Prevalence of ultra-rare undifferentiated round cells sarcoma of bone and soft tissue after genomic classification.

11559 Background: Over the last decade, the category of undifferentiated round cell sarcomas (URCS), defined by the absence of Ewing sarcoma-associated translocations, has emerged. Aim of this study was to assess prevalence of each entity and outcome after genomic classification. Methods: Ewing sarcoma and other URCS diagnosed between 1920 and 2020 were reviewed. All URCS with available material were analyzed with FISH, RT-qPCR and/or Archer FusionPlex Sarcoma Panel. Demographic and treatment were collected. Survival was analyzed in patients with available follow-up. Results: 1995 cases identified, 20 cases lacked material for further genetic analysis and were excluded. 1975 cases were classified as follows: 1925 Ewing sarcomas (97.47%), 25 CIC-rearranged sarcomas (1.27%), 16 BCOR-CCNB3 rearranged sarcomas (0.81%), 2 EWSR1-NFATC2 sarcoma (0.1%), one each as CIC-LEUTX and FUS-NFATC2 rearranged sarcoma (0.05% each), and 5 as unclassified URCS (0.25%). A different presentation according to tumor type was shown in 43/50 ultra-rare tumors (Table). Forty-one/50 cases had available follow-up: 20/41 patients underwent surgery, 14/41 surgery+radiotherapy, 6 radiotherapy only, and no local treatment for 1 patient. Chemotherapy was administrated to 36/41 patients (Ewing sarcoma drugs in 16/22 CIC-DUX-4 and 8/11 BCOR-CCNB3; osteosarcoma drugs in 2/11 BCOR-CCNB3, and doxorubicin/ifosfamide in 2/22 CIC-DUX4 and 2/5 URCS; not specified in 6 cases). The 3-years overall survival (OS) was 32.7%f for CIC-rearranged sarcomas (75% in localize disease, 7,7% for the advanced disease, p 0.0084), 81.8% for BCOR-CCNB3 sarcomas (87.5% localized, 66.7% advanced; p 0.0734), and 60% for URCS (p 0.057). 1 patient with CIC-LEUTX sarcoma presenting with metastases died 13 months from diagnosis, 1 patient with FUS-NFATC2 and 1 with EWSR1-NFATC2 rearrenged sarcomas were alive without disease at 8 and 5 years from onset. Conclusions: Prevalence of URCS characterized by a combination of morphologic observation ad molecular techniques is provided. The majority of the cases underwent surgery or surgery combined with radiotherapy, and Ewing-like chemotherapy. The survival difference among different entities underscores the need of accurate subclassification of round cell sarcomas. Novel drugs for CIC-DUX-4 sarcomas presenting with metastases are needed. [Table: see text]