A 22-year-old woman with a left perirenal tailgut cyst.A. Plain radiograph shows a large radiopaque mass in the left abdomen (arrows).B. Contrast-enhanced CT at the level of the kidneys depicts a homogeneous unilocular cystic mass anterior to and compressing the left kidney, without invasion of surrounding structures.C. CT scan at the level of aortic bifurcation shows that the left ureter is displaced by the mass (arrow). Note the presence of a well-enhanced cystic rim (arrowheads).D. The lower margin of the mass extends to the level of bifurcation of the left common iliac artery.E. Multiplanar reformation (MPR) image shows the extent of the mass.F. Photomicrograph shows papillary fronds of glandular epithelium with a ciliated border lining the cystic wall, which contains arrays of smooth muscle (original magnification, ×40; hematoxylin-eosin staining).G. Photomicrograph of higher magnification than that of the cystic wall depicts benign glandular epithelium with a ciliated border (original magnification, ×400; hematoxylin-eosin staining).