Clear cell adenocarcinoma of the tunica vaginalis of the testis with an adjacent uterus-like tissue.

Testicular and paratesticular neoplasms that resemble the common epithelial type of ovarian tumor are quite rare. Paratesticular clear cell carcinoma is very uncommon in the testis, with no reported cases of a tumor arising from the tunica vaginalis in the literature to our knowledge. The present case shows that it is highly malignant and metastatic. The differential diagnosis of the tumor was made after thorough clinical, pathological and immunohistochemical investigations, from the mesothelioma of the tunica vaginalis, paratesticular serous papillary carcinoma, carcinoma of the rete testis, epididymal adenocarcioma, yolk sac tumor of the testis and metastatic carcinoma. The tumor showed Bcl-2 and Her-2/neu immunoreactivity, but was non-reactive for p53. This tumor, with a uterus-like structure as a paratesticular tumor-like mass, was composed of endometrial-type glands and stroma surrounded by bundles of smooth muscle, and is the third example of this kind of structure in English written literature. The patient, having normal external genitalia and fertility, represents the first reported case of paratesticular malignant differentiation of müllerian-type epithelium in the normal gonadal state. Müllerian-type epithelium located in the vicinity of the testis and/or endometriotic metaplasia of the mesothelium of the tunica vaginalis might be the possible origins for this uterus-like structure, and as a result, for this tumor.