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

Rhabdomyosarcoma
I. Leuschner
Definition
Rhabdomyosarcoma is a sarcoma
occurring in the urinary bladder that
recapitulates morphologic and molecular
features of skeletal muscle.
ICD-O code 8900/3
Epidemiology
They are the most common urinary bladder
tumours in childhood and adolescence.
Almost all bladder rhabdomyosarcomas
are of embryonal subtype,
whereas the genetically distinct
alveolar subtype is extremely rare in this
site {1887}. In adults rhabdomyosarcoma
is rare and usually of the pleomorphic
type.
Fig. 2.74 Rhabdomyosarcoma of the bladder.
Macroscopy
Growth pattern of embryonal rhabdomyosarcoma
in urinary bladder has
two basic forms with prognostic impact:
polypoid, mostly intraluminal tumours
associated with a favourable prognosis
(botryoid subtype) and deeply invasive
growing tumours involving the entire
bladder wall and usually adjacent organs
showing a worse prognosis.
Histopathology
Tumour cells of embryonal rhabdomyosarcoma
are usually small, round
cells, often set in a myxoid stroma. Some
cells may have classic rhabdomyoblastic
appearance with abundant eosinophilic
cytoplasm and cross striations. Botryoid
subtype of embryonal rhabdomyosarcoma
has a condensation of tumour cells
beneath the covering surface epithelium,
called the cambium layer. Deeper parts
of the tumours are often hypocellular. The
Fig. 2.73 Embryonal rhabdomyosarcoma.
botryoid subtype of embryonal rhabdomyosarcoma
is the end of a spectrum
of polypoid growing embryonal rhabdomyosarcomas
sharing a similar
favourable prognosis {1482}. Primarily
deep invasive growing tumours of the urinary
bladder wall have usually a low
degree of differentiation and are associated
to a similar worse prognosis as seen
for embryonal rhabdomyosarcoma of
prostate.
Immunohistochemically, the tumour cells
express myogenin (myf4) and MyoD1 in
the nucleus {612,1404}. This is assumed
to be specific for a skeletal muscle differentiation.
Highly differentiated tumour
cells can lack myogenin expression.
Desmin and pan-actin (HHF35) can also
be detected in almost all rhabdomyosarcomas
but it is not specific. Staining for
myosin and myoglobin can be negative
because it is usually found only in well
differentiated tumour cells. Recurrences
of embryonal rhabdomyosarcoma can
show a very high degree of differentiation
forming round myoblasts.
Carcinoid / Rhabdomyosarcoma 139