Microscopic description:

The tumor on low power shows papillary architecture with fibrovascular cores.

The tumor cells are arranged in irregular cords, nests and sheets growing downwards from overlying benign urothelium.

Few foci with microcysts formation are present.

On high power the cells show peripheral palisading, small nuclei with minimal nuclear atypia and no mitotic activity.

The stroma is loose edematous with no frank stromal invasion.

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

References:

Amin MB, Gómez JA, Young RH. Urothelial transitional cell carcinoma with endophytic growth patterns: a discussion of patterns of invasion and problems associated with assessment of invasion in 18 cases. Am J Surg Pathol 1997;21:1057-68

Roychowdhury M. Inverted urothelial papilloma. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/bladderinvertedpapilloma.html. Accessed February 7th, 2022.

Guo A, Liu A, Teng X. The pathology of urinary bladder lesions with an inverted growth pattern. Chin J Cancer Res. 2016;28:107–2

CASE IMAGES