Endometriosis in Peritoneal Washings–A Potential Diagnostic Pitfall in a Patient with Known Endometrial Adenocarcinoma

Endometriosis is characterized by the presence of ectopic endometrial tissue outside the uterus, and is an uncommon benign finding in peritoneal washing cytology. Diagnosis of this process is challenging, especially in the context of a known endometrial malignancy, where the presence or absence of peritoneal involvement is believed to have prognostic implications. In such cases, it is important to accurately distinguish between endometriosis and welldifferentiated metastatic malignancy. We discuss the case of a 68-year old woman who underwent staging surgery, including peritoneal washing cytology, for biopsy-proven endometrioid adenocarcinoma of the endometrium. Peritoneal washing cytology showed hallmark features of endometriosis, namely occasional ball-like clusters lined by columnar cells and containing central endometrial stromal-like cells, on a background of blood and haemosiderophages. Positive CD10 immunocytochemistry confirmed the presence of endometrial stroma. These findings, when taken together with cytological features and lack of significant cytologic atypia, further supported a diagnosis of endometriosis. Review of histopathology slides from the THBSO specimen confirmed the presence of ovarian and cervical endometriosis. In conclusion, distinguishing between endometriosis and endometrial adenocarcinoma in peritoneal washing cytology requires a combination of identification of salient cytological features and correlation with clinical information and histopathological findings.