Figure 13a. Venous hemangioma of the renal sinus in a 33-year-old man with gross hematuria and left flank pain. (a) Axial unenhanced CT scan shows a well-defined high-attenuation lesion (arrows) adjacent to the left renal pelvis. (b) Axial contrast-enhanced CT scan shows that the mass (arrows) has low attenuation and poor enhancement. Because of the possibility of malignancy, left nephrectomy was performed. At pathologic examination, the lesion was composed of multiple vascular channels of variable sizes beneath the pelvic mucosa. Renal hemangioma is seldom diagnosed preoperatively but should be included in the differential diagnosis when CT demonstrates poor enhancement of a renal mass located at the pelvicaliceal junction or in the inner medulla. (Reprinted, with permission, from reference 22.)