Tohru Nakagawa

Introduction Retroperitoneal tumors account for 0.2% of all neoplasms. Among these tumors, retroperitoneal vascular malformations are particularly rare, with most previously reported cases being venous malformations. Case presentation A 72‐year‐old woman was diagnosed with a retroperitoneal tumor on abdominal computed tomography. The 27‐mm diameter tumor was located away from the right kidney and major vessels in the right perirenal adipose tissue. Contrast‐enhanced computed tomography revealed a heterogeneously enhanced tumor with well‐defined borders. Dynamic contrast‐enhanced magnetic resonance imaging revealed rapid enhancement in the arterial phase and a progressive filling‐in pattern in the delayed phase. Although vascular malformation was suspected, a definitive diagnosis could not be established. The retroperitoneal tumor was excised laparoscopically for therapeutic and diagnostic purposes, and the histopathological diagnosis confirmed it as a capillary arteriovenous malformation. Conclusion Herein, we presented a rare case of retroperitoneal capillary arteriovenous malformation that was difficult to definitively diagnose preoperatively.