Summary Primary seminal vesicle carcinoma is a rare entity whose diagnosis can be achieved by ruling out the main carcinomas that commonly invade the seminal vesicles. Although a panel of immunohistochemical markers (cancer antigen 125, cytokeratin [CK] 7, CK20, prostate-specific antigen, and prostate-specific acid phosphatase) has been proposed as unique for primary seminal vesicle carcinoma, a reliable positive marker is lacking. In this article, we report a case of primary seminal vesicle carcinoma in a 57-year-old man. The tumor was localized to the left seminal vesicle and histologically characterized by papillae lined by broad eosinophilic cells with pleomorphic nuclei. The neoplastic cells expressed cancer antigen 125 and CK7, whereas CK20, prostate-specific antigen, and prostate-specific acid phosphatase were negative. A strong and diffuse nuclear labeling for PAX8 was detected. Because carcinomas of the colon, bladder, and prostate, the main differential diagnosis in this setting, have been reported consistently to be PAX8 negative, this marker may be very useful for a prompt diagnosis of seminal vesicle carcinoma. Highlights PAX8 is strongly positive in seminal vesicle carcinoma. PAX8 is consistently negative in carcinomas of the colon, bladder, and prostate. PAX8 is very useful in this differential diagnosis.
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