Serous borderline tumor of the paratestis

Reported herein is a case of serous borderline tumor (SBT, ovarian epithelial type tumor) of the paratestis, involving the tunica vaginalis, in a 64‐year‐old man. The patient complained of right hydrocele; puncture cytology of the turbid fluid pointed to an adenocarcinoma. Right orchiectomy was performed and multiple micronodules were grossly observed in the paratestis. On microscopy small papillary epithelial lesions were found with psammoma bodies and intraglandular papillary lesions were irregularly recognized in the stroma of the paratestis, similar to SBT of the ovary. The tumor cells had often short microvilli. Mucin production was evident on PAS and colloid iron staining. Both papillary and glandular epithelial cells were positive on immunohistochemistry for Ber‐EP4/epithelial antigen, low‐molecular‐weight cytokeratin (CAM5.2), cytokeratin 7 and estrogen and progesterone hormone receptors, but negative for CEA, cytokeratin 20 and calretinin. The average proliferative index was approximately 10.5% as assessed on Ki‐67 (MIB‐1) staining. Ultrastructurally, the cells did not demonstrate any well‐developed microvilli or secretory granules and immunohistochemical findings supported SBT of Müllerian type (ovarian epithelial type tumor), while excluding a papillary type of malignant mesothelioma. The lesion in the present case was concluded to be a testicular serous tumor of Müllerian type, similar to SBT of the ovary.