A growing transplanted kidney.

Sir,

Renal sinus lipomatosis (RSL) is a rare disease, described by replacement of parenchyma by sinus and/or perirenal fat. We report here an exceptional case of RSL on kidney graft after corticosteroid treatment was stopped, leading to functional graft transplantectomy.

A 74-year-old man received a first renal allograft in 2004. He was maintained on tacrolimus and mycophenolate mofetil with no rejection. Corticosteroid was stopped at the sixth month. The lowest serum creatinine level was 100 μmol/L. On February 2007, renal ultrasonography, performed for acute renal failure, showed dilatation of renal cavities. Contrast-enhanced computed tomography (CT) showed an enlarged kidney (14.3 cm) with an increased amount of fatty-like tissue in renal sinus and perirenal area (Figure 1). For 6 months, he experienced several obstructive renal failures and infections, leading to permanent nephrostomy. Biopsy of the sinus mass revealed an extensive fatty tissue with fibrosis. Finally, because of bilateral pulmonary embolism secondary to vena cava compression, persistent obstruction and continuous graft enlargement without conclusive histology, and despite good renal function, he underwent transplantectomy. Histological examination showed RSL (immunophenotypical labelling and biomolecular investigations negative for sarcoma) and atrophic chronic pyelonephritis graft.

Fig. 1

Filling defects secondary to thrombosis in the right RV and VCI (white arrows) after contrast administration from vena cava superior.

RSL is essentially described on native kidneys [1,2]. Possible risk factors for RSL include ageing, obesity and pathologic states that cause renal inflammation such as intense corticotherapy and/or early rejection for transplants. Often, chronic or repeat urinary tract infections have been found, and one hypothesis is that RSL may be secondary to periodic leakages of urine into the peripelvic tissues [2]. RSL can also be part of replacement of destroyed or atrophic renal tissue [3]. It has also been suggested that high-dose steroid treatment by itself, Cushing's syndrome or obesity may have contributed to the development of fibrolipomatosis [4]. Intriguingly, a very few cases of RSL have been reported in the renal transplant population [5]. This case is intriguing for several reasons: The time between transplantation and the first symptoms was quite long (3 years). The recipient of the contralateral kidney did not develop any RSL. The patient experienced his first urinary tract infection at the same time as the first acute obstructive renal failure. No history of earlier acute rejection episodes has been found. He did not have a long-term steroid treatment and was not obese. Finally, our case is also exceptional because it led to transplantectomy. Transplantectomy was decided in view of the rapidly increasing graft mass, responsible of vena cava compression and suspicion of malignity.

In conclusion, our case underlines that diagnosis of RSL, although exceedingly rare, should be considered when imaging exams demonstrate a fat-containing renal sinus mass. In transplant recipients, immunosuppressive regimen and malignancy risk underline the importance to establish differential diagnosis between fibrolipomatosis and fibrosarcoma, renal tumour or post-transplant lymphoproliferative disorder.

Conflict of interest statement. None declared.