Endometriosis in a mesothelial cyst of tunica vaginalis of the testis. Report of a case.

A rare case of endometriosis occurring in paratesticular mesothelial cyst is presented. It was found in a 7 mm mesothelial inclusion cyst of tunica vaginalis in a 46-years-old old man who underwent a radical orchiectomy for seminoma. It showed a typical histologic pattern with endometrioid cylindrical epithelium and cellular stroma. The lesion was immunohistochemically positive for estrogen receptors and progesterone receptors, in contrast with the adjacent mesothelium. However, rare endometrioid epithelial cells expressed mesothelial markers calretinin and cytokeratin 5/6. This immunohistochemical overlap with mesothelium and morphological transition between endometrioid epithelium and mesothelium favor metaplastic pathogenesis of the lesion. In differential diagnosis, it is important to distinguish paratesticular endometriosis from tissue of teratoma (especially when a germ cell tumor is present in the testis, as was seen in this case).