Discussion:

Inverted papilloma in uncommon (<1%) benign neoplasm of bladder.

They are common in elderly men (60-70s) but have been described in wide age range.

Common location for these tumors in bladder are trigone, bladder neck and prostatic urethra.

Clinically similar symptoms as that of urothelial carcinoma-intermittent painless hematuria.

Treatment is excision (TURBT-Transurethral resection of bladder tumor).

Gross:

• Inverted papillomas are commonly solitary polypoid sessile or pedunculated lesions with smooth surface ranging in size from3-8 cm.

Microscopy:

• Inverted papillomas are commonly solitary polypoid sessile or pedunculated lesions with smooth surface ranging in size from3-8 cm.
• The classic/ trabecular subtype, glandular subtype and mixed subtype Classic subtype with normal urothelium on the surface and endophytic growth of nested tumor cells from mucosa to lamina propria.
• Some tumor cell nests are interconnected forming trabeculae with peripheral palisading.
• Cells in the center of the nests are thin spindled and arranged parallel to basal lamina.
• Mitotic figures are absent.
• The stroma is commonly loose but can show fibrosis.
• No significant cytologic atypia in tumor cells.
• Glandular subtype morphologically overlaps with cystitis cystica/glandularis.

For the pathological diagnosis of inverted papilloma Amin et al.(1) proposed 9 strict diagnostic criteria

1. Inverted growth pattern
2. Smooth surface lined with mature urothelium
3. Uniform epithelial morphology
4. Tumor cells with consistent arrangement
5. No/few mitotic figures
6. Possible presence of microcyst formation
7. Possible presence of non keratinizing squamous metaplasia
8. No exophytic component
9. Non invasive