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Mixed epithelial and stromal tumor of the kidney (MESTK) is a rare renal tumor for which there is limited clinical case experience. The aim of this study was to investigate the clinicopathological features, imaging manifestations, immunohistochemical phenotype, diagnosis, and differential diagnosis of MESTK. We systematically evaluated the clinicopathological characteristics of 13 patients diagnosed with MESTK from 2016 to 2022. There were 12 female cases and 1 male case, and their age ranged from 24 to 62 (mean age 47 years). The tumor was located in the left kidney in 11 patients. Seven of these patients were located in the upper pole of the left kidney. The mean diameter of the tumor was 4.6 cm (range 1.3–8.6 cm), and the main body of the tumor was mainly located in the medulla or medulla. In 7 cases, the tumors were clearly outlined, 4 cases had clear but irregular borders, and in 3 cases, the masses broke through the renal capsule but did not invade the surrounding organs. All tumors had varying proportions of cystic and solid components. In most tumors, the epithelial cell component was predominant. Among the epithelial components, we observed a predominance of large cysts. Small cysts and tubular structures were also seen; the least common were papillary structures. The most common types of lining epithelium were flat and cuboidal. The cell cytoplasm was predominantly eosinophilic with hyaline degeneration. The tumor cells were composed of different proportions of sparse hypocellular areas and dense hypercellular areas between these cells. In most cases, the cellular stroma had an ovarian-like appearance. It is characterized histologically by mature adipocytes, thick-walled blood vessels, and chronic inflammatory cells. Calcification and collagenization were seen in a few cases. In all female patients, estrogen and progesterone receptors in the interstitial component were almost always positively expressed. In male patients, all interstitial components showed locally positive expression of androgens. MESTK had unique characteristics with complex and variable lesion shapes. There was a high degree of overlap with cystic kidney cancer, and the rate of missed and misdiagnosis was extremely high. The diagnosis could not be confirmed by preoperative imaging. The final diagnosis depended on pathomorphology. • Mixed epithelial and stromal tumor of the kidney (MESTK) is a rare renal tumor for which there is limited clinical case experience. • The incidence of MESTK is low compared to other renal tumors. The relatively limited knowledge of MESTK and its uncommon incidence has led to a high rate of misdiagnosis and underdiagnosis of this tumor. • So far, there is no large sample of data available for clinician pathologists. [ABSTRACT FROM AUTHOR]

To explore the clinicopathological characteristics, immunohistochemical phenotype, diagnosis and differential diagnosis of renal cell carcinoma associated with Xp11.2 translocation/TFE3 gene fusion. The clinical history, pathological morphology, immunohistochemical phenotype and related molecular test results of 13 cases of Xp11.2 translocation/TFE3 gene fusion-related renal cell carcinoma were retrospectively analyzed, and the relevant literature was reviewed. Of the 13 patients, 5 were males and 8 were females. The age of onset ranges from 8 to 73 years old, most of which were middle-aged and elderly patients. Among them, there were 3 cases of left kidney tumor and 10 cases of right kidney tumor. In the treatment method, 2 of the 13 patients underwent partial nephrectomy, and 11 underwent radical nephrectomy. Histopathological morphology showed papillary, nested, tubular and acinar structures. The cytoplasm was transparent or eosinophilic, and the interstitial fibrosis was accompanied by chronic inflammatory cell infiltration, hemosiderin deposition and foam cell aggregation. The immunohistochemical analysis of 13 patient specimens all expressed TFE3 antibody, and the expression intensity was strongly positive; gene FISH detection technology revealed the breakage and rearrangement of TFE3 gene in 12 assessable cases. One of thirteen patients had a metastasis at follow-up from 3 to 69 months. This type of kidney cancer was a rare subtype. Because of its complex and changeable shape, it has a high degree of overlap with other kidney cancer subtypes, and the missed diagnosis rate and misdiagnosis rate are extremely high. The diagnosis is mainly based on pathomorphology and immunohistochemistry, TFE3 positive expression and TFE3 gene destruction and rearrangement. • The diagnosis of renal cell carcinoma is mainly based on pathological morphology, immunohistochemical TFE3 strong positive expression and TFE3 gene disruption and rearrangement. • Due to the high cost of TFE3 gene testing, due to various factors, the diagnosis of this tumor in many domestic hospitals is limited to the level of immunohistochemical detection, and the 13 cases of tumors reported in this report are all genetically tested at the molecular level. • This study found that when the immunohistochemical TFE3 is more than 80% strong, it can replace FISH to a certain extent. [ABSTRACT FROM AUTHOR]