Small Round Blue Cell Sarcoma Other Than Ewing Sarcoma: What Should an Oncologist Know?

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Sarcoma (SH Okuno, Section Editor)

Small Round Blue Cell Sarcoma Other Than Ewing Sarcoma: What Should an Oncologist Know? Jessica L. Davis, MD1 Erin R. Rudzinski, MD2,* Address 1 Department of Pathology, Oregon Health & Science University, Portland, OR, 97239, USA *,2 Department of Laboratories, Seattle Children’s Hospital, Seattle, WA, 98105, USA Email: [email protected]

* Springer Science+Business Media, LLC, part of Springer Nature 2020

This article is part of the Topical Collection on Sarcoma Keywords Round cell sarcoma I CIC-rearranged sarcoma I BCOR-altered sarcomas I Sarcomas with non-ETS gene fusions

Opinion statement The diagnosis of round cell sarcomas has changed rapidly over the last decade, causing much diagnostic confusion for pathologists and oncologists. The advances in diagnosis are largely due to the advent of next-generation sequencing techniques, which allowed the recognition of novel gene fusions in round cell sarcomas. The new 5th edition of the WHO Classification of Tumors of Soft Tissue and Bone recognizes four subgroups of undifferentiated round cell sarcomas: Ewing sarcoma, CIC-rearranged sarcomas, BCORaltered sarcomas, and sarcomas with EWSR1-non-ETS fusions, in addition to desmoplastic small round cell tumor. This classification is based on a variety of publications showing that each of these molecular subtypes has unique clinical and prognostic characteristics distinct from Ewing sarcoma, therefore supporting the validity of recognizing these as discrete diagnostic entities. Despite our improved ability to diagnose these new round cell sarcomas, there remains confusion on how best to identify and treat these tumors. However, several key clinicopathologic features can point the physician toward the correct diagnosis. The goal of the following article is to emphasize the key clinical, pathologic, molecular, and prognostic differences between Ewing sarcoma and these non-Ewing round cell malignancies to improve recognition of these rare diseases.

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Curr. Treat. Options in Oncol.

(2020) 21:90

Introduction Round cell sarcomas are a challenging group of tumors to properly diagnose and classify. Historically, most tumors were classified as either Ewing sarcoma (ES) or undifferentiated round cell sarcoma. However, with the evolution of more sophisticated molecular techniques, including RNA sequencing and expression profiling, several unique diagnostic categories (sometimes referred to collectively as Ewing-like sarcomas) have emerged. [1] While these tumors are rare, and there is still much to learn, these new subtypes demonstrate their own morphologic, immunophenotypic, molecular, and clinical signatures that are important for the pathologist and oncologist to

recognize. To this end, the term Ewing-like sarcoma is no longer applied generically, and these diagnoses are individually recognized in the new 5th Edition World Health Organization Classification of these round cell tumors. [2•] The emphasis of this article is on the clinicopathologic features