Cystic hamartoma of the renal pelvis.

We describe three hitherto undocumented cases of renal lesion in the adult age group that share a similar site and histological features. They are three adult women, with a short clinical history of pain and an abdominal mass. A partial or complete nephrectomy resulted in clinical cure. All cases showed an intrarenal multicystic mass situated adjacent to the pelvicalyceal system. These vaguely circumscribed lesions had no true capsule and blended in with the adjacent renal parenchyma. The histological appearance was distinctive and characterised by disorderly biphasic proliferation of epithelial and mesenchymal elements. The epithelial component consisted of tubules and cysts lined by cuboidal and columnar epithelium showing focal oncocytic changes. The stroma was cellular and predominantly fibroblastic with scattered bundles of smooth muscle cells. Despite extensive sampling, blastemal cells were not identified. The tubular epithelium was positive for CAM 5.2, epithelial membrane antigen, carcinoembryonic staining, and vimentin immunostaining. The stroma stained positively for vimentin and smooth muscle bundles for alpa smooth muscle actin and desmin. The cytological appearances of these lesions were benign. We propose that these are benign hamartomatous lesions arising as a result of faulty focal embryogenesis. They are distinct from well recognised lesions such as multilocular cysts, partially differentiated nephroblastomas, mesoblastic nephromas, and nephrogenic adenofibromas.