We present a rare case of unilateral intratesticular cystadenocarcinoma with mucinous differentiation in a 71-year-old male patient. The orchiectomy specimen revealed a 3.5-cm well to moderately differentiated cystic neoplasm with elongated fine papillary structures lined by columnar epithelium with alternating goblet and ciliated cells completely replacing the testicular parenchyma. Immunohistochemical studies showed positivity for cytokeratin 20, carcinoembryonic antigen, and mucin 2, and negativity for cytokeratin 7, mucin 5AC, vimentin, thyroid transcription factor 1, Wilms' tumor 1, and cancer antigen 125. The current case shows a focal cyst-lining component with benign appearance. Imaging studies and colonoscopy showed no evidence of other suspicious lesions. This tumor resembles morphologically to an intestinal-type ovarian surface epithelial tumor.
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