Mixed epithelial and stromal tumor in combination with oncocytoma of the kidney: a rare clinical case and review of the literature.

Introduction. Mixed epithelial and stromal tumors (MEST) of the kidney are rare neoplasms and, along with adult cystic nephroma, are grouped together called mixed epithelial and stromal tumors (World Health Organization Classification of Tumors of the Kidney, 2022). Clinically and radiologically, they mimic other cystic renal neoplasms, thus definitive diagnosis requires histological and immunohistochemical examination. Despite the benign course, MEST can undergo malignancy; isolated cases of recurrence have also been documented. Clinical case. Patient X, 65 years old, underwent robot-assisted left-side partial nephrectomy for cystic formations of the left kidney with a hypervascular soft tissue component along the medial semicircle of one of the cysts. According to histological examination, MEST of the left kidney in combination with oncocytoma was diagnosed. Conclusions. This article presents probably the first published case of MEST in combination with oncocytoma of the kidney.