Bladder, ureter & renal pelvis
Urothelial neoplasms-noninvasive
Flat hyperplasia

Topic Completed: 1 December 2014

Revised: 20 December 2018

Copyright: (c) 2003-2018,, Inc.

PubMed Search: Bladder [title] Flat hyperplasia

Monika Roychowdhury, M.D.
Page views in 2019: 1,317
Page views in 2020 to date: 858
Cite this page: Roychowdhury M Flat hyperplasia. website. Accessed August 7th, 2020.
Definition / general
  • Markedly thickened mucosa (up to 10 or more cell layers) without cytologic atypia
  • May be adjacent to low grade papillary urothelial neoplasm
  • By itself, has no malignant potential and requires no treatment
  • True incidence is not known due to lack of large scale screening studies
Clinical features
  • Asymptomatic
Prognostic factors
  • Isolated lesion does not have a premalignant potential
  • Molecular studies suggest a preneoplastic potential in flat hyplerplasia concurrent with low-grade papillary tumors (Hum Pathol 2010;41:155)
Microscopic (histologic) description
  • May occur adjacent to low-grade papillary tumors or as an isolated lesion
  • Marked thickening of mucosa up to 10 or more cell layers
  • Maintains morphologic evidence of maturation from basal cells to superficial cells
  • Absence of cytologic atypia or mitoses
Positive stains
  • CK20 limited to umbrella cells (also stains deeper cells in dysplasia)
  • CD44 staining limited to basal cells (stains all layers in reactive atypia but no staining in dysplasia)
Negative stains
Molecular / cytogenetics description
  • Genetic alterations in chromosome 9 are frequent in flat hyperplasia with concurrent low-grade papillary tumors
Differential diagnosis
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