Soft Tissue Tumors in Children and Adolescents:
A Morphologic Pattern Oriented Approach with Molecular and Genetic Correlations
Section 2 -
Cheryl M. Coffin, M.D.
David M. Parham, M.D.
A 15-year old male presented with an expansile mass arising from soft tissue of thenar eminence. The
lesion had been present for several weeks and was growing steadily. On examination it was fixed and
firm, with indistinct margins. MRI examination revealed an infiltrative, intramuscular lesion that did
not invade bone. An incisional biopsy was performed. Following diagnosis, the patient was placed on
combination chemotherapy with vincristine, actinomycin D, and cyclophosphamide. An experimental agent
was added as "window" therapy. The lesion decreased in size, and a wide surgical excision was performed,
followed by external beam irradiation and consolidation chemotherapy. Two years later, the patient
presented with bone marrow and pulmonary metastases and expired after several months on a salvage
Discussion and Differential Diagnosis:
The lesion represents an alveolar rhabdomyosarcoma. The points for discussion include diagnosis and
subclassification of rhabdomyosarcoma, ancillary studies to consider, and the molecular biologic features
of the types of rhabdomyosarcoma.
The lesion consists of a small round cell tumor that infiltrates tendinous and fibroadipose tissue.
There is partial encapsulation, but the lesion primarily has infiltrative borders. It comprises sheets
of primitive cells focally subdivided by thin fibrous septa. There is extensive cracking artefact, with
cells forming floating aggregates. The tumor cells primarily contain round nuclei of variable size and
chromatin density; nucleoli are not conspicuous. Cells contain modest and variable amounts of eccentric
eosinophilic cytoplasm. Neither cross striations nor overt myogenesis are apparent, and giant cells are
rare. There is a high mitotic index, and numerous karyorrhectic bodies are present. Yellow-brown
crystals of zinc pigment are present, reflecting prior immersion in B5 fixative. Immunohistochemical
stains revealed strong, diffuse nuclear positivity for MyoD and myogenin and cytoplasmic staining for
desmin and muscle-specific actin (clone HHF-35). Reverse transcriptase polymerase chain reaction
(RT-PCR) studies were positive for PAX3/FOXO1 (formerly PAX3/FKHR).
Rhabdomyosarcomas are the most common soft tissue malignancies seen in childhood and comprise the bulk
of these lesions in children aged <10 years. In adolescents, other soft tissue sarcomas as a group
outnumber rhabdomyosarcomas, but they are still the most common sarcoma. Although they are often
described as tumors of skeletal muscle, in fact rhabdomyosarcomas often arise within viscera and other
diverse sites. The tendency of the tumor cells to undergo myogenesis, however, defines the lesion. This
rhabdomyopoietic tendency is usually only partially expressed, however, and most cells often appear
undifferentiated at the cytologic level. However, this trait can be used for immunohistochemical
confirmation of the diagnosis, rather that the strictly historical use of "cross-striations". MyoD and
myogenin stains are most useful in this respect, and they should stain nuclei, not only cytoplasm, as
they function as transcription activators that promote creation of muscle-related proteins such as
myosin. Desmin and muscle specific actin stains are also useful but are less specific or sensitive. Of
particular note is the importance of disregarding cytoplasmic positivity in MyoD, which is a non-specific
It is also of primary importance to subclassify rhabdomyosarcomas, as the evidence indicates that at
least two different entities comprise this tumor. The International Classification of Diseases published
by the World Health Organization separates rhabdomyosarcoma into embryonal, alveolar, and pleomorphic
subtypes. The most critical decision to make is whether a given lesion is an alveolar tumor. In the
currently published WHO criteria, even the presence of a focal alveolar component is enough to make this
distinction, although most are probably "pure" in nature. Recently, however, this has been modified to
"at least 50% alveolar histology" to qualify at alveolar rhabdomyosarcoma, in order to prevent
over-diagnosis. The other major subtype, embryonal rhabdomyosarcoma, resembles embryonic muscle both
morphologically and biologically and contains delicate spindle cells that form focal aggregates in a
myxoid stroma rich in connective tissue mucins. Some embryonal tumors consist primarily of these dense
cellular aggregates, superficially resembling "solid alveolar" tumors, but they retain the cytologic
features of embryonic muscle. Some embryonal tumors may be further subdivided into a superior prognosis
subgroup containing botryoid and spindle cell subtypes, but this has no clinical utility at present. One
feature that may have importance in embryonal rhabdomyosarcoma is anaplasia, or the presence of enlarged
cells with irregular hyperchromatic nuclei and atypical mitoses. Reviews by the Children's Oncology
Group (unpublished data) and the Intergroup Rhabdomyosarcoma Study indicate that anaplasia is associated
with poor outcome, but it is not used to stratify patients for treatment purposes at present.
Alveolar rhabdomyosarcomas, on the other hand, comprise highly cellular aggregates of round cells that
cytologically resemble hematopoietic cells rather than embryonic myotubes. As such they possess round
nuclei, condensed chromatin, and usually only modest amounts of cytoplasm. The classic pattern,
comprising fibrous septa that subtend nests of cells and are lined by rows of myoblasts, is often not
present, particularly with limited biopsies. Instead one may encounter solid sheets of round cells with
the so-called "solid variant", but cracking artefact and clumping are features of both patterns. The
undifferentiated nature of the cells usually leads to use of ancillary techniques such as
immunohistochemistry for definitive diagnosis.
Differential diagnosis includes the list of small round blue cell tumors as discussed in the following
section. It must also be stated that other embryonal tumors, such as Wilms tumor, hepatoblastoma, and
pleuropulmonary blastoma, may show myogenesis, as may other soft tissue sarcomas, particularly malignant
peripheral nerve sheath tumor. These can be excluded by careful clinical correlation and histologic
Genetic studies should also be considered for diagnosis of alveolar rhabdomyosarcoma. Most tumors are
characterized by a genetic fusion between the PAX3 gene on chromosome 2q and
the FOXO1 gene (formerly FKHR) on chromosome
13q. The t(2;13) translocation that produces this fusion may be identified with standard cytogenetics,
which typically require two weeks for analysis and often yield negative results because of no growth or
fibroblast contamination. Quicker and more reliable methods include RT-PCR and fluorescence in situ
hybridization (FISH), which are now available at regional and commercial laboratories for referral
testing. Approximately 20% of alveolar rhabdomyosarcomas contain an alternate fusion between the PAX7 and FOX01 genes, which must also be tested. A
minority of alveolar rhabdomyosarcomas are fusion-negative and have been the subject of continuing
biologic investigations. Embryonal rhabdomyosarcomas are PAX/FOXO1-negative
and usually show loss of heterozygosity of genes on chromosome 11p, caused by allelic deletions or
uniparental isodisomy. Typically they also manifest abnormalities in genomic imprinting of these genes.
Regardless of biologic considerations, patients with alveolar rhabdomyosarcomas generally have a worse
outcome than those with embryonal tumors, independent of tumor stage or site. The outcome for children
with rhabdomyosarcoma has progressively improved, but largely this has resulted from better survivals
with embryonal subtypes. Because of the characteristic molecular signature of alveolar rhabdomyosarcoma,
it is hoped that the development of gene-based therapy will lead to improvement in that tumor as well.
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