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

Abstract

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.