Malignant Sertoli cell tumors of the testis: a study of 13 examples of a neoplasm frequently misinterpreted as seminoma.

The distinction of Sertoli cell tumors from seminoma is critical to ensure proper treatment. Although usually straightforward, we highlight herein 13 malignant Sertoli cell tumors of the testis with light microscopic features that mimicked seminoma. All of the cases were received in consultation, 10 with submitting diagnoses of seminoma, usually of classic type, but three cases of spermatocytic type. Patients ranged from 15 to 80 years of age (median 37 years); all presented with testicular masses. The tumors were typically firm, white to yellow-tan, and often had foci of hemorrhage. The dominant microscopic pattern was nested or sheet-like, with some tumors having secondary patterns of trabeculae-solid tubules, hollow tubules, and pseudofollicles. Tumor cells were polygonal with conspicuous clear cytoplasm in 12 cases; the cytoplasm was focally eosinophilic in 10 cases, but this was never conspicuous. Nine tumors had cytoplasmic vacuoles, and three of four that were investigated stained for intracytoplasmic glycogen. Nuclei were small (5) to medium-sized (8), round-to-oval (13), and vesicular with irregular contours (11). Nucleoli were present in 11 tumors (six small; five large). Stromal fibrosis (12) and lymphoid infiltrates (10) were conspicuous, and tumor necrosis (11) and vascular invasion (8) also were seen. Mitotic figures ranged from <1 to 21/10 high power fields (HPF) (median 1/10 HPF). Staining for inhibin-alpha, epithelial membrane antigen, and cytokeratin (AE1/AE3) was positive in four of four, six of six, and three of six cases, respectively; placental alkaline phosphatase was negative in all five tumors investigated. The nested growth pattern, prominence of clear cells, lymphoid infiltrate, inconspicuous tubular differentiation, cytoplasmic glycogen, and prominent nucleoli caused these tumors to be mistaken for seminomas. The smaller, less pleomorphic nuclei of Sertoli cell tumors, their lower mitotic rate, and the absence of intratubular germ cell neoplasia are helpful differential features. Immunohistochemistry is a useful adjunct in confirming the diagnosis of Sertoli cell tumor, but only if the overlapping features are appreciated by conventional microscopy and the diagnosis of Sertoli cell tumor included in the differential.