A Case of Prostatic Mixed Germ Cell Tumor Showing Pagetoid Spread Into the Vas Deference in a Patient With Klinefelter Syndrome

Primary germ cell tumors (GCTs) of the prostate are extremely rare in Klinefelter syndrome (KS), with only four cases having been reported. Here, we report a case of mixed GCT characterized by pagetoid spread into the epithelium of the vas deference in a patient with KS. A 34‐year‐old man diagnosed previously with KS presented with gross hematuria. Radiological examination revealed a tumor occupying the prostate and bilateral seminal vesicles. Histologically, the tumor predominantly exhibited the pattern of a yolk sac tumor and focally presented as a seminoma. Notably, the tumor cells exhibited pagetoid spread in the epithelium of the left vas deferens, mimicking germ cell neoplasia in situ (GCNIS). Isochromosome 12p was detected by fluorescence in situ hybridization analysis of the tumor cells, but no tumor cells were found in either testis. Immunohistochemically, the GCNIS‐like component was positive for SALL4 and c‐kit but negative for α‐fetoprotein, glypican‐3, SOX17, OCT3/4, and SOX2, which is atypical for GCNIS. Based on these findings, we concluded that the tumor primarily originated from the prostate and then extended into the left ductus deferens. This case highlights a rare presentation of mixed GCT with unusual anatomical distribution in KS.