Objective:To investigate the CT features and surgical treatment of mixed epithelial and stromal tumor of the kidney (MESTK).Methods:From April 2015 to August 2018, 13 patients with MESTK confirmed by pathology at the First Affiliated Hospital, College of Medicine, Zhejiang University, were enrolled in this study and their clinical data were analyzed retrospectively. These patients included 3 males and 10 females, with age ranging from 22 to 80 years old and the median age of 37 years old. Two patients complained of lumbar discomfort with urinary urgency and another 2 patients presented with hematuria. Nine patients were asymptomatic and their renal lesions were detected by abdominal ultrasonography for physical examination or other reasons. There were 2 cases had a history of polycystic kidney disease, 1 patient took contraceptive for 2 years, and 3 cases took antihypertensive agents for 5-10 years. The other patients denied any administration of hormone or other medicine for long time. The urine routine test was normal for these patients except for 2 patients did not take this test before operation. The creatinine value ranged from 45 to 486 μmol/L, with the median value of 67 μmol/L. The scope of glomerular filtration rate (GFR) was 6.98 to 132.28 ml/min, with the median value of 109.28 ml/min. The preoperative computer tomography (CT) showed single neoplasm in 11 patients and no obvious neoplasm was found in 2 patients who had polycystic kidney disease. Moreover, these lesions presented cystic neoplasm for 8 cases, cystic-solid neoplasm for 2 cases and solid neoplasm for 3 cases. Long diameter was measured about 2.5 to 7.5 cm, with the median of 5 cm, and calcification was found in 4 cases. These tumors manifested mild to moderate enhancement and delayed-enhancement for tumor parenchyma and intracapsular septum. The CT attenuation value was from 20 to 55 Hounsfield unit (HU). Base on the R. E.N.A.L. score system, these renal lesions got 4 to 10 points, with the median value of 7 points. Before operation, just 1 case was diagnosed as angiomyolipoma and 2 cases were diagnosed as polycystic kidney disease. No definitive diagnosis was made for the other 10 cases by CT imaging. In addition, the tumor staging based on preoperative imaging was made at T1a for 8 cases and T1b for 3 cases. Among these cases, 2 patients with polycystic kidney disease underwent open radical nephrectomy due to recurrent hematuria and uremia. One patient underwent laparoscopic radical nephrectomy due to perirenal fat adhesion. Partial nephrectomy was performed in remaining 10 patients by open surgery for 6 patients, robotic surgery for 3 patients and laparoscopic surgery for 1 patient. These renal tumors and 0.5-1.0 cm surrounding normal renal parenchyma were removed during operation.Results:In this study, 8 patients had blood loss of 30 to 100 ml during open operation, and 5 patients had blood loss of 20 to 100 ml during laparoscopic or robotic surgery. The warm ischemic time during partial nephrectomy was around 17 to 40 min, with the median value of 20 min. All of 13 cases were diagnosed as MESTK by postoperative pathology and the surgical margin was negative. There were no serious complications or special treatment after operation. The follow-up time ranged from 12 to 41 months, with the median time of 21 months. Obvious signs of tumor recurrence or metastasis were not detected in 13 cases during follow-up.Conclusions:CT images of MESTK mostly presented cystic or cystic-solid lesions, and solid lesions were relatively rare. This disease always showed mild to moderate enhancement and delayed enhancement of septal or tumor parenchyma. If there is coarse calcification in the lesions, it is more likely to be diagnosed as MESTK. Partial nephrectomy is preferred and radical nephrectomy should be considered when perirenal fat adhesion is suggested by the preoperative CT images.