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Soft Tissue Tumors Part 3 - Muscle, Vascular, Nerve, Other

Embryonal rhabdomyosarcoma not otherwise specified (NOS)

Author: Nat Pernick, M.D, PathologyOutlines.com, Inc.

Revised: 18 July 2009, last major update July 2009

Copyright: (c) 2002-2009, PathologyOutlines.com, Inc.

Epidemiology

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● Most common subtype (75%)

● Mean age 7 years, 2/3 male

● Usually head and neck, GU, extremities

Clinical

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● Intermediate prognosis

● Treatment with multiple agent chemotherapy, surgery and radiation

Gross images

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Left atrial tumor with                                         parotid tumor and various images

hemorrhage and necrosis

Other images: intramuscular white tumor mass with focal necrosis

Micro description

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● Dense zones of undifferentiated, differentiating and well differentiating cells

● Cells are immediately beneath epithelium (Nicholson’s cambium layer - resembles hypercellular zones that produce growth rings in trees, image)

● Undifferentiated cells are blue cells with minimal wispy cytoplasm but no skeletal differentiation, and central nuclei

● Differentiating cells have moderate amphophilic or eosinophilic cytoplasm, often fibrillar, may have tadpole shape (tapering bipolar cytoplasm), often with nuclei arranged in tandem

● Well differentiated cells have cytoplasmic cross-striations

● Matrix is collagenous or myxoid

● Nuclei usually are dense and smooth with indistinct nucleolus, moderate nuclear pleomorphism but no anaplasia

● Mitotic figures are associated with less differentiated cells

● Rarely rhabdoid features (abundant eosinophilic cytoplasm, round/oval cytoplasmic inclusions)

Micro images

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Dense condensations of rhabdomyoblasts                Cells in sheets

with myxoid stroma

Cells in anastomosing fascicles                                    Individually dispersed cells

Cells in abundant                                                                Paucicellular tumor due to markedly           

fibromyxoid stroma                                                           edematous or myxoid stroma

Most tumors contain undifferentiated round or oval cells with sparse clear or amphophilic wispy cytoplasm, dense chromatin and irregular nuclear membranes

Diagnostic cells have increased eosinophilic or basophilic cytoplasm that displace the dense hyperchromatic nucleus

Differentiating cells have tapered, bipolar cytoplasm or a tadpole shape with the nuclei at one end; nuclei may be arranged in tandem (like cars on a train)

Well differentiated rhabdomyoblasts (found in 1/3)                  Cells may have abundant amphophilic

have eosinophilic cytoplasm with cross striations                   or clear cytoplasm, fine chromatin

and small nucleoli, resembling lymphoma

Cells often have clear cytoplasm due to glycogen,

rarely causing a spiderweb appearance

Tumors rarely consist primarily of cells with circumscribed, pink, ground glass or fibrillar cytoplasmic inclusions but no cross striations, nuclei may be more vesicular than usual with prominent nucleoli

Part A: nuclei are small with pleomorphism

Myogenin

Desmin staining:

Cross striations                  Muscle differentiation                       Spindle cells       

in rhabdoid cells

Reticulin

Cytology description

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● Noncohesive undifferentiated cells with minimal cytoplasm, variable cytoplasm in differentiating cells

● Cross striations only in well differentiated cells

● Nuclei are dense with indistinct nucleoli

● Rarely intranuclear cytoplasmic inclusions (Diagn Cytopathol 2009 Jun 15 [Epub ahead of print])

Cytology images

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Noncohesive, poorly differentiated cells with no/minimal

cytoplasm or differentiating cells with variable eosinophilic cytoplasm

Positive stains

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● Desmin, HHF35 (90%); also myogenin and MyoD1

● S100 (scattered in 20%), CAM5.2 (6%)

EM images

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Molecular

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● No recurrent structural chromosome rearrangements, but has frequent chromosome gains in #2, 8, 11, 12, 13 and 20
● Frequently a loss of one of the two parental alleles at one or more contiguous chromosomal 11 loci (Atlas Genet Cytogenet Oncol Haematol January 2009)

● Congenital cases are associated with t(2;8)(q35;q13) involving PAX3 gene (Cancer Genet Cytogenet 2009;191:43)

● PAX3 translocation identified with spectral karyotyping in single case (Cancer Genet Cytogenet 2009;189:98)

Molecular images

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Allelic loss of imprinted region at 11p15.5

End of Soft Tissue Tumors Part 3 - Muscle, Vascular, Nerve, Other > Embryonal rhabdomyosarcoma not otherwise specified (NOS)

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