Can postoperative radiotherapy be omitted in localized standard-risk Ewing sarcoma? An observational study of the Euro-EWING Group.

10518 Background: Uncertainty exists on the most appropriate use of surgery, radiotherapy (RT) or both for treatment of primary Ewing sarcoma (ES). The objective of this study was to assess the impact of postoperative RT on local control in pts with localized ES and good (<10% residual cells) histologic response to chemotherapy (CT). Methods: We analyzed data of all pts included in the EE99-R1 trial (comparing 2 consolidation CT regimens) undergoing surgery after induction CT. Local recurrence (LR) cumulative incidence (CI) was estimated using a competing risk approach. As local therapy was not randomized but tailored to the individual patient and tumor characteristics, impact of RT was assessed in multivariable models, adjusted for possible confounders: country, age, tumor type, site and volume, quality of resection and histologic response. We also evaluated the heterogeneity of RT effect by patient and tumor characteristics (interaction). Results: Among the 599 pts included from 1999 to 2009, 142 (24%) ...