Undifferentiated small round cell sarcoma – A WHO 2020 update

Undifferentiated small round cell sarcomas of bone and soft tissue.

1. Ewing sarcoma

2. Round cell sarcoma with EWSR1-non-ETS fusions.

3. CIC-rearranged sarcomas

4. Sarcoma with BCOR genetic alterations

1. EWINGS SARCOMA

Ewing sarcoma (ES) and peripheral primitive neuroectodermal tumor (PNET) comprise the same spectrum of neoplastic diseases known as the Ewing sarcoma family of tumors (EFT). The characteristic translocations involve the EWSR1 gene at 22q12 and either the FLI1 gene at 11q24 or the ERG gene at 21q22.

2. Round cell sarcoma with EWSR1-non-ETS fusions.

They are of two types (a) NFATC2 sarcoma may exhibit remarkable epithelioid features.

(b) PATZ1 sarcomas are composed of a rather undifferentiated round to ovoid cell population, often characterised by the presence of a sclerotic background.

3. CIC-Rearranged Sarcomas

CIC sarcomas always exhibits focal pleomorphism, and at times epithelioid morphology can predominate. CIC-DUX4 fusion from either a t(4;19)(q35;q13) or a t(10;19)(q26;q13) translocation.

4. Sarcoma with BCOR genetic alterations

BCL6 transcriptional corepressor (BCOR) is located at Xp11.4 locus

BCOR sarcomas (despite being allocated to the round cell sarcoma family) more often tend to exhibit a spindled morphology.

Rearrangements occurring in this novel disease mainly include BCOR-CCNB3, BCOR-MAML3 and ZC3H7B-BCOR

Reference: The 2020 WHO Classification of Soft Tissue Tumours: news and perspectives Marta Sbaraglia1, Elena Bellan1, Angelo P. Dei Tos.