of cellular anaplasia is not higher than in the classical type. High proliferative activity measured by counting mitoses or cells positive for proliferation markers (Ki 67) does not, however, negatively influence the course of the disease (Table 4.5).

Seminoma cells react with many different antibodies. For diagnostic purposes, however, only a few are really important. The most frequent misdiagnosis is to confound seminoma with embryonal carcinoma, which usually happens when the surgical specimen is improperly fixed.

Seminoma cells stain with CD 117 (c-kit) and do not react with CD 30 antibodies, whereas the cells of embryonal carcinoma are CD117-negative and CD30-positive.

In some rare seminomas, single tumor cells are cytokeratin- and CD 30-positive. OCT4 is typically positive in seminoma and embryonal carcinoma. Likewise, yolk sac tumor, spermatocytic seminoma, and choriocarcinoma are all OCT4 negative. Almost all seminomas (and cases of IGCNU) express podoplanin on the cytoplasmic membranes, whereas embryonal carcinomas are either negative or, in about 30 % of the cases, show limited reactivity that is restricted to the apical surfaces of the cells. SOX1 occurs in about 95 % of seminomas and is negative in embryonal carcinoma. CD 30 positive reactivity occurs in 93–100 % of embryonal carcinomas but the other germ cell tumors, including seminoma, are either negative or show only rare positive cells.

Rarely, these tumors are mistaken for lymphoma. This typically spread interstitially among the entrapped, atrophic seminiferous tubules.

Seminomas are, without exception, aneuploid tumor with low copy number of i(12p).

The most important prognostic factors are the diameter of the tumor and the invasion of the rete testis, whereas vascular invasion seems not to have the same prognostic importance as in NSGCT (Table 4.6)

The cut surface is described as mucoid, gelatinous or edematous showing cystic, necrotic, and even hemorrhagic areas. The tumor tissue is yellowish or cream-colored, well circumscribed and can be multinodular. The tumor grows in an expansive and not infiltrative manner.

Microscopically, the tumor borders are sharp. Three different tumor cell types are present. The large (15–20 μm) cells with rather regular round nucleus predominate. The small cells (6–8 μm) resemble lymphocytes and the single scattered giant cells (50–150 μm). All have nuclei with a coarse chromatin.

Lymphocytes are scanty and in the neighboring tubules IGCNUs cells are missed. Intratubular spread of tumor cells is frequent. The polymorphous micromorphology with many mitotic figures gives the impression of an extremely aggressive tumor (Fig. 4.8, 4.9 and 4.10).

The so-called anaplastic spermatocytic seminoma contains a monomorphous population of large cells with big nucleoli that has no prognostic significance. Spermatocytic seminoma combined with sarcoma, mostly rhabdomyosarcoma, has been described and is a very aggressive disease.

The tumor cells are negative for all markers which are commonly found in GCTs. The PLAP reaction is, with very few exceptions, negative. Forty percent of spermatocytic seminomas are CD117+ and some few show a dot-like focal perinuclear reaction for low molecular-weight cytokeratin (CAM 5.2). Spermatocytic seminoma is OCT4 negative, a fact that can be used to differentiate from seminoma and embryonal carcinoma in selected cases, since both are positive.

The tumor cells are polyploid or aneuploid. Gain of chromosome 9 combined with an unchanged chromosome 12 seems to be specific for this tumor.

Usually these tumors are confined to the testis and never relapse and only rarely metastasize. The sarcoma component in mixed tumors metastasizes primarily to the lung and may be responsible of patient dead.

The tumors are usually small and the cut surface is granular. Necrosis and small hemorrhagic spots give the grayish colored tumor. The growth pattern can be solid, papillary, glandular, or tubular (predominant architectural pattern occupying at least 50 %). Less common primary patterns include nested, anastomosing glandular, micropapillary, sieve-like glandular, pseudopapillary, and blastocyst-like. Rarely, embryonal carcinoma develops in the context of polyembryoma-like (Fig. 4.11, 4.12, 4.13, 4.14, 4.15, 4.16, 4.17, 4.18 and 4.19).

Occasionally, EC may present significant chronic inflammation and granulomatous lesion suggesting seminoma. Also, features suggesting endodermal sinus tumor or teratoma may be seen.

The tumor cells are large primitive epithelial cells with a cytoplasm varying from amphophilic to basophilic or clear (seminoma-like cells). Finding intratubular spreading tumor masses with central necrosis is common.

The large pleomorphic and hypercromatic nuclei contain one or more large eosinophilic nucleoli. Mitoses are abundant. Syncytiotrophoblast giant cells may be present. Half of cases show vascular invasion.

The positive staining with cytokeratin antibodies (AE1/AE3; CAM 5.2) confirms the epithelial nature of these tumors. Unlike with seminomas, the PLAP reaction is only focally positive.

The strong and specific staining with CD 30 (Ki 1, Ber-H2) can lead to a wrong diagnosis of giant cell (immunoblastic) lymphoma. Nuclear expression of OCT4 and SALL4 is common in EC. As already mentioned, ECs are negative for CD 117, whereas seminomas do not react with cytokeratin antibodies and CD 30. SOX1 occurs in about 95 % of seminomas and is negative in embryonal carcinoma.

The aneuploid or even triploid tumor cells show manifold chromosomal aberrations, most constant being the presence of i(12p). The number of i(12p) copies seems to correlate with the aggressiveness of the EC.