Abstract
Mesonephric-like adenocarcinoma is a recently described adenocarcinoma of the uterine body and ovary with overlapping features of mesonephric adenocarcinoma and endometrioid carcinoma. It is thought to be a mullerian adenocarcinoma that has differentiated along mesonephric lines. A 71-year-old woman had a 3-cm endometrial mass that invaded the myometrium without gross or microscopic evidence of cervical involvement. The tumor had a variety of architectural patterns and produced prominent stromal hyalinization containing embedded cords and trabeculae of tumor cells. No squamous or mucinous differentiation or associated mesonephric remnants or hyperplasia was identified. The tumor was positive for TTF1 and GATA3, very focally and weakly positive for estrogen receptor and negative for progesterone receptor and nuclear expression of β-catenin. An unusual inverse pattern of TTF1 and GATA3 immunoreactivity was observed. DNA analysis by digital droplet polymerase chain reaction and quantitative polymerase chain reaction identified an activating KRAS (G12A) mutation. The tumor was interpreted as corded and hyalinized mesonephric-like adenocarcinoma that mimicked corded and hyalinized endometrioid carcinoma.