Seminoma with tubular, microcystic, and related patterns: a study of 28 cases of unusual morphologic variants that often cause confusion with yolk sac tumor.

We report 28 testicular seminomas with cystic spaces of variable nature, sometimes accompanied by solid and hollow tubular patterns (12 cases). The spaces often suggested reticular or microcystic patterns of yolk sac tumor, and the solid and hollow tubular patterns often added to the diagnostic confusion. The tumors occurred in men 21 to 55 years old and on gross examination had the typical appearance of seminoma. On microscopic examination, the spaces ranged from small, closely packed and relatively regular to dilated, more dispersed and somewhat irregular. The hollow tubules appeared to result from central discohesion within nests of tumor. The spaces, particularly when large, often contained occasional tumor cells or inflammatory cells within pale edema fluid. The cytologic appearance of the cells lining the spaces, and in the surrounding tumor, retained the typical features of seminoma cells. Thirteen tumors (46%) either lacked (8 cases) or had very scant (5 cases) lymphocytes in the cystic and tubular areas, and hyaline globules were absent. Thirteen of 13 tumors were immunopositive for OCT-3/4 in the nontypical and typical areas; 9 of 10 were placental alkaline phosphatase positive, and 7 of 10 were c-Kit (CD117) positive. The same 13 cases were negative with cytokeratin (AE1/AE3) and alpha-fetoprotein stains. Distinction from yolk sac tumor is aided by the observation that the spaces of yolk sac tumor are often more irregular in their individual shapes and frequently form anastomosing channels. Additionally, the spaces of yolk sac tumor randomly merge with various other yolk sac tumor patterns. The cells lining spaces in yolk sac tumor are often flattened with compressed nuclei and lack the typical prominent nucleoli of seminoma cells. Paucity of lymphocytes and intracystic edema, however, are not differentially helpful, although basophilic fluid favors yolk sac tumor. A panel of immunostains (AE1/AE3, OCT-3/4, and alpha-fetoprotein) is helpful in the differential with yolk sac tumor in especially problematic cases. The edema and paucity of lymphocytes may suggest spermatocytic seminoma, but the varied cell types of that neoplasm are absent.