Malignant Cells in Urine Cytology: Not Always High-Grade Urothelial Carcinoma

Abstract The main purpose of urine cytology is the detection of high-grade urothelial carcinoma (HGUC). When malignant cells of nonurothelial origin are present, it is often misdiagnosed as HGUC due to the rarity of this event and difficulty in distinguishing subtle cytomorphologic differences. To arrive at the appropriate diagnosis, evaluation of urine cytology specimens requires correlation with clinical and radiologic findings and utilization of cytologic-histologic correlation. We present the case of a 64-year-old woman with gross hematuria. Imaging located an infiltrative mass involving the left renal pelvis, highly suggestive of urothelial carcinoma. Lymphadenopathy and multiple bony lesions in the spine were concerning for metastatic disease. Urine cytology specimens taken during cystoscopy contained malignant cells, which were interpreted as HGUC. The workup of a subsequent lymph node biopsy showed a metastatic high grade carcinoma of renal origin with SMARCB1 loss. This finding led to the reexamination of the urine cytology specimens, revealing the malignant cells were not HGUC but rather renal carcinoma. This case highlights a potential pitfall in the evaluation of urine cytology and the importance of cytologic-histologic correlation.