Discussion:

Nephrogenic adenoma (also known as nephrogenic metaplasia) is a rare benign lesion that can be found anywhere in the urinary tract and arises from a proliferation of implanted renal tubular cells (1). It has a male predominance and is more common in adults with an average range of prestation of 61 years old (2). However, approximately 10% of nephrogenic adenomas can affect children and, also, have been reported to occur in a large rage of age groups (from 4 to 81 years-old) (3).

It is a condition that can be associated and triggered by a large variety of insults to the bladder like recurrent urinary tract calculi, recurrent urinary tract infections, intravesical therapy with BCG or mitomycin, diverticula of the urinary bladder, kidney transplantation, previous cystoscopy, bladder surgery, bladder augmentation, chronic catheterization, foreign bodies, radiotherapy, chemical agents, and other irritative factors (1, 3).

Clinically, the nephrogenic adenoma of the bladder can be asymptomatic or can present with irritative bladder symptoms like urinary frequency and urgency. Very rarely can present with hematuria. The cystoscopy usually shows a velvety appearance of the vesical mucosa that commonly is mistaken with urothelial carcinoma (1, 3).

Gross examination of nephrogenic adenomas usually demonstrates a polypoid, sessile or papillary lesion. The histopathologic sections can show several histologic patterns like tubular, tubulocystic, polypoid, papillary, fibromyxoid, and flat. Generally, the nephrogenic adenomas are composed of small hollow tubes that look similar to mesonephric tubules and are lined by a single layer or bland cuboidal or hobnail cells which surround eosinophilic or basophilic secretions (1, 3, 4). Given the several histologic patterns of nephrogenic adenomas, the differential diagnosis can be broad. For papillary nephrogenic adenomas (like our case report), the differential diagnosis includes urothelial papilloma, papillary urothelial neoplasm with low malignant potential, and low-grade papillary urothelial carcinomas (5). Finally, the immunohistochemistry stains positively with AE1/AE3 (pancytokeratin), CAM5.2, CK7, CK20, EMA, CA-125, PAX8 and PAX2. It also has a weak positive staining with PSA or PAP and a variable positivity with P504S. On the other hand, stains negatively with CK903, p63, and CD10 (3).