SEMINOMA (USUAL TYPE)

The almost invariable framework of delicate fibrous septa with associated blood vessels and sprinkling of lymphocytes should ideally be present to diagnose seminoma with confidence as other tumors may mimic seminoma on low power but usually lack the septal framework. High power scrutiny to make sure the typical cytologic features of seminoma cells are present is always advisable. A rare seminoma of the usual type has inconspicuous septa and the diagnosis of seminoma should be made cautiously in such cases. Although seminomas usually obliterate the underlying parenchyma, they may have a conspicuous interstitial growth pattern with tubular preservation, particularly peripherally, and rarely the cells are so uniformly widely dispersed in the interstitium that a tumor is grossly inapparent (19).

One aspect of the morphology of seminomas of otherwise typical type that may be confusing is the presence in some cases of solid or hollow tubules or others spaces of various types (Fig. 1) (37). The spaces range from small, closely packed, and relatively regular to dilated, more dispersed, and somewhat irregular. These tumors often lack or have scant lymphocytes. These tumors may have edema and that finding and the paucity of lymphocytes may suggest the erroneous diagnosis of spermatocytic seminoma, but the varied cell types of that neoplasm are absent. A microcystic pattern of seminoma is unassociated with other features, such as hyaline bodies, that would make yolk sac tumor a more realistic consideration and seminomas with tubules (that may be confused with glands) do not have as pleomorphic nuclei as seen in embryonal carcinoma. The lesional cells in these patterns, however, have the typical features of usual seminoma cells which is crucial in the differential diagnosis. Staining for OCT-4 (positive in seminoma but not yolk sac tumor) may help in the differential with a yolk sac tumor. Mistaking a malignant Sertoli cell tumor for a seminoma also occurs (see the section on Sertoli cell tumors). In occasional seminomas with syncytiotrophoblasts giant cells prominent intracytoplasmic lacunae may form small cysts which occasionally may be striking on low power (14). In the differential diagnosis of seminoma with embryonal carcinoma, and for that matter certain other issues, immunohistochemistry may play a role. An excellent discussion of this appeared in 2005 in Seminars in Diagnostic Pathology, and the contribution of Dr. R.E. Emerson and Dr. T. M. Ulbright (9) is strongly recommended. An additional rare feature of seminomas is the presence of signet ring cells (40).