Polyphenotypic small round-cell tumors

Figure 1. Polyphenotypic small round cell tumor from the abdomen of a 4-year-old boy. (a) The biopsied material stained weakly for CD99, whereas (b) the excised tumor showed diffuse and strong positivity for the same antigen. The staining procedure used was identical.

Peripheral neuroectodermal tumors expressing DES, MSA, or MYOGN are classified as polyphenotypic small round cell tumors by many observers. 

Desmoplastic small round cell tumor variants of polyphenotypic small round cell tumors are generally negative for CD99. 

Polyphenotypic small round cell tumor (DSMCT) Mesenchymal chondrosarcoma... 

The latter observation also recalls a notable pitfall in the diagnosis of E-RMS namely, polyphenotypic small round cell tumors with partial myogenic differentiation. These lesions coexpress several markers of divergent cellular lineages. We will discuss them in greater detail later in this section (Fig. 4.13). 

The term polyphenotypic small round cell tumor seems most appropriate to describe the general attributes of these lesions. The desmoplastic small round cell tumor (DSRCT) is the best-recognized representative of this group. It is characterized by an EWS/WTl chimeric transcript, and is a morphologically distinctive neoplasm that is characterized by reactive fibrosis surrounding discrete nests of tumor cells (Fig. 4.15). DSRCT has a relatively complicated immunoprofile. 

Desmoplastic small round cell tumor SRCs in nests with prominent stromal desmoplasia, central necrosis, variable cystic degeneration, and epithelial differentiation. Polyphenotypic cytokeratins, epithelial membrane antigen, vimentin, desmin, WT1 (C-terminus), CD99. t(11 22)(p13 q12)with EWS/WTl fusion. 

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