Soft Tissue Tumors in Children and Adolescents:
A Morphologic Pattern Oriented Approach with Molecular and Genetic Correlations
Section 3 -
Cheryl M. Coffin, M.D.
David M. Parham, M.D.
A 16-year old male presented with a large soft tissue lesion of the chest wall, not directly arising
from a rib. The lesion has been growing steadily for several weeks and involved the musculature of the
chest and axilla. It was associated with pleuritic pain, and the overlying skin was slightly reddened.
No metastatic disease was found on chest films, or bone scans. On MRI and CT scans, the tumor did not
appear to involve the bony structure of the rib. Following an incisional biopsy, the patient was placed
on a regimen of doxorubicin, vincristine, cyclophosphamide, and dactinomycin, alternating with ifosfamide
and etoposide. The lesion dramatically decreased in size, with alleviation of symptoms. After 12 weeks
of therapy, an excision of the mass was performed, and the patient is currently free of disease 98 months
Discussion and Differential Diagnosis:
This is an example of an extraosseosseus Ewing sarcoma. Because of morphologic overlap, clinical
similarities, and biologic identity, Ewing sarcoma and peripheral primitive neuroectodermal tumors
(PNET), are presently considered the same lesion. This is reflected in the new International
Classification, which defines them as a single entity with varying degrees of neuroectodermal
differentiation. The presence of distinct neural differentiation, manifested by Homer Wright rosettes,
immunohistochemical positivity for multiple neural markers, and ultrastructural evidence of
neurosecretory granules within dendritic processes, has been used to separate PNET from Ewing sarcoma.
The clinical utility of this distinction has been debated, but evidence derived from homogenously treated
patients indicates no difference in their outcome. The points for discussion in this case include
differential diagnosis, ancillary tests, clinical features, and current outcome.
Histologically, this example consists of an undifferentiated small round cell tumor that infiltrates
skeletal muscle and fibrous tissue. The entrapped skeletal muscle shows degeneration, and areas of dense
fibrous connective tissue are present. The bulk of the tumor exhibits degenerative changes and necrosis
with discohesive and cellular eosinophilia. Preserved viable cells of cells are cohesive and contain
round, regular nuclei, slightly granular chromatin, inconspicuous nucleoli, and minimal cytoplasm.
Rosettes are not identified. Immunostains revealed diffuse, membranous positivity for CD99, focal
cytoplasmic positivity for synaptophysin, and negativity for desmin, myogenin, CD45, and CD43. RT-PCR
revealed an EWS/FLI1 fusion.
Ewing sarcoma has traditionally been considered the archetypal "small round blue cell tumor",
reflecting its content of primitive, undifferentiated cells that form solid sheets of tumor without
distinguishing characteristics at the light microscopic level. These lesions comprise a large proportion
of solid tumors in children, where the most common entities on the differential diagnosis list primarily
consists of Ewing sarcoma, undifferentiated neuroblastoma, rhabdomyosarcoma (particularly the solid
alveolar subtype), and non-Hodgkin's lymphoma, including lymphoblastic and Burkitt subtypes. Table 1
lists the pathologic features of the first four entities, along with less common lesions, desmoplastic
small round cell tumor, clear cell sarcoma of tendon sheath, extraskeletal myxoid chondrosarcoma, and
rhabdoid tumor (see case 8). Lymphoblastic lymphoma can pose particular difficulties, as it may be
CD45-negative and CD99-, Fli-1-, and rarely even cytokeratin-positive, like Ewing sarcomas. For this
differential, positivity for markers such as TdT, CD79a, CD10, and CD43 is useful to indicate lymphoma.
Other soft tissue tumors to consider include poorly differentiated synovial sarcoma, malignant
peripheral nerve sheath tumor, extraskeletal mesenchymal chondrosarcoma, and extramedullary hematopoietic
lesions including ALL, AML, and Langerhan cell histiocytosis. Even benign lesions such as diffuse and
localized forms of giant cell tumor of tendon sheath and chronic inflammatory lesions may at times
resemble undifferentiated round cell sarcoma. Also to be considered within visceral tumors are organ
specific blastomas, such as Wilms tumor, hepatoblastoma, pleuropulmonary blastoma, and sialoblastoma, and
more differentiated neuroectodermally-derived lesion such as melanoma and paraganglioma.
