121P Radiological staging with FDG-PET vs WB-MRI in Ewing sarcoma in the era of precision treatment

Baseline accurate assessment of metastatic disease is crucial in Ewing sarcoma (ES) for determining treatment strategies. Advanced imaging, including 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) with CT or MRI, enhance metastasis detection and can be used alongside imaging for staging of skeletal metastases, such as whole-body MRI (WB-MRI). CT chest scans are performed to detect lung metastases. We share our experience staging ES patients referred through the London Sarcoma Service (LSS). We reviewed staging investigations from January 2018 to December 2022 for patients referred via LSS with confirmed ES, using electronic patient records. Among 182 identified patients, 95 underwent FDG-PET imaging, consisting of PET-CT (n=78) and PET-MRI (n=17). In this cohort, 49/95 (52%) had localised disease, and 46/95 (48%) had locally advanced or metastatic disease. Using a gold standard of PET, WB-MRI and CT chest combined, the sensitivity for metastasis detection was 75% (6/8 patients) with PET-CT/MRI and 50% (4/8) with WB-MRI. PET-CT/MRI demonstrated a negative predictive value of 91% (21/23), while WB-MRI showed 84% (21/25). In depth comparison of the 29 patients with both WB-MRI and FDG-PET (24 PET-CT, 5 PET-MRI) baseline staging imaging was performed. Additional lymph node metastases were present in 5 patients and not seen on WB-MRI. 10 had indeterminate findings on WB-MRI requiring further imaging (4 with metastatic bone disease, and 6 requiring further FDG-PET or dedicated imaging of the site of concern). WB-MRI's utility in baseline staging was limited in this cohort, offering no substantial incremental information compared to PET-CT/MR. It exhibited shortcomings in detecting lymph node metastases, with no instances of metastasis solely identified by WB-MRI without concurrent detection on PET-CT/MR. Based on this dataset, PET-CT/MR emerges as a superior tool for baseline staging, while the role of WB-MRI should be reserved for specific cases contingent upon clinical considerations. Importantly, staging and treatment should not be delayed if WB-MRI is not readily available. Acknowledging the study's limited sample size, larger investigations are needed for comprehensive insights.