RENAL TRACT IMAGINGImaging of renal masses and staging of renal tumoursA J BRADLEY, MRCP, FRCR and Y Y LIM, FRCS, FRCRRadiology Department, University Hospital of South Manchester NHS Foundation Trust,Manchester, UKSummary Renal masses are a common, often incidental finding on cross-sectional imaging. Ultrasound can readily characterize a mass as solid or cystic. The Bosniak classification is based on CT but can equally well be applied to MRI orcontrast-enhanced ultrasound. CTisthemainmethodforstagingrenal cell carcinoma(RCC), withMRI usedinspecific indications, such as to assess the extent of venous involvement. MostsolidrenalmassesareassumedtobeRCC,althoughapproximately20%ofmasses under 3 cm will be benign. The seventh edition of the American Joint Committee on Cancer TNM staging forrenal cancer has been presented. An overview of the therapeutic options for renal cancer is discussed.doi: 10.1259/img.20110081 2014 The British Institute ofRadiologyCitethisarticleas:BradleyAJ,LimYY.Imagingofrenalmassesandstagingofrenaltumours.Imaging2014;23:20110081.Abstract. Renal masses are commonly encountered in adultradiological practice, with a range of appearances from entirelycystic to solid. Characterization of more complex cystic massesis a challenge, to which the now considerably refined Bosniakclassification has been applied. The Bosniak classification willbe discussed in detail, with the role of ultrasound, CT and MRdescribed. Dealing with the indeterminate masses presents anadditional challenge, which is greatly aided by dedicated CTand MR protocols. Benign tumours of the kidney includeangiomyolipomas and oncocytomas. Prior to surgery, it is notalways possible to confidently define a lesion as benign,although certain features, such as focal fat, are important.Radiology plays a vital role in imaging renal cell carcinoma(RCC). These tumours account for approximately 90% of allmalignant renal masses. The incidence of RCC is increasing,which, at least in part, is a consequence of the exponentialincrease in cross-sectional imaging. This has resulted in theidentification of many incidental tumours, which overall are ofa lower stage. The epidemiology, presentation and relevance ofthe various surgical options will be reviewed. The imagingmodalities applied to staging will be discussed, with anemphasis on multidetector CT. The TNM staging classificationin its most recently revised form is described, with discussionof the challenges this presents to cross-sectional imaging. Therole of ultrasound, CT, MR and positron emissiontomography-CT in imaging RCC will be discussed. The use ofimaging-guided needle biopsy and percutaneous ablation in themanagement of RCC is addressed. A brief mention is made ofother cases of malignant renal masses, including transitionalcell carcinoma and lymphoma.Massesof therenal parenchymacanbeclassifiedassimple cystic, complex cystic, solid or indeterminate.Benignor simple renal cysts are verycommon, witharoundhalf thepopulationoverage50havingsimplerenal cysts.1,2Prevalence increases with age, from,10%under40yearsto.60%over80years,and theyaresig-nificantly more common in males than females.1Theprevalenceof renal cancer alsoincreases withage, and1015% of renal cancers are cystic in nature.2,3Use of the Bosniak classification allows stratification ofcysticlesionsintothosethatcanbeignored, thosethatneed to be followed up and those that require urologicalreferral forconsiderationofremoval.4,5Forall solidtu-mours, approximately 50%are detected incidentally,usuallyat asmaller size andinanearlier stage thansymptomatic tumours.68When considering masses ,4 cm,6070%are nowdetected incidentally.911Increasedincidental detection of small tumours has shown a rise inthe incidence of renal cancer, and consequently a rise inboththenumberoftreatedcancersand5-yearsurvivalrate.12However, approximately 20% of small renalmasses are benign,13,14which complicates optimum man-agementofsuchlesions. IntheUK, all presumedrenalcancers are discussed at a multidisciplinary team meetingwith input from urologists, radiologists, histopathologistsand oncologists to optimize treatment decisions.Address correspondenceto: Dr AlisonJ. Bradley. E-mail: alison.bradley@uhsm.nhs.ukImaging, 23 (2014), 20110081birpublications.org 1 of 12Alarger, solidlesionarisingfromthe renal paren-chyma almost always represents renal cell carcinoma(RCC) and should be staged appropriately. The AmericanJointCommitteeonCancerstagingofrenal cancerwasrevised in the seventh edition in 2010, to take into accountthe prognostic features available from studies publishedsince the sixth edition in 2002.15Cystic renal masses and theBosniak classificationAccurate characterization of cystic renal masses deter-minesthemanagement of suchlesionsanduseof theBosniak classification enables this (Table 1).4,5It was de-velopedforCTbut canequallybeappliedtoMRI, orultrasoundtoo, providedthat contrast enhancement isused.1618TheBosniakclassificationisbasedonthefol-lowingfeaturesasseenonCT: fluiddensity, wall andseptal thickness, enhancement, calcificationandthepo-sition of the cystic mass within the kidney. An up-to-dateversion is given in Table 1.UltrasoundUltrasoundisgoodfordifferentiatingbetweencysticand solid lesions, the hallmark of the cystic lesion beingan anechoic structure demonstrating strong posterioracoustic enhancement. Provided that such a lesion has nointernal contents, anda sharplymarginatedposteriorwall, it can be categorized as Bosniak I or simple in na-ture.8Althoughposterioracousticenhancementisocca-sionally seen in solid lesions too, these will haveechogenic contents distinguishing themfromcysts.Harmonic imaging may be helpful in defining cysticcomponents, particularly in larger patients.19,20TheBosniakcategoryIIcystisslightlycomplexwithone or two thin septa. The finding of more complex fea-tures on ultrasound, such as nodularity, calcification,multipleseptationsandseptal orwall thickening, war-rants a CT (or MRI) scan for a more accurate assessmentofthelesion(Figure1). RoutineuseofcolourorpowerDopplerforcomplexcystscanhelptocategorizecom-plexity,21suchthat anylesionshowingflowwithinin-ternal septa must be at least category III. The absence offlow, however, cannot betakenasevidenceofalowercategory.Use ofcontrastfurther clarifies,withaccuracyrates approaching those of CT.17,18CT imaging techniqueCT scanning plays the key role in the accurate assess-mentofcomplexcysticorsolidlesions. Toevaluateen-hancementwithinalesion, anunenhancedscanshouldbeperformedfollowedbyascaninthenephrographicphase(90120 sdelay). Thenephrographicphaseistheoptimum phase to characterize renal masses.22As thereis maximal andhomogeneous renal parenchymal en-hancement, this allows detection of renal masses, whichnormally do not enhance to the same degree.23The meanattenuation value of the lesion should be obtained on theunenhanced and post-contrast scan. Measurement shouldbe made on the thickest part of the septa or walls, usingthesmallest possibleregionof interest (ROI) cursor. Alesion is considered to be enhancing if there is an increaseof >20 HU between the pre- and post-contrast scans. Anincrease of 1020 HU should be regarded as in-determinate, and further imaging is required for accurateevaluation.24For image review, multiplanar reconstruction (MPR) isimportant to fully assess these lesions. Evaluation of thevolumedataset, ratherthanjust theaxial images, hasbeen shown to reduce interobserver variation and resultinassignment of different Bosniakclassifications, com-pared with using axial images alone.25For optimal MPR,thinslicesof