Eble JN, Sauter G., Epstein JI, Sesterhenn IA - iarc

Inverted papilloma
G. Sauter
Definition
Benign urothelial tumour that has an
inverted growth pattern with normal to
minimal cytologic atypia of the neoplastic
cells.
Epidemiology
The lesion occurs mostly solitary and
comprises less than 1% of urothelial neoplasms
{1843}. The male: female ratio is
about 4-5:1. Ages of affected patients
range from 10 years {2861} to 94 years
{1309} with a peak frequency in the 6th
and 7th decades.
Etiology
The etiology of inverted papilloma is
unknown. Hyperplasia of Brunn nests and
chronic urothelial inflammation have been
suggested as possible causes.
Localization
More than 70% of the reported cases
were located in the bladder but inverted
papillomas can also be found in ureter,
renal pelvis, and urethra in order of
decreasing frequency. The trigone is the
most common location in the urinary
bladder {363,596,1037,1049,1071,1190,
2416,2494}.
urothelial cells invaginate extensively from
the surface urothelium into the subadjacent
lamina propria but not into the muscular
bladder wall. The base of the lesion
is well circumscribed. Anastomosing
islands and cords of uniform width distribution
appear as if a papillary lesion had
invaginated into the lamina propria. In
contrast to conventional papillary urothelial
neoplasms, the central portions of the
cords contain urothelial cells and the
periphery contains palisades of basal
cells. The relative proportion of the stromal
component is mostly minimal but
varies from case to case, and within the
same lesions.
A trabecular and a glandular subtype of
inverted papilloma have been described
{1409}. The trabecular type is composed
of interanastomosing sheets of urothelium
sometimes including cystic areas. The
glandular subtype contains urothelium
with pseudoglandular or glandular differentiation.
Foci of mostly non-keratinizing squamous
metaplasia are often seen in inverted
papillomas. Neuroendocrine differentiation
has also been reported {2534}.
Urothelial cells have predominantly
benign cytological features but focal
minor cytologic atypia is often seen
{363,1409,1843}. Mitotic figures are rare
or absent {363,1409}.
It is important to not extend the diagnosis
to other polypoid lesions with predominantly
subsurface growth pattern such as
florid proliferation of Brunn nests or areas
of inverted growth in non-invasive papillary
tumours.
Clinical features
Hematuria is the most common symptom.
Some cases have produced signs of
obstruction because of their location in
the low bladder neck or the ureter {503}.
Dysuria and frequency have been recorded
but are uncommon {376}.
Macroscopy
Inverted papillomas appear as smoothsurfaced
pedunculated or sessile polypoid
lesions. Most are smaller than 3 cm
in greatest dimension, but rare lesions
have grown to as large as 8 cm
{363,596,1071,1190,2101}.
A
Histopathology
Inverted papilloma has a relatively smooth
surface covered by histologically and
cytologically normal urothelium.
Randomly scattered endophytic cords of
B
Fig. 2.34 Noninvasive urothelial neoplasm. A, B Inverted papilloma. C Most urothelial cells in this example
of inverted papilloma are immunohistochemically reactive with antibodies anti-cytokeratin 7.
C
114 Tumours of the urinary system