a 69-year - old female visited the nephrology clinic owing to microscopic hematuria detected by a routine health check - up . a ct scan revealed a 3.33.0 cm slightly highly attenuated cyst in the lower pole of the right kidney on the precontrast image , which had no enhancement ( fig . the cyst was compatible with a bosniak category ii renal cyst . however , some portions of the renal cyst were enhanced on the gadolinium - enhanced t1-weighted mr image ( fig . 1c ) . under the impression of renal cell carcinoma ( rcc ) , partial nephrectomy was performed . the gross specimen was a yellowish solid mass that microscopically was compatible with papillary type rcc , fuhrmann nuclear grade iii ( fig . a 42-year - old female visited the nephrology clinic because of an incidental renal cyst detected by ultrasonography . the cyst showed internal calcification and amorphous echogenic portions without definite solid mass or increased vascularity ( fig . 3 ) . a ct scan demonstrated a 3.73.57.2 cm ovoid - shaped cystic lesion medially located in the mid portion of the right kidney , which showed calcifications that were not contrast - enhancing ( fig . 4a and 4b ) . on the basis of these findings , the patient was diagnosed with a bosniak category ii renal cyst . however , t1-weighted gadolinium - enhanced mr imaging demonstrated an enhancing portion in the cyst compatible with bosniak category iv ( fig . radical nephrectomy was then performed and the pathologic examination revealed a yellowish solid mass with mucinous material that was microscopically compatible with mucinous adenocarcinoma ( fig . 5 ) . at present , the patient has remained stable for 3 years postoperatively . a 69-year - old female visited the nephrology clinic owing to microscopic hematuria detected by a routine health check - up . a ct scan revealed a 3.33.0 cm slightly highly attenuated cyst in the lower pole of the right kidney on the precontrast image , which had no enhancement ( fig . the cyst was compatible with a bosniak category ii renal cyst . however , some portions of the renal cyst were enhanced on the gadolinium - enhanced t1-weighted mr image ( fig . 1c ) . under the impression of renal cell carcinoma ( rcc ) , partial nephrectomy was performed . the gross specimen was a yellowish solid mass that microscopically was compatible with papillary type rcc , fuhrmann nuclear grade iii ( fig . a 42-year - old female visited the nephrology clinic because of an incidental renal cyst detected by ultrasonography . the cyst showed internal calcification and amorphous echogenic portions without definite solid mass or increased vascularity ( fig . 3 ) . a ct scan demonstrated a 3.73.57.2 cm ovoid - shaped cystic lesion medially located in the mid portion of the right kidney , which showed calcifications that were not contrast - enhancing ( fig . 4a and 4b ) . on the basis of these findings , the patient was diagnosed with a bosniak category ii renal cyst . however , t1-weighted gadolinium - enhanced mr imaging demonstrated an enhancing portion in the cyst compatible with bosniak category iv ( fig . radical nephrectomy was then performed and the pathologic examination revealed a yellowish solid mass with mucinous material that was microscopically compatible with mucinous adenocarcinoma ( fig . the bosniak classification for cystic renal masses was first introduced in 1986 , and many urologists and radiologists use the classification as a means of assessing and managing cystic renal disease , with general agreement between observers in most cases [ 1 - 3 ] . during the past two decades , the bosniak classification has been updated and refined . although the bosniak classification was developed on the basis of ct findings , it is also commonly applied with the use of other imaging modalities . mr imaging is useful for characterizing complex cystic renal masses because the vascularity ( enhancement ) of a lesion can be evaluated , and it has been shown that the bosniak classification can be applied to mr imaging . it should be noted that mr imaging has better contrast resolution compared with ct , and occasionally there will be findings on mr imaging scans that are not correlative with ct images . israel et al .   compared the findings of both ct and mr imaging in the evaluation of cystic renal masses by using the bosniak classification system . in their series , the enhancement characteristics during ct and mr imaging differed in 2 ( 3% ) of the 69 lesions . the first was a cyst that contained a mural nodule , which was enhanced by 13 hu on ct after intravenous contrast material administration . although it was believed during ct that this lesion represented a hypovascular neoplasm , mr imaging showed definitive enhancement in the mural nodule , a finding that supported a diagnosis of malignancy . the second lesion was characterized as category iif by ct and had a thick calcified wall . however , on mr imaging , the wall of the lesion appeared enhanced , thicker , and more irregular , and the lesion was characterized as category iii . placing a lesion into a specific category can be difficult in some cases , in particular , differentiating between category iif and iii . our cases displayed a renal cystic mass that showed a category change from category ii on the basis of ct imaging to category iv after further mr imaging evaluation . in both cases , the masses one factor that has not changed since the inception of the bosniak classification system is the importance of lesion enhancement . the most important criterion used to differentiate surgical lesions from nonsurgical lesions is the presence or absence of tissue vascularity ( enhancement ) . our two patients were diagnosed with the papillary type of rcc and mucinous adenocarcinoma , respectively . these cases were relatively hypovascular tumors compared with the clear cell - type rcc , which may result in difficulties in accurate diagnosis based on enhanced ct . the superior contrast resolution of mr imaging may be ideal for the further characterization of renal lesions categorized as ii and iif by ct . have reported that diffusion - weighted imaging may help to differentiate benign and malignant renal tumors and could be added to routine kidney mr imaging protocols . this technique could potentially be used as an additional parameter to help to determine appropriate clinical management . also , measuring the signal intensities of trace diffusion images and apparent diffusion coefficient values may be useful for subgrouping renal cysts .
the bosniak renal cyst classification has been accepted by urologists and radiologists as a way of diagnosing cystic renal masses and determining the management approach . 
 we report two cases of a renal cystic mass that showed a category change from category ii on the basis of enhanced computed tomography to category iv after further gadolinium - enhanced magnetic resonance imaging . in both cases , 
 the cysts were later confirmed as kidney cancer by pathology .