Mixed epithelial and stromal tumor of the kidney with polypoid component extending into renal pelvis and ureter

Mixed epithelial and stromal tumor (MEST) of the kidney is an unusual benign neoplasm that predominantly occurs in middle-aged females. It typically appears as a well-circumscribed multiseptate mass with solid components on computed tomography (CT) or magnetic resonance image (MRI), reflecting its characteristics of an admixture of stromal proliferation and epithelial cells consisting of multiple cysts. We present a rare case of 61-year-old woman with MEST, which manifested as a multilocular cystic mass with a polypoid component protruding into the renal pelvis and ureter. To our best knowledge, this is the first case of MEST extending into the ureter.