Renal replacement lipomatosis with interstitial nephritis and secondary degenerative changes: A rare entity

Renal replacement lipomatosis (RRL) is a rare benign condition characterized by the replacement of the renal sinus, hilum, and perirenal space with extensive fatty tissue. It is also known as replacement lipofibromatosis and is generally observed in individuals aged 60-70 years. RRL is typically associated with renal calculi, chronic inflammation, and hydronephrosis, leading to severe atrophy of the renal parenchyma and, ultimately, a non-functional kidney. A study indicated that in 3,500 intravenous pyelogram cases, the incidence of RRL was 0.66%, with no significant gender preponderance. We report a case of a 65-year-old male patient who presented with a left renal mass and hematuria. Non-contrast CT of the kidneys, ureters, and bladder (NCCT KUB) revealed a 23 mm calculus at the pelvi-ureteric junction, with extensive peripelvic and periureteric fat stranding and a prominent renal fascia. Left nephroureterectomy was performed, and the excised specimen was submitted for histopathological examination. On gross examination, the specimen measured 11.5x7.5x4.5 cm; the ureter measured 7 cm in length and 0.6 cm in diameter. The outer surface was unremarkable. On sectioning, the pelvicalyceal system was markedly dilated and replaced by fat.