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Embryonal Rhabdomyosarcoma :
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Age and site: Usually occurs in
children (befor the age of 10 years).
Common sites include head and
neck region, genitourinary tract and retroperitoneum.
Pathological features- Alternating cellular and myxoid areas.
Classical form is composed of undifferentiated round or spindle cells
together with a few scattered eosinophilic rhabdomyoblasts. Some
elongated rhabdomyoblasts show angulation of muscle fibres (broken
straw sign). Multivacuolated or spider web cells may be present.
Cytoplasmic cross striations are noted in about 20 -30% cases.
Intracellular glycogen is more prominent in well-differentiated
forms.
Immunohistochemistry plays an important role in establishing the
diagnosis. Desmin and
muscle actin are positive.
MyoD1
and myogenin (myf4) are also useful markers.
Variants- 1. Botryoid
2. Spindle cell 3. Anaplastic
Botryoid
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Located in mucosa lined
hollow organs, eg. nasal cavity, nasopharynx, bile duct, urinary
bladder and vagina.
Grossly, the tumour has a polypoid grape like
appearance.
Microscopically the tumour arises in the submucosa and is
covered by hyperplastic epithelium.
Subepithelial cambium layer composed of condensed undifferentiated
tumour cells is a characteristic feature.
The rest of the tumor consists of a relatively sparse population of
cells in a backround of mucoid stroma. Scattered spindle shaped or
rounded rhabdomyoblasts may be present.
Spindle
cell-
Located
in the paratesticular region (rarely head & neck region).
Characterized by eosinophilic spindle cells arranged in a fascicular
or storiform pattern. D/D-
Nerve sheath tumour , leiomyosarcoma
,
fibrosarcoma.
Anaplastic-
Characterized by
areas of bizarre cell with giant nuclei in a collagenous
backround, together with foci of identifiable embryonal areas. Atypical
mitotic figures are present.
Alveolar
Rhabdomyosarcoma :
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Age and site - Usually occurs
between 10 and 25 years. Commonly located in the upper and lower
extremities.
Pathological features - Alveolar pattern - The tumour consists of
aggregates of undifferentiated tumour cells arranged in a alveolar
pattern. Eosinophilic rhabdomyoblasts and giant cells with peripheral
wreath- like nuclei are prominent. The cellular aggregates are
separated by dense hyalinized fibrous septa. The tumour cells at the
periphery are well preserved. The cells at the centre of the alveolar
spaces are loosely arranged and poorly preserved. Cross striations are
noted in only 10% cells. Solid pattern - Solid packed masses of tumour
cells resembling round cell areas of embryonal rhabdomyosarcoma (may
resemble carcinoma or malignant melanoma). A microalveolar pattern has also
been described.
Immunohistochemistry - Demonstrate strong and widespread myogenin expression
compared to
embryonal type.
The majority of the alveolar
rhabdomyosarcomas are associated with specific chromosomal
translocations (pARMS). Because alveolar variant is much more
aggressive than embryonal variant, RMS subclassification has clinical
relevance.
Pleomorphic
Rhabdomyosarcoma :
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Pleomorphic rhabdomyosarcoma
is considered rare and controversial.
Age and site - Occurs in adults
(above 45 years). Usually located on the large muscle of the
extremities specially on the thigh.
Pathological features - The tumour consists of large polygonal or
spinde shaped rhabdomyoblasts. The backround stroma is collagenous.
Storiform areas may be present (resemble
pleomorphic MFH).
The tumour cells contain glycogen. Cells with cross striations are
usually absent.
Rare variant :
Sclerosing
Rhabdomyosarcoma-
It is not clear where this
variant belong in the current classification of Rhabdomyosarcoma.
Site: Usually located in the
extremities.
Characterised by prominent
hyaline sclerosis and a pseudovascular growth pattern. Histologically,
the neoplasms are composed of round/polygonal and spindle-shaped
tumour cells including typical rhabdomyoblasts.
The cells are primitive appearing
with scanty eosinophilic cytoplasm and irregular nuclear outline and
coarse chromatin. The stroma divides the tumour mass into lobules and
small nests.
Tumour cells stained
positively for desmin (focal and sometimes dot-like), strongly
positive for smooth muscle actin (HHF35) and also for markers of
striated muscle differentiation (myogenin, MyoD1). This tumour may
mimic
extraskeletal myxoid chondrosarcoma, mesenchymal chondrosarcoma
osteosarcoma,
sclerosing epithelioid fibrosarcoma and even
angiosarcoma.
Cytogenetics: Cytogenetics play an important
role in confirming the diagnosis rhabdomyosarcoma. Alveolar
rhabdomyosarcoma is associated with a specific translocation,
t(2;13)(q37;q14) or its variant t(1;13)(p36;q14). Embryonal
rhabdomyosarcoma often shows loss of heterozygosity for 11p, but there
is no specific cytogenetic or molecular marker comparable to those for
alveolar RMS.
Mimics of Rhabdomyosarcoma:
Fetal rhabdomyoma
; pseudosarcomatous fasciitis;
genital rhabdomyoma ; atypical
spindle cell lesions of the bladder and atypical cells in nasal
polyps.
International
Classification of Rhabdomyosarcoma:
Superior prognosis:
Botryoid RMS ; Spindle cell RMS
Intermediate prognosis:
Embryonal RMS
Poor prognosis:
Alveolar RMS ; Undifferentiated sarcoma
The
combination of histology, immunohistochemistry, chromosome analysis,
and molecular cytogenetics played an important role in the diagnosis
of Rhabdomyosarcoma.
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