Endometriosis of sigmoid colon mimicking malignant tumor diagnosed by intraoperative imprint cytology

A case of endometriosis of the sigmoid colon on imprint cytology from an intraoperative biopsy is discussed. Cytologic specimens showed sheets or tubular epithelial clusters and stromal fragments. The epithelial cell nuclei were small and round to ovoid with finely granular chromatin and inconspicuous nucleoli. The background showed a few scattered spindle-type stromal cells without pigment-laden histiocytes. A definitive diagnosis of endometriosis can be based on cytology, provided that the cytologic findings are interpreted in the appropriate clinical context.