Pathology Thread

III. Benign Tumors

Objectives:

After completing this tutorial the student will be able to:

• list benign epithelial tumors of the urothelium
• describe clinical features of benign tumors of the urothelium
• describe gross and histologic features and identify lesions
• state the distinguishing features between transitional cell papilloma and grade I papillary transitional cell carcinoma

Benign transitional epithelial tumors are uncommon in the bladder. They take the form of transitional cell papillomas, which may be exophytic (up to 3% of tumors) or endophytic (inverted). An uncommon benign tumor seen mainly in the bladder but also described in the ureter is the nephrogenic adenoma.

1. Exophytic Transitional Cell Papilloma

These are defined as papillary proliferations composed of histologically normal transitional epithelium that is less than 7 cells in thickness. Papillary tumors that meet this criterion are very rare and the majority of papillary tumors will be carcinomas.

The tumors are seen incidentally or present with painless hematuria. They arise as single isolated lesions but multiple and sequential lesions are not uncommon. Grossly, they appear as small, delicate, soft, branching lesions, which are attached to the mucosa by a thin stalk.

Histologically, the individual branching papillary fronds have a central, loose fibrovascular core covered by normal-looking transitional epithelium.

Papillary fronds of a transitional cell papilloma.

Higher magnification of a frond showing bland-looking transitional epithelium within normal thickness (i.e. not more than 7 cells thick).

Transitional cell papillomas are benign but have a high frequency of recurrence. In up to 7% of recurrences the epithelial cells show cytologic features consistent with malignancy. The recurrence rate of papillomas is almost identical to that of low-grade transitional cell carcinoma and so some authors consider all papillary neoplasms of the bladder as carcinomas.

2. Inverted Transitional Cell Papilloma

These are very rare tumors and are seen in the renal pelvis and ureter but most commonly in the bladder, especially in the trigone. They occur more frequently in males than in females with peak incidence in 60 to 70 years. They present with painless hematuria and occur as solitary mucosal nodules or sessile polypoid lesions 1 to 3 cm.

The microscopic features show a normal surface urothelium from which solid branching and anastomosing cords of cytologically bland urothelial cells extend into the lamina propria to form a complex endophytic growth. Cystic change as well as focal colonic metaplasia may be seen. The intervening stroma is usually edematous.

Inverted transitional cell papilloma. There is normal surface transitional epithelium from which solid broad sheets of epithelium with cystic change grow downwards into an edematous stroma.

Inverted TCP follows a benign course.

Question: In which other part of the body are inverted papillomas found?

3. Nephrogenic Adenoma

Rare tumor or tumor-like lesion of the bladder. It is usually in the trigone but can occur in other parts of the bladder. Patients present with dysuria or gross or microscopic hematuria. It has no age predilection.

Histologically, the lesion is characterized by numerous small tubules lined by cuboidal epithelium surrounded by thick basement membrane lying in edematous and congested lamina propria. These tubules resemble different segments of the renal tubules, hence the name nephrogenic. There is no significant cellular atypia and mitoses are absent. The overlying mucosal epithelium comprises a single layer of cuboidal or low columnar cells or normal urothelium.

The lesion behaves in a benign fashion but recurrences are frequent after transurethral resection.