Borderline Mucinous Neoplasm of the Renal Pelvis with Osseous Metaplasia

Primary mucinous cystic tumours of the renal pelvis are extremely rare neoplasms. Herein, we describe the case of a 63-year-old man with a chronic history of nephrolithiasis, presented with long-standing flank pain and progressive abdominal distension. Radiological investigations demonstrated a multicystic, lobulated lesion extensively involving the left kidney. The laparoscopic nephrectomy specimen showed a mucin-containing multicystic tumour that involved the renal pelvis and medulla. Histologically, the cystic spaces were lined with stratified, multi-layering, mucin-containing columnar neoplastic epithelium with no evidence of an invasive growth pattern into the kidney parenchyma. An osseous metaplasia was evident in the tumour. A pathological diagnosis of a borderline mucinous tumour arising from mucinous metaplasia of the renal pelvis was rendered. No adjuvant therapy was given. The patient showed no evidence of recurrent or metastatic disease 58 months after surgery. This case highlights clinical and radiological diagnostic difficulties and contributes to the histopathological spectrum of an extremely rare kidney tumour.