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| This
is the most common primary mesenchymal orbital neoplasm of childhood.
Clinically, patients often have a rapid onset of unilateral
proptosis with chemosis and redness (#22113).
This tumor does not arise from differentiated muscle, but rather
arises from primitive undifferentiated mesenchyme which may be located
anywhere in the orbit (#22114). The
neoplastic cells tend to differentiate toward cells resembling those of
the rhabdomyoblast of a seven to ten week old fetus.
Three cytologic variances are recognized.
The most common variant in the orbit is the "embryonal"
type
(#22115) in
which the cells are elongated with an abundant eosinophilic cytoplasm
assuming a racquet or strap shape, with occasional tadpole-shaped cells.
The cells are arranged in a loose syncytium.
Mitotic figures are frequent.
Characteristic cross- striations representing Z-bands in the
cytoplasm are often not present and special stains (trichrome) or EM
studies may be necessary to make the diagnosis.
The "alveolar variant" shows fibrovascular septae or
trabeculae dividing the neoplastic cells into clusters resembling lung
alveoli and tend to be more aggressive.
The "adult pleomorphic variant" is the least common and
consists of more differentiated cells with obvious cross-striations.
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| Clinical
#22113 |
Gross
#22114 |
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| High
Power #22115 |
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