Low-grade mucinous adenocarcinoma of the uterine corpus: a rare and deceptively bland form of endometrial carcinoma

Uterine corpus mucinous epithelial proliferations present diagnostic challenges due to histologic similarities to cervical lesions. We present a rare but distinctive endocervical-like mucinous carcinoma of the uterine corpus resembling adenoma malignum of the cervix that can be mistaken for a benign endometrial process. The clinical-pathologic features of 16 endometrial carcinomas exhibiting a pure endocervical-like mucinous proliferation were evaluated. Hysterectomy and available prehysterectomy specimens were assessed for architectural complexity, nuclear pleomorphism, macronucleoli, nuclear pseudostratification, mitotic index, necrosis, prominent neutrophils, and voluminous extracellular mucin (mucin encompassing >50% of a ×40 field). Cases involving the cervix or lower uterine segment were confirmed as endometrial in origin based on immunohistochemical stains (estrogen receptor, progesterone receptor, p16, and vimentin). Patient age ranged from 45 to 70 years; 6 of 16 (38%) were premenopausal, 11 of 16 (69%) had abnormal bleeding, and 7 of 16 (44%) had a history of hormonal therapy. Prehysterectomy diagnoses were benign in 2 of 16 (13%) cases, borderline in 9 of 16 (56%) cases, and carcinoma in 5 of 16 (31%) cases, whereas 8 of 16 (50%) hysterectomy specimens showed myoinvasive adenocarcinoma. With the exception of 2 cases, architectural complexity was low-to-moderate and no specimens showed marked nuclear pleomorphism. Macronucleoli and abundant mitotic activity were absent. Nuclear pseudostratification was present in 7 of 16 (44%) cases, necrosis in 1 of 16 (6%) cases, prominent neutrophils in 7 of 16 (44%) cases, and voluminous extracellular mucin in 9 of 16 (56%) cases. Cytologically bland mucinous epithelial proliferations should be diagnosed with caution in endometrial samplings. The presence of an endocervical-like mucinous epithelial process in association with voluminous extracellular mucin should prompt consideration for a low-grade mucinous adenocarcinoma of the uterine corpus.