ESTRO 2021 Abstract Book

S1164

ESTRO 2021

STFS was 13.3 months with a 2-year rate of 32.4%. In univariate analysis, Karnofsky index, FNCLCC grade, CTV size, EQD2, AJCC T stage were significantly related to the four clinical outcomes . In multivariate analysis, higher grade and AJCC T stage were significantly related to lower LPFS, PFS, OS and STFS, whereas higher EQD2 was positively correlated with LPFS, OS and STFS. Twenty-three patients (22%) experienced severe acute adverse events related to RT (n=22 grade 3, n=1 grade 5), mostly radiation dermitis (n=12, 11.1%). Conclusion In patients presenting unresectable STS, dose escalation using definitive modern photon radiotherapy is a safe treatment, and can delay initiation of systemic treatment. Further investigation are necessary to identify patients who benefits the most for this modality. PO-1417 Impact of preoperative radiotherapy in surgically treated Ewing Sarcoma M. Ganovelli 1 , G. Frosini 1 , M. Loi 2 , D. Greto 2 , B. Guerrieri 1 , C. Mattioli 1 , C. Cerbai 1 , V. Lorenzetti 1 , D.A. Campanacci 3 , C. Caporalini 4 , F. Muratori 5 , G. Scoccianti 5 , M. Mangoni 1 , L. Livi 1 1 University of Florence, Department of experimental and clinical biomedical sciences "M. Serio", Florence, Italy; 2 Azienda Ospedaliero-Universitaria Careggi, Radiation Oncology Unit, Florence, Italy; 3 University of Florence, Department of Orthopaedic Oncology, Florence, Italy; 4 Azienda Ospedaliero-Universitaria Careggi, Pathology Unit, Florence, Italy; 5 Azienda Ospedaliero-Universitaria Careggi, Department of Orthopaedic Oncology, Florence, Italy Purpose or Objective Ewing sarcoma (EWS) is a heterogeneous and aggressive group of disease with a major incidence in adolescent and young adults. In patients eligible for upfront chemotherapy followed by local excision, pathological response is correlated with disease control and improved resection quality. Radiotherapy is frequently combined to surgery, in particular following poor response to chemotherapy and marginal resection, although it is unclear if preoperative administration may result in improved tumor regression and increased local control. The main aim of the study is to evaluate the determinants of local control in surgically treated EWS and the impact of preoperative radiotherapy in terms of pathologic response and resecability. Materials and Methods Data of consecutive EWS patients treated with induction chemotherapy and surgical excision between July 2002 and January 2020 were retrospectively reviewed. Local Control (LC) was defined as the time interval from diagnosis to local relapse. Tumor response was graded according to the Picci score. Statistical analysis was performed to assess correlation between clinical and pathological features and outcome. Results Thirty-nine patients were included in our study. Skeletal and extra-skeletal disease were found in 33 (84%) and 6 (16%) patients, respectively. Primary tumor was located on the trunk in 46% of cases (n=18) and limbs in 54% of cases (n=21). Metastatic disease was present at diagnosis in 38% (n=15) of patients. All patients received preoperative chemotherapy followed by surgical excision, resulting in a 90% R0 resection rate. Radiotherapy was administered in 21 (54%) patients to a median dose of 54 (range 42-54) Gy in 36 (range 28-36) fractions, consisting of preoperative and postoperative irradiation in 10 (26%) and 11 patients (28%), respectively. Pathological response was scored grade 1 (n=30, 77%), 2 (n=2, 5%) and 3 (n=7, 18%). After a median follow-up of 22 months (range 8-72 months), 6 local recurrence occurred. Local control (LC) was 92% at 1 year and 85% at 2 years. At univariate analysis, only primary tumor location in the trunk was correlated with impaired LC (median not reached, p=0.018). Use of preoperative radiotherapy was neither significantly correlated with grade 3 complete tumor response (24% versus 20% p=0.79) nor positive margin status (8% versus 20%, p=0.24).