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Background: Undifferentiated round-cell sarcomas (URCSs) of soft tissue and bone are a group of clinically heterogeneous tumors. Diagnosis of these malignancies is based mainly on recurrent genetic alterations. The most common and the best known representative of this group is Ewing sarcoma (ES) which is characterized by gene fusions including EWSR1 or FUS and ETS transcription factors family. Other newly described entities are CIC-rearranged sarcoma, sarcoma with BCOR genetic alterations, and round-cell sarcoma with EWSR1-non-ETS fusions. All these novel tumors are known as Ewing-like sarcomas., Summary: It is believed that morphologic features of ES and Ewing-like sarcomas vary only slightly or even that cytomorphology is not relevant. But differences are usually obvious, and some cytologic findings, such as spindle cells, connective tissue fragments, or myxoid stroma, are typical for Ewing-like sarcomas but not for ES. Each of these entities is also characterized by different immunoprofiles. The aim of this review was to summarize cytomorphologic and immunohistochemical features of URCS and compare them with other small round-cell tumors., Key Messages: Cytology can be successfully used in URCS diagnosis as a complementary tool for core-needle biopsy or even alone in selected cases, especially in recurrent and metastatic tumors. Knowing the morphologic and immunohistochemical differences between URCS is essential to provide appropriate ancillary studies and make a definitive diagnosis., (© 2021 S. Karger AG, Basel.)

Objectives: BCOR-CCNB3 sarcoma (BCS) is an undifferentiated tumor that has some clinical and morphologic similarity to classic Ewing sarcoma, but it is characterized by a distinct BCOR-CCNB3 gene fusion. There are no reports describing its cytomorphologic findings., Methods: We describe cytologic findings of five molecularly proven BCS cases (four males and one female, aged 8.5-22 years)., Results: Smears showed mainly round cells, but some spindle cells and rhabdoid-like cells were also observed. Dispersed cells dominated in smears, but also distinct pseudopapillary structures with vascular cores were noted in four cases. Scant connective tissue fragments were found in four cases. There was no rosette formation in any case., Conclusions: BCS should be differentiated from other round cell tumors. Some cytologic features, especially rhabdoid-like cells, connective tissue fragments, and pseudopapillary formations, combined with immunohistochemical and molecular studies, may be helpful in making the appropriate diagnosis., (© American Society for Clinical Pathology, 2019. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Background: Recently, a new World Health Organization Reporting System for Soft Tissue Cytopathology (WHO System) was introduced. To analyze the value of this system, routine fine‐needle aspiration soft tissue tumor (STT) cases were reviewed. Methods: Cytology samples of STTs collected between 1954 and 2022 at the Institut Curie were used (2214 cases, including 1376 primary tumors). All specimens were classified according to the predominant cytomorphological pattern and the WHO System. The diagnostic accuracy and risk of malignancy (ROM) in each category were calculated. Results: Final diagnoses revealed 1236 malignancies and 978 benign or low‐risk tumors. The original cytological evaluation led to 21 false‐negative results (0.85%) and 29 false‐positive results (1.17%). Sensitivity, specificity, positive predictive value, and negative predictive value were 98.3%, 92.1%, 97.5%, and 94.2%, respectively. Overall diagnostic accuracy was 94.2%. The ROM calculated according to the WHO System was 29.87%, 2.49%, 39.62%, 51.43%, 68.42%, and 97.69% in the nondiagnostic, benign, atypical, soft tissue neoplasm of uncertain malignant potential, suspicious for malignancy, and malignant categories, respectively; however, it varied broadly depending on the morphological pattern (62.78% in spindle cell tumors, 84.58% in myxoid tumors, 3.00% in lipomatous tumors, 78.15% in epithelioid tumors, 94.26% in pleomorphic tumors, and 100% in round cell tumors). Conclusions: Cytology of STTs is a powerful diagnostic method. Some cytological patterns overlap in different morphological groups, and the possibility of false‐negative and false‐positive diagnoses may persist. This analysis evidenced utility of the WHO System, especially when combined with morphological pattern assessment. Subclassification in particular diagnostic categories allowed for calculation of the ROM, which is crucial for optimal patient management. The cytological evaluation of soft tissue tumors seems to be a powerful diagnostic method and can be successfully applied in adult and pediatric populations. However, some cytological patterns may overlap in different morphological groups of lesions and the possibility of a false‐negative and false‐positive diagnosis may persist. [ABSTRACT FROM AUTHOR]