Extragenital endometrial stromal sarcoma of transverse mesocolon: A diagnostic conundrum

Endometrial stromal sarcoma (ESS) is a rare uterine neoplasm infrequently arising in extra-genital sites. Herein, we report an extremely rare case of primary extra-genital ESS of transverse mesocolon occurring in a 51-year-old female presenting with gradually increasing abdominal mass. The clinical diagnosis considered was a gastrointestinal stromal tumor. Intra-operatively, the mass was confined exclusively to the transverse mesocolon. Microscopy revealed a cellular tumor composed of oval to elongate neoplastic cells with hyperchromatic nuclei, inconspicuous nucleoli and were immunoreactive for CD10, progesterone receptor (PR), estrogen receptor (ER), and PAX8; negative for KIT, CD34, SMA, S100, synaptophysin, chromogranin, WT1, and calretinin. A distinct arborizing network of arterioles along with foci of endometriosis was also seen. We present this case for its extreme rarity and the challenges entailed in its diagnosis.