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Small, Blue, Round Cell Tumors of ChildhoodSmall, blue, round cell tumors (SBRCTs) pose a particularly great challenge to the surgical pathologist, owing to the considerable histologic overlap in these tumors, and the growing disparity in the treatment modalities, and hence, clinical outcome in the different subsets of SBRCTs. Fortunately, along with breakthroughs in our understanding of the cell and molecular biologic basis of these tumors have come novel immunohistochemical markers that can reliably distinguish among the different SBRCTs. Some common markers, in use for many years, must be applied with caution, owing to their 'infidelity' in this context. For example, desmin, long considered a marker of rhabdomyosarcoma, can also be expressed in a subset of PNET/ Ewings sarcoma. And CD99, often considered a marker of PNET/ Ewings sarcoma, is not lineage specific and can also be expressed in other SBRCTs. Two special categories of markers are most useful in this set of tumors. The first are antibodies to nuclear transcription factors such as myogenin, a DNA-binding protein that acts as a 'switch gene', turning on expression of other muscle-restricted proteins. The second is a set of proteins to proteins overexpressed as a consequence of chromosomal translocations that define these tumors. In the case of PNET/Ewings sarcoma and desmoplastic small round cell tumor, two important members of the SBRCT group, traditional 'cell type specific' markers are not of great utility because these tumors are best defined by their unique chromosomal translocations, as shown in the table below:
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