We report four cases of Leydig cell tumor of the testis with a microcystic pattern that mimicked yolk sac tumor. The patients ranged in age from 27 to 35 years and, except for one tumor that was discovered incidentally, presented with testicular masses. All tumors were intratesticular, and three were well circumscribed by a rim of fibrous tissue, whereas one showed minor, focal extension into the adjacent testis. The tumors typically had a vaguely lobular architecture subdivided by fibrous bands. Three of the cases had a complex microcystic appearance caused by individually vacuolated cells and coalescent cystic spaces; this pattern accounted for the majority of two tumors. Another case had focal collections of Leydig cells with prominent cytoplasmic vacuoles but lacked the coalescent spaces. The microcyst contents ranged from optically clear to eosinophilic or lightly basophilic, with the latter having the staining qualities of acid mucopolysaccharide. Three tumors had uniform, bland nuclei and low mitotic rates (1 mitotic figure per 10 high power fields), but one had marked, random nuclear pleomorphism and an average mitotic rate of five mitotic figures per 10 high power fields. By immunohistochemistry, all were diffusely positive for vimentin; two of three were positive for inhibin, and one showed focal positivity for cytokeratin (CAM 5.2). All were negative for alpha-fetoprotein and placentalike alkaline phosphatase and, apart from having microcystic and solid areas, lacked other features typical of yolk sac tumor. Clinical follow-up ranged from 2 months to 2 years with no patient having recurrence or metastasis. The distinction of Leydig cell tumor from yolk sac tumor has important clinical implications because patients with the former usually receive only clinical follow-up, but the latter often requires chemotherapy.
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