RESUMEN
Tumors of ovarian epithelial type of testis are rare entities. After ruling out metastasis, especially from gastrointestinal origins, these neoplasms are categorized according to their ovarian correspondents, now including a new group of seromucinous tumors, which was introduced by the World Health Organization in 2013. Here we present the case of a 60-year-old man with a testicular tumor, showing focally stratified, immunohistochemical PAX8-positive epithelium and endometrioid-like stroma, supporting the diagnosis of seromucinous borderline tumor of ovarian-type surface epithelium. The two main competing hypotheses for the etiology of these neoplasms are development either by metaplasia of the mesothelium of the tunica vaginalis testis, or from remnants of Mullerian duct, which is supported by the notable positivity for PAX8 in our case. To our knowledge, we are the first to describe this newly defined entity of seromucinous borderline tumor of the testis in the English-speaking literature.