Lest the unwary pathologist be overwhelmed with the challenge presented by this daunting array of
entities, the good news is that the first four entities mentioned make up the overwhelming predominance
of these lesions, and they each have characteristic immunohistochemical and genetic features that make
ancillary testing critical for precise categorization and treatment. In spite of the undifferentiated
nature of small round blue cell tumors, electron microscopists recognized in the 1970s that focal
differentiation was often apparent at the ultrastructural level. That finding was exploited by
immunohistochemists, who systematically identified a panel of markers that can be used for ancillary
diagnosis, and refinement of that panel continues to this day. Finally, cytogeneticists recognized the
existence of tumor-specific karyotypic aberrations, and these findings were exploited by molecular
geneticists who developed genetic tests that can be similarly used for diagnostic confirmation. The
important lessons to remember is that no single marker can be relied on exclusively, and that
careful histologic examination and clinical correlation remain the cornerstone of diagnosis.
With Ewing sarcoma, an immunohistochemical panel is essential, because none of the markers are
specific enough to be relied upon as a single reason for diagnosis. That stated, CD99 and/or FLI-1
immunostains are useful as sensitive markers and help to confirm the diagnosis. Diffuse membranous
positivity is seen with CD99, and nuclear staining is seen with Fli-1; the latter marker is not currently
in widespread usage. Nevertheless, combined FLI-1 and CD99 immunostaining appears to improve the
sensitivity and specificity of histologic diagnosis. As noted above Ewing sarcomas often stain with
neural markers such as synaptophysin or neuron-specific enolase, particularly if rosettes are evident.
Chromogranin is less useful, because of the low neurosecretory granule content of individual cells.
Neural positivity is also present with neuroblastoma, but CD99 negativity is usually seen with that
tumor. Cytokeratin is positive in about 10% of cases, reflecting their ectodermal differentiation. Rare
tumors may exhibit unexpected staining with markers such as desmin, GFAP, and S100, also reflecting the
pluripotent nature of neuroectoderm. Vimentin is universally positive but useful only to confirm the
suitability of tissue for staining. Electron microscopy may be helpful in cases with unexpected or
incongruent immunohistochemical results.
Genetic testing is also recommended in round cell tumors, particularly if the diagnosis is difficult.
The characteristic finding in Ewing sarcoma is the EWS/FLI-1 fusion,
reflected by the t(11;22) translocation seen on cytogenetic examination. This finding can be detected by
FISH or RT-PCR studies, even on paraffin sections, but use of fresh tissues gives better results. Touch
preps can be used for FISH analysis, and they are also useful for detecting the cellular cohesion that
distinguishes Ewing sarcoma from lymphoma. About 10% of cases have an alternate fusion, in which another
member of the ETS gene family fuses with EWS. In these cases, "pull-apart" FISH performed on the EWS
gene can detect a translocation but will not identify the fusion partner. One particular tumor to
exclude is desmoplastic small round cell tumor (DSRCT), which possesses a fusion of EWS and the WT1 gene. This tumor also shows a
characteristic striking desmoplasia with tumor cells in nests rather than sheets, and the cells show
polyphenotypia with positivity for cytokeratin, neuron-specific enolase, vimentin, and desmin. Rare
tumors have the phenotypic features of DSRCT but a genetic fusion typical of Ewing sarcoma. In my
opinion, these should be treated as Ewing sarcoma in spite of the potential diagnostic confusion, but
fortunately these lesions are too rare for meaningful follow up analysis. FISH appears to be a more
sensitive and reliable than RT-PCR for ancillary diagnosis, although complex tumors often benefit from
Ewing sarcomas and PNETs occur in a diversity of anatomic sites, even dura mater, kidney, and skin,
but they are most common in axial locations such as the chest wall, pelvis, and paraspinal region. Ewing
sarcomas now enjoy a much better clinical outcome, thanks to the identification of active
chemotherapeutic agents. In a recent large, multi institutional study, there was a five-year event-free
survival of 69 per cent among patients with non-metastatic disease treated with ifosfamide plus etoposide
in addition to a regimen. Survival worsened with metastatic disease, as that group had an event-free
five-year survival of 22 per cent. Other factors associated with worse outcome are pelvic location, size
of tumor, proximal vs distal tumors, and lack of a significant chemotherapy response on histologic
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