Mixed epithelial and stromal tumor of the kidney: a case series of clinical diversity in atypical presentations

Background: This study explores the clinicopathological features, diagnostic criteria, treatments, and prognosis of mixed epithelial and stromal tumor of the kidney (MESTK), with a focus on atypical cases (males and young females)-filling the gap in the perception that "only perimenopausal women are susceptible" in MESTK epidemiological data. Case Description: A retrospective analysis of 4 MESTK patients (admitted between January 2019 and December 2024) was conducted, including epidemiological data, imaging findings, pathology, immunohistochemistry (IHC), treatments, and follow-up outcomes. This analysis was combined with a literature review to summarize diagnostic and treatment patterns. Among the 4 patients, 3 were male (21, 29, and 54 years old) and 1 was female (22 years old). All were asymptomatic except 1, who reported occasional low back pain; the tumors were detected incidentally in all cases. Imaging showed cystic or cystic-solid masses (3 classified as Bosniak IIF; 1 with fat density). Pathologically, all tumors exhibited biphasic epithelial-stromal differentiation. Immunohistochemically, epithelial components expressed paired box gene 8 (Pax-8) and cytokeratin 7 (CK7); stromal components expressed estrogen receptor (ER) and progesterone receptor (PR). All patients underwent robot-assisted laparoscopic partial nephrectomy; postoperatively, physician assessment showed no significant decline in renal function, and patients reported no discomfort such as low back pain, with uneventful follow-ups. Conclusions: MESTK presents diverse clinical features, and male or young patients are not rare. Diagnosis requires integration of non-specific imaging findings with pathological and immunohistochemical examination (positive biphasic markers). Robot-assisted partial nephrectomy is effective, and benign cases have a good prognosis. This study enriches MESTK clinical data, aiding in the differentiation of atypical cases and providing clinical evidence for standardized diagnosis and minimally invasive treatment of such cases.