(P 026) IS THERE STILL A PLACE FOR BONE MARROW SAMPLING IN VERY HIGH RISK EWING SARCOMA? ROLE OF FDG PET/CT IN THE INITIAL STAGING OF VERY HIGH RISK EWING SARCOMA IN A PROSPECTIVE PHASE II MULTICENTRIC STUDY.

Objective: Ewing Sarcoma Family of Tumours (ESFT) are rare tumours, with metastatic spread at diagnosis in one out of three patients. Bone and/or bone marrow involvement strike markedly prognosis. Accurate initial staging is therefore fundamental for treatment adaptation. 18F-FDG PET/CT is the current modality for the evaluation of disease extension. The aim of this study is to assess the prospective value of FDG PET/CT compared to bone marrow cytology/biopsy (BMCB) staging in very high risk ESFT patients.

Methods: Pediatric and adult patients from 15 French sarcomas referral centers with a diagnosis of extrapulmonary disseminated ESFT were prospectively included from 2017 to 2021, in the COMBINAIR3 phase II trial. This study aimed to evaluate a strategy combining dose dense induction chemotherapy, high dose consolidation and prolonged maintenance for these very high risk patients. All patients underwent baseline full body 18F-FDG PET/CT with centralized real-time imaging review and BMCB sampling at diagnosis consisting in both bone marrow cytology and biopsy (at least 2 different sites per patient). Patients management was similar wether bone spread was presenting as lytic or intra-medullarry lesion. We report here the baseline bone and bone marrow (BM) analysis.

Results: Out of 43 patients included (aged 6 to 47 years-old), 11 were positive on BMCB staging (cytology n=6, biopsy n=1, both n=4), all consistant with bone/BM extension defined on baseline 18F-PET/CT: 6 had BM involvement on PET (5 also had bone lesions), 5 had bone involvement only.
PET/CT identified metastatic spread to bone and/or BM in 35 patients (81%), including 24 with negative BMCB. Twenty-three had bone lesions, 11 had both bone and BM involvement and one had BM extension alone. Bone/BM involvement was confirmed either by lytic bone destruction on the CT-component or on MRI imaging.

Conclusion: FDG PET/CT detects 3 times more lesions than BMCB in the initial staging of very high risk Ewing Sarcoma. Our data suggest that BMCB evaluation can be avoided in favor of non-invasive PET/CT staging.