All forms of testicular germ cell tumors -- except for spermatocytic seminoma -- arise from a common precursor lesion known as intratubular germ cell neoplasia (ITGCN). An easy way of thinking about this is to consider it the germ cell equivalent of “carcinoma in situ”.
Histologically, ITGCN resembles seminoma, but keep in mind that upon invasion, ITGCN may differentiate into any form of germ cell tumor (embryonal, chorio, yolk sac, and/ or seminoma)
Histologic features:
basilar proliferation of large cells with clear cytoplasm
one or two prominent nucleoli may be seen
stains for placental alkaline phosphatase (PLAP) are strongly positive in the neoplastic cells (like seminoma)
cells may spread in a pagetoid fashion (creeping along beneath the normal epithelium) to involve the rete testis
when the cells fill and distend the tubules, the better diagnosis is intratubular seminoma
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