IMAGING TEACHING CASE Extensive Infiltrating Renal Cell Carcinoma With Minimal Distortion of the Renal Anatomy Mimicking Benign Renal Vein Thrombosis Elizabeth M. Hecht, MD, 1 Nicole Hindman, MD, 1 William C. Huang, MD, 2 Andrew B. Rosenkrantz, MD, 1 and Jonathan Melamed, MD 3 INDEX WORDS: Renal cell carcinoma; renal vein thrombosis; computed tomography; magnetic resonance imaging. INTRODUCTION Malignancies of the kidney can present with an infiltrative appearance and may include lym- phoma; metastatic disease; epithelial tumors, such as invasive transitional cell carcinoma; medul- lary carcinoma; renal sarcoma; and occasionally, aggressive renal cell carcinoma (RCC). 1,2 Al- though an infiltrative appearance is not common for RCC, it can occur in up to 6% of cases. 2 These tumors do not present as discrete expan- sile masses, but instead show an infiltrative pat- tern of growth preserving the overall size and contour of the kidney. The normal internal archi- tecture of the kidney is replaced and obliterated by tumor. This infiltrative appearance can even mimic benign infectious and inflammatory pro- cesses, such as bacterial and xanthogranuloma- tous pyelonephritis 2 or benign renal vein throm- bosis. In this case study, we discuss and compare the imaging features of infiltrative renal tumors and renal vein obstruction and discuss the potential pitfalls of diagnosis. CASE REPORT Clinical History and Initial Laboratory Data A 45-year-old man with no significant medical or surgical history presented to the emergency department with flank pain for 2 weeks and fever. Initial laboratory data, including a basic metabolic panel, complete blood cell count, liver enzymes, and lipase, were normal, including serum urea nitrogen level of 18 mg/dL (6.4 mmol/L) and serum creati- nine level of 1.3 mg/dL (114.9 mol/L; estimated glomeru- lar filtration rate, 63 mL/min/1.73 m 2 [1.1 mL/s/1.73 m 2 ]). Prothrombin time was mildly increased to 13.1 seconds (reference range, 9.7-12.8 seconds), but international normal- ized ratio and partial thromboplastin time were normal. There was an increased erythrocyte sedimentation rate of 61 mm/h (reference range, 0-10 mm/h) and C-reactive protein level of 109 mg/L (reference, 10 mg/L). Urinalysis was remarkable for high specific gravity 1.030 g/mL (refer- ence, 1.030 g/mL), and there were trace proteins in urine. Physical examination findings were unremarkable, except for temperature to 99.8°F. Urinalysis results were unremark- able. On the basis of the computed tomographic (CT) findings described next, a hematologic workup was per- formed to determine whether the patient had an underlying coagulopathy. Coagulation profile showed factor VIII level of 170 IU/dL (normal, 65-150 IU/dL), but factor V, prothrom- bin, protein C and S, antithrombin III, anticardiolipin antibod- ies, dilute Russel viper venom screen, and activated protein C resistance assay results were normal. Based on CT find- ings and laboratory data, the diagnosis of a benign renal vein thrombosis was believed to be most likely. The patient was treated with anticoagulation therapy and discharged to home 2 days later. Two weeks later, the patient experienced wors- ening abdominal pain and was readmitted. Imaging Studies Initial contrast-enhanced CT scan showed a thrombus in the right renal vein that protruded slightly into the inferior vena cava (IVC). The right kidney was enlarged, with a delayed nephrogram and perinephric fat stranding without evidence of discrete renal mass and relatively normal corti- comedullary differentiation characteristic of acute renal vein obstruction (Fig 1A and B). CT scan repeated 2 weeks later showed no change in appearance of the kidney and renal vein despite anticoagulation. Magnetic resonance imaging (MRI) was performed 1 day later to exclude underlying malignancy. On MRI, no internal enhancement was detected in the thrombus, favoring a nontumoral thrombus, although there was minimal disruption of the corticomedullary nephro- gram anteriorly (Fig 1C and D). The CT and MRI findings of preserved corticomedullary differentiation with an extensive nonenhancing thrombus supported a diagnosis of a benign renal vein thrombosis. However, in the absence of a defini- tive underlying cause and because there was no evidence of From the Departments of 1 Radiology, 2 Urology, and 3 Pathology, New York University Langone Medical Center, New York, NY. Received May 15, 2009. Accepted in revised form Septem- ber 3, 2009. Originally published online as doi:10.1053/j. ajkd.2009.09.030 on December 7, 2009. Address correspondence to Elizabeth M. Hecht, MD, Department of Radiology, University of Pennsylvania Health System, 3400 Spruce St, Philadelphia, PA 19104. E-mail: elizabeth.hecht@uphs.upenn.edu © 2010 by the National Kidney Foundation, Inc. 0272-6386/10/5505-0022$36.00/0 doi:10.1053/j.ajkd.2009.09.030 American Journal of Kidney Diseases, Vol 55, No 5 (May), 2010: pp 967-971 967