(P 431) TREATMENT STRATEGY AND PROGNOSTIC FACTORS IN EXTRASKELETAL EWING SARCOMA: AN INSTITUTIONAL RETROSPECTIVE ANALYSIS

Objective: Extraskeletal Ewing sarcoma (EES) is an aggressive and rare small round cell neoplasm, more commonly marked by EWSR1-2::FLI1 translocation. Whether it is associated with a different prognosis comparing to its skeletal, more common, counterpart is still to be defined. The aim of our study is to analyze treatment strategy, prognostic factors, and survival rates in patients (pts) with EES.

Methods: We conducted a retrospective analysis searching for all consecutive adult pts treated with chemotherapy (CT) for EES at the Fondazione IRCCS Istituto Nazionale dei Tumori, Milan, Italy. Data on pts characteristics were retrieved and analyzed. A univariate survival analysis with log-rank test was made, and overall survival (OS) and event-free survival (EFS) were estimated using the Kaplan–Meier method.

Results: Between 1983 and 2022, we identified 122 adult pts (median age = 34) with diagnosis of EES; herein, we present the data of the 56 pts with a diagnosis confirmed by the identification of EWS/ETS family transcript. Patient characteristics, treatment and responses are summarized in Table 1 and 2. At a median follow-up of 73.5 mos, the 5-year OS was 73.5% and the 10-year OS was 64%. The 5-year OS was 84% in pts with localized disease and 25% in metastatic pts. The univariate analysis was performed on pts with localized disease. A trend to worse OS was observed in pts with tumor size ≥8 cm at the diagnosis (p= 0.12). Complete R0 resection was significantly associated with better OS and EFS compared to R1 or R2 (p < 0.0001 and p= 0.03 respectively). No difference in outcome was seen between different local treatment modalities (surgery alone, definitive radiotherapy or surgery plus radiotherapy). Median EFS rate was significantly less favorable in pts with tumors ≥8 cm (p=0.02). Pts achieving CR/PR/SD according to RECIST following induction CT had respectively 100/80/40% 5-year EFS (p=0.01). No difference in OS was seen between pts with CR and PR. Complete pathological response (pCR) was not statistically significantly associated with a better outcome compared to no pCR.

Conclusion: EES is generally underrepresented in clinical trials and/or is analyzed together with its bone counterpart. In localized disease, tumor size, adequate surgical resection and radiological response to induction CT were significantly related to outcome. We found a strong correlation between clinical response and the outcome, apparently higher than for pathologic response. The choice of local treatment was less critical for outcome than expected, confirming that also in EES a multidisciplinary treatment may be effective in selected cases also without surgery.