Endometrial cysts within the liver: a rare entity and its differential diagnosis with mucinous cystic neoplasms of the liver.

Endometrial cysts within the liver are rare but can present as diagnostic challenges on small biopsies or frozen sections and may mimic mucinous cystic neoplasms (MCN) of the liver. Five cases of endometrial cysts and 6 cases of MCNs within the liver were collected. The clinicopathological, imaging, and immunohistochemical features were systematically reviewed and compared. The average size of the endometrial cysts was 8.3 cm. Four patients had a prior pelvic operation and coexisting endometriosis at other sites. All 5 cases of endometrial cysts had positive ER staining within both the epithelium and the stroma. PR was also positive in both epithelial and stromal cells in 4 cases. Four cases had additional immunostains performed, which all showed cytokeratin 19 and cytokeratin 7 positivity (only in epithelium) and CD10 positivity (only in stroma). α-Inhibin and calretinin were negative for both the epithelium and the stroma in all 4 cases. All 6 MCN cases (mean size, 11.1 cm) had positive ER, PR, and α-inhibin staining only in the stroma. ER and PR were positive in both the epithelium and stromal cells in endometrial cysts, whereas they were positive only in the stromal cells of MCNs. The stromal cells were CD10 positive and α-inhibin negative in endometrial cysts as opposed to the opposite staining pattern in MCNs. Awareness of this distinct staining pattern and the possibility of endometrial cysts in the liver can lead to accurate diagnoses and appropriate treatment modalities.