A Case of Pediatric Mixed Epithelial and Stromal Tumor of the Kidney With Atypical Stromal Cells.

Mixed epithelial and stromal tumor of the kidney (MESTK) is a rare renal tumor characterized by both cystic and solid components. Although typically benign, its components can undergo malignant transformation, manifesting as a sarcomatous feature. Carcinomatous transformations are exceedingly rare. MESTK predominantly affects perimenopausal women, with male patients being rare and often associated with a history of hormonal therapy. Pediatric MESTK is extremely rare, with few reports. We herein report an eight-year-old child diagnosed with MESTK, exhibiting typical histological findings and featuring a few stromal cells with atypia in a focal area. These atypical cells exhibited bizarre nuclei and were positive for p53, although no mitotic figures were observed, and the Ki-67 labeling index was not elevated compared with the surrounding areas. The follow-up period was relatively short and there was no evidence of recurrence or metastasis. The patient remains under careful observation.