Pitfalls in Urine Cytology: A Case of Fumarate Hydratase‐Deficient Renal Cell Carcinoma Initially Diagnosed as High‐Grade Urothelial Carcinoma

Fumarate hydratase (FH)-deficient renal cell carcinoma (RCC) is a rare, aggressive, hereditary subtype of renal cancer that requires careful diagnostic considerations. We report a case of a 33-year-old Asian woman who presented with a 20-day history of hematuria. Imaging studies revealed a solid tumor in the lower pole of the right kidney with lymph node metastases. Urinary cytology revealed benign squamous cells, inflammatory cells, and atypical epithelial cells, suggestive of high-grade urothelial carcinoma. Following a right nephrectomy, the tumor displayed papillary structures composed of cells exhibiting atypical, elongated nuclei with eosinophilic nucleoli and peripheral halos. Immunohistochemical staining demonstrated negative FH expression. Genetic analysis identified a somatic missense mutation in the FH gene, confirming the diagnosis of FH-deficient RCC. This case highlights the importance of integrating cytological, histological, and genetic analyses for accurate diagnosis of FH-deficient RCC.