Clear Cell Renal Cell Carcinoma in a 60-Year-Old Man Arising From Congenital Left-Sided Ectopic Right Kidney (Crossed Fused Renal Ectopia) With Invasion Into the Contralateral Kidney.

BACKGROUND Crossed fused renal ectopia (CFRE) is an uncommon congenital renal anomaly in which 1 kidney crosses the midline and fuses with the contralateral kidney. Although CFRE is often asymptomatic, its rare association with renal malignancy poses substantial diagnostic and surgical challenges due to abnormal anatomy and vascular supply. This report describes clear cell renal cell carcinoma (RCC) arising in CFRE with direct invasion into the contralateral, normally positioned kidney. CASE REPORT A 60-year-old man presented with a 2-month history of mild left loin pain without urinary or systemic symptoms. Initial laboratory findings were unremarkable. Imaging studies, including contrast-enhanced computed tomography, demonstrated a congenitally ectopic right kidney fused to the left kidney, consistent with CFRE, and a large heterogeneously enhancing mass replacing most of the ectopic kidney and extending into the lower pole of the normally located left kidney. Positron emission tomography showed no evidence of distant metastasis. The patient underwent radical nephrectomy of the ectopic right kidney combined with partial nephrectomy of the invaded segment of the left kidney, with preservation of renal perfusion. Histopathologic examination confirmed clear cell RCC, World Health Organization/International Society of Urological Pathology grade 3, staged as pT3a, with negative surgical margins. The postoperative course was uneventful, and renal function was preserved. CONCLUSIONS This is the first documented report of RCC arising in CFRE with invasion into the contralateral normal kidney. It emphasizes the importance of detailed preoperative imaging, individualized surgical planning, and nephron-sparing strategies in the management of complex renal anomalies with malignancy.