a 73-year - old female was admitted for surgical removal of a left renal pelvic mass . she has taken antihypertensive agents for 30 years and received percutaneous coronary intervention due to stable angina 3 years ago . during routine follow - up , a renal mass was incidentally detected 2 months ago . laboratory values revealed a marginal elevation of serum creatinine ( 1.21 mg / dl ) . the ultrasonography showed an echogenic mass within the renal pelvis and mild dilation of several calyces . the calcified lesion was not found on plain radiograph of the abdomen ( kidneys , ureters , and bladder ) . an abdominopelvic computed tomography revealed a noncalcified soft tissue mass in the left renal pelvis on precontrast enhancement scan . the cut surface of the resected specimen revealed a 532 cm - sized , well - defined ovoid , cystic mass which was compactly filled with white and reddish tan - colored amorphous materials in the pelvis . histologically , the cystic lumen contained multiple keratinous materials and much of the cystic lining epithelia were denuded . , santa cruz , ca , usa ) and cytokeratin 7 ( ck7 ) ( 1:100 ; dako , glostrup , denmark ) was performed to identify epithelium - specificdifferentiation . in contrast , most of the cells in the lining of squamous epithelia were negative for these urothelial specific antibodies . however , some portion of the squamous epithelia interestingly contained a few uroplakin - positive cells ( fig . even though the epidermoid cyst of the kidney is a very rare to be characterized , the typical features of the lesion can be summarized as follows . the clinical symptoms are flank pain , gross hematuria , or lower urinary tract symptoms . the calcified inhomogeneous lesion , combined renal stone , and previous renal surgery are other important clinical clues for the renal epidermoid cyst ( table 1 ) . to date , two cases of epidermoid cyst in the renal pelvis have been reported . they also showed a typical calcified lesion combined with renal stone like other renal epidermoid cysts . with these reasons , it seemed hard to think of epidermoid cyst as a presumptive diagnosis before surgery in this case . to the best of our knowledge , this case of a noncalcified epidermoid cyst in the renal pelvis is the first report in the english literature . the histology of a typical epidermal cyst that was lined by stratified squamous cells with laminated keratins . although epidermoid cyst of the skin is a common lesion resulted from implantation of epidermis into the dermis , histogenesis of epidermoid cyst in the kidney remains uncertain . as a possible histogenetic mechanism , either epidermal embryonic remnants of wolffian ducts or traumatic implantation of transformed epithelial cells during renal operation have been proposed . the extension of squamous metaplastic changes from the upper ureter to the pelvocalyceal system in prolonged irritative condition induced mostly by stones is considered as the other plausible explanation . however , the cellular basis of squamous metaplasia is not fully understood in this rare condition . the apical surface of urothelium is highly specialized as it is covered almost by urothelial plaques which are composed of uroplakins . four uroplakins ( ia , ib , ii , and iii ) have been isolated and are considered to be biochemical markers of terminal urothelial differentiation . ck7 is an intermediate filament proteins expressed in a wide variety of simple epithelia but not in the squamous epithelia . it is well known that vitamin a deficiency can induce keratinizing squamous metaplasia in a variety of epithelia including urothelium . the urothelial keratinization in the vitamin a - deficient mice bladder occurred heterogeneously adjacent to areas lined with normal appearing urothelium . the sharp boundary between the keratinized epithelium and the seemingly normal urothelium was maintained with no intermediated cells . according to the our histologic examination , nearly almost of the urothelial cells in the cystic lining were positively stained against both uroplakin ii and ck7 , but most of the squamous epithelial cells were negative for these antibodies . the two epithelia were sharply delineated and more distinctly distinguishable in the immunohistochemical stained sections . these features were similar with heterogenous metaplasia of the animal urothelium in the previous study . however , some squamous epithelia contained a few uroplakin - positive cells and ck7-positive cells to some extent . even though the significance of few uroplakin expressions in squmous epithelium should be evaluated by other studies , is this an indirect evidence of the direct transformation of umbrella cells to squamous metaplasia or dedifferenctiation of metaplastic squamous epithelium to urothelium ?
since the renal epidermoid cyst is too rare , the mechanisms of squamous morphogenesis have not well characterized . 
 a 73-year - old female was referred with an incidentally detected renal pelvis mass . 
 abdominopelvic computed tomography scan revealed a noncalcified soft tissue mass in the renal pelvis . 
 total nephroureterectomy was performed under the impression of a renal pelvis malignancy . 
 the patient was discharged without postoperative complication . 
 the outer surface of mass lesion was lined with urothelia and squamous epithelia , containing keratinous materials . 
 the urothelia were positively stained against uroplakin ii and cytokeratin 7 , whereas almost of the squamous epithelia were negative with uroplakin ii . 
 the two different epithelia were generally sharply demarcated . 
 interestingly , some part of squamous epithelia contained uroplakin - positive and many more cytokeratin 7-positive cells . 
 the atypical clinical features in our case can reconsider the diagnostic clues of renal epidermoid cysts that have been reported before , and the unique immunohistochemical results may understand the histogenetic implications of the lesion .