a 66-year - old african - american female with a history of end - stage renal disease secondary to diabetes and hypertension underwent a cadaveric kidney transplant . the post - operative course was unremarkable with excellent graft function and serum creatinine ranging from 1.4 to 1.7 mg / dl for many years . seven years later , serum creatinine was noted to increase to as high as 3.6 mg / dl over a few days from presentation . there were neither specific complaints nor any localizing physical examination findings . a kidney ultrasound showed a hyperechoic solid mass in the renal medulla , with central compression and resultant hydronephrosis . follow - up imaging with computed tomography ( ct ) of the abdomen showed an 11  10  8.5 cm fatty mass at the center of the transplant kidney , causing compression of the central renal pelvis and dilatation of peripheral calyces ( figure 1 ) . mycophenolate mofetil was held for initial suspicion of malignancy and was then restarted once malignancy was ruled out after the medullary mass biopsy . 
 fig . 1.ct of the abdomen showing a fatty mass ( arrow ) at the center of the transplant kidney . ct of the abdomen showing a fatty mass ( arrow ) at the center of the transplant kidney . due to the obstruction and renal deterioration , a percutaneous nephrostomy was placed with subsequent nephrostogram showing dilatation and distortion of the renal allograft collecting system . after decompression , the patient had an improvement in renal function with serum creatinine decreasing to 1.6 mg / dl ( figure 2 ) . renal cortex biopsy showed low - range borderline acute tubulointerstitial rejection with mononuclear infiltrates , extensive arteriolar hyalinosis and moderate interstitial fibrosis and tubular atrophy . a biopsy of the renal medullary mass revealed mature adipocytes with no evidence of hyperchromasia and increased vascularity ( figure 3 ) . a dose of methylprednisolone was administered intravenously followed by an oral steroid taper for the acute rejection changes seen in the biopsy . renal sinus lipomatosis refers to a moderate increase in sinus fat . in some cases , the whole renal parenchyma may be replaced by fat and is known as renal replacement lipomatosis . renal replacement lipomatosis is a rare condition , characterized by fatty proliferation of the renal sinus with atrophy of the renal tissue . the extent of renal involvement is variable , with renal sinus lipomatosis being the mildest form and renal replacement lipomatosis being the most severe . some differential diagnoses include liposarcoma , lipoma , angiomyolipoma and transitional cell carcinoma of the renal sinus . in contrast to renal lipomatosis , these tumors are usually located intrarenally or extrarenally outside the renal sinus . . the formation of renal lipomatosis may be due to chronic inflammation , 70% of which are associated with a calculus . renal calculi have been found to cause obstruction of the renal collecting system , which subsequently results in hydronephrosis , chronic infection and eventual atrophy of the renal parenchyma . the development of renal lipomatosis has also been attributed to long - term steroid administration , chronic leakage of urine in the perinephric tissue , renal tuberculosis   and renal infarction . other possible risk factors include aging , obesity , atherosclerosis   and chronic urinary tract infection . lipomatosis in the transplant kidney had been reported within 6 months to 8 years of transplantation . in the case of our patient , the most common investigational tools used to detect renal lipomatosis include a kidney ultrasound and a ct scan , both of which were done on our patient . ultrasound often shows a hypoechoic rim of residual renal parenchyma with a hyperechoic mass at the center , representing the lipomatous tissue . the ct scan demonstrates the destroyed renal parenchyma and the lipomatous mass with a negative attenuation similar to normal adipose tissues . when present in a native kidney that is already deemed non - functional , nephrectomy is often the adequate treatment . this principle , however , may not necessarily apply to a functional transplant kidney . in the case of our patient , since there was note of compression of the surrounding renal tissue with concomitant dilatation of the renal pelvis , nephrostomy tube placement was deemed to be necessary . in fact , the patient 's serum creatinine level gradually decreased after nephrostomy tube placement relieved the urinary obstruction . the presence of a mass infiltration in the patient 's renal medulla compelled us to obtain a tissue biopsy in order to rule out malignancy . in this case , pathology showed mature adipocytes with no malignant features . incidentally , a biopsy of the patient 's renal cortex revealed low - range borderline rejection , which we addressed with steroid therapy . this may be related to holding of mycophenolate mofetil during the period when malignancy was not ruled out yet . although percutaneous nephrostomy placement is not an ideal solution to relieve the obstruction of the renal collective system , a more permanent solution remains elusive . impact of lipomatosis on the renal cortex and the long - term prognosis in other situations suggest a dismal prognosis . utilizing the native ureter to bypass obstruction can be a possibility if it is not involved in the pathology . in this case , however , extensive intrarenal involvement effectively precludes surgical excision of the lipomatosis as well as distal relief of obstruction with an indwelling stent , thereby leaving proximal diversion with a nephrostomy tube as the only option . in summary , renal lipomatosis in a kidney transplant is a very rare entity with limited or no treatment options . the exact etiology is unclear , and it remains a consideration whether the type of immunosuppression can contribute or reverse the formation . early diagnosis and relieving calyceal obstruction can improve renal function and hopefully prevent progressive cortical atrophy .
renal lipomatosis was diagnosed in a kidney transplant recipient who presented with acute kidney injury ( aki ) several years after transplantation . 
 the patient had an odd - looking kidney transplant on ultrasound and computed tomography ( ct ) scan , showing a medullary mass with resultant compression of the surrounding renal parenchyma . 
 a biopsy of the renal medulla confirmed fatty infiltration of the renal parenchyma . 
 the patient underwent percutaneous nephrostomy and aki resolved with relief of the obstruction . 
 renal lipomatosis is a rare condition that should be differentiated from other neoplasms of the kidney . 
 when it occurs in a functioning transplant kidney , the treatment approach proves to be very challenging .