RENAL MASS WITHIN A HORSESHOE KIDNEY: PREOPERATIVE EVALUATION WITH THREE-DIMENSIONAL HELICAL COMPUTED TOMOGRAPHY CHERYL T. LEE, SUSAN HILTON, AND PAUL RUSSO ABSTRACT Tumors arising within a horseshoe kidney are uncommon and may be difficult to characterize. We report a case of transitional cell carcinoma in a horseshoe kidney. Although several conventional imaging techniques were used to define the mass, each provided limited information. Recent advances in computer technology permit radiologic imaging to provide detailed examination of the horseshoe kidney with a single study. We describe the use of three-dimensional multislice helical computed tomography as the preferred modality to clarify the suspected neoplasm, vasculature, and collecting system of a horseshoe kidney. High-quality anatomic characterization will certainly enhance the opportunity for nephron-sparing surgery within this population. UROLOGY 57: 168vi–168ix, 2001. © 2001, Elsevier Science Inc. T he incidence of horseshoe kidney (HSK) is es- timated to be 1 to 4 in 1000 persons. 1 This renal fusion anomaly occurs between the fourth and eighth weeks of gestation when the ureteric bud has entered the renal blastema but before the kidneys have rotated on their long axis and begun their ascent. 2 The etiology of abnormal fusion may be due to the aberrant migration of posterior neph- rogenic cells to form a connection between the two developing kidneys, creating the horseshoe shape. 3 This type of fusion may create an isthmus contain- ing true renal parenchyma. Alternatively, the HSK may result from median fusion of the metanephric tissues during an early embryologic stage, produc- ing a fibrous isthmus. 4 The incidence of neoplasms in the HSK is esti- mated to be 1% to 12%, 5–7 although this likely rep- resents the upper limit, in view of the potential reporting and selection bias. Though renal cell car- cinoma is the most common type of neoplasm, ac- counting for 50% of cases, 5–7 it does not occur at a greater incidence in the HSK population than in the general population. In contrast, transitional cell carcinoma of the renal pelvis or upper collect- ing system, which accounts for 28% to 40% of cas- es, 5,8 does appear to have an increased incidence in patients with an HSK. 9 The traditional evaluation of an HSK mass has included intravenous urography to outline the excre- tory system, renal angiography to demonstrate arte- rial anomalies, traditional two-dimensional com- puted tomography (CT) to illustrate the location and extent of the mass, venography if specific abnormal- ities are suspected, and, often, renal ultrasound to further define any cystic components. This constella- tion of studies can be time consuming and expensive. Recent technological advances have allowed the pro- duction of high-quality three-dimensional (3D) im- ages using multidetector helical CT scanning and 3D imaging with display workstations. The multidetec- tor helical scanners use multiple rows of detector el- ements, rather than single rows as in traditional scan- ners. This imaging strategy, in combination with 3D technology, provides effective and accurate assess- ment of the renal parenchyma and of the venous, arterial, and excretory systems related to the kid- ney, 10 and avoids multiple radiologic procedures and exposures to intravascular contrast. Furthermore, in- formation from the conventional CT scan source im- ages and the 3D model is readily applied to the pa- tient undergoing nephron-sparing surgery. The following case demonstrates a role for 3D CT imaging in the preoperative evaluation of an HSK mass. S. Hilton is a member of the medical advisory board of the Vital Images Company, manufacturer of the three-dimensional work- station. From the Departments of Urology and Radiology, Memorial Sloan-Kettering Cancer Center, New York, New York Address for correspondence: Cheryl T. Lee, M.D., Section of Urology, University of Michigan Hospitals, 1500 East Medical Center Drive, Box 0330, Ann Arbor, MI 48109 Submitted: June 30, 2000, accepted: August 17, 2000 CASE REPORT © 2001, ELSEVIER SCIENCE INC. 0090-4295/01/$20.00 168vi ALL RIGHTS RESERVED PII S0090-4295(00)00857-8