Small (£4 cm) cortical renal tumors: characterization with multidetector CT Ghazi Alshumrani, 1 Martin O’Malley, 1 Sangeet Ghai, 1 Ur Metser, 1 John Kachura, 1 Antonio Finelli, 2 Kamal Mattar, 2 Tony Panzarella 3 1 Department of Medical Imaging, University of Toronto, Toronto, Canada 2 Department of Urology, University of Toronto, Toronto, Canada 3 Department of Biostatistics, University of Toronto, Toronto, Canada Abstract Background: To determine if the pathology of small (£4 cm) solid renal tumors can be predicted from findings on multidetector CT. Methods: This retrospective study included 46 patients (median age, 60 years; range, 32–91 years; 27 males, 19 females) with 47 tumors who underwent triphasic renal CT with pathology correlation. Two radiologists reviewed CT studies blinded to pathology results and recorded the morphologic and enhancement features of the tumors. Results: The 47 tumors (median diameter, 2.5 cm; range, 0.6–4.0 cm) included: 26 (55%) clear cell renal cell carcinomas; 9 (19%) oncocytomas; 7 (15%) papillary renal cell carcinomas; 2 (4%) chromophobe renal cell carcinomas; 2 (4%) inflammatory pseudotumors; and 1 (2%) angiomyolipoma with minimal fat. Amongst the three commonest tumors, heterogeneity was seen in 23/ 26 (88%) clear cell renal cell carcinomas, 6/9 (67%) oncocytomas, and 2/7 (29%) papillary renal cell cancer. Median (minimum–maximum) absolute nephrographic phase enhancement (nephrographic minus unenhanced phase) was: clear cell renal cell carcinomas 65 HU (34– 120), oncocytomas 80 HU (51–111), and papillary renal cell carcinomas 16 HU (7–32). Conclusion: Absolute nephrographic phase enhancement of £32 HU distinguished papillary renal cell carcinomas from clear cell renal cell carcinomas and oncocytomas. Key words: Renal—CT—Characterization—Tumor Approximately 50% of all renal cell cancers (RCCs) are discovered as an incidental finding on a cross-sectional imaging study [1]. The management of a suspected RCC is evolving with options including open or laparoscopic radical or partial nephrectomy, percutaneous image- guided ablative procedures such as radiofrequency ablation, biopsy of the mass for further evaluation, or imaging surveillance [2, 3]. The size of the renal mass, patient factors such as age and associated medical con- ditions, and local expertise will influence management decisions. The size of an incidentally detected renal mass, suspected to be RCC, is of particular importance. In one large study of solid renal masses that were surgically removed, smaller masses were more likely to be benign or of a lower nuclear grade than larger masses [4]. Although most solid, enhancing renal masses represent RCCs, the differential diagnosis of a solid, enhancing renal mass without gross fat on CT includes malignant (e.g., RCC) and benign (e.g., oncocytoma and angiomyolipomas with minimal fat) tumors. Several studies have evaluated CT for characterization of renal tumors but to our knowledge, none have focused exclu- sively on small (£4 cm), solid, enhancing renal masses using multidetector CT. The purpose of this study was to determine if mor- phologic features and/or enhancement patterns using a triphasic renal CT protocol could predict the pathologic diagnosis of solid, enhancing renal masses £4 cm. This size was chosen since it corresponds to stage T1a using the TNM staging system for RCC [5]. Materials and methods This retrospective study was approved by our Research Ethics Board. Informed consent was not required. An electronic search was performed of patients who had a radical or partial nephrectomy or percutaneous biopsy of Correspondence to: Martin O’Malley; email: martin.o’malley@uhn.on.ca ª Springer Science+Business Media, LLC 2009 Published online: 18 June 2009 Abdominal Imaging Abdom Imaging (2010) 35:488–493 DOI: 10.1007/s00261-009-9546-3