Nephrogenic adenoma: A report of 3 unusual cases infiltrating into perinephric adipose tissue

Nephrogenic adenoma of the urinary bladder, where they present most frequently, are typically confined to the lamina propria but can on occasion focally involve the superficial muscularis propria. Less commonly, nephrogenic adenoma involves the renal pelvis and ureter where again they almost always only involve the lamina propria. We identified 3 consult cases in which tubules of nephrogenic adenoma extensively involved the muscularis propria and focally infiltrated the perinephric adipose tissue, for which the contributing pathologists considered the diagnosis of adenocarcinoma. In 1 case, that of a 71-year-old man, the lesion was associated with a hemorrhagic renal cyst (3.0 cm) and a spontaneous retroperitoneal bleed (6.0 cm) of unknown origin. In the second case, that of a 73-year-old woman, 2 foci (2.2, 1.6 cm) were present in the renal pelvis. They developed after biopsy of a low-grade noninvasive papillary urothelial carcinoma in the same site complicated by perforation. The third case was that of a 20-year-old woman with ureteropelvic junction obstruction and severe hydronephrosis associated with renal calculi. In all cases, the lesions were positive for CK7 and PAX8. These 3 cases report the novel finding that nephrogenic adenoma can occasionally have a deep infiltrative pattern into perinephric adipose tissue, possibly as a result of either biopsy-associated perforation or extensive disruption due to hemorrhage or mechanical obstruction. Awareness of this worrisome infiltration pattern, although rare, can prevent a misdiagnosis of an infiltrating carcinoma.