[F]FDG PET/CT quantitative parameters for the prediction of histological response to induction chemotherapy and clinical outcome in patients with localised bone and soft-tissue Ewing sarcoma.

Objective: The application of [F]FDG PET/CT in predicting histologic response to induction chemotherapy in patients with Ewing sarcoma (EWS) has been proposed using the values of pre-post treatment SUV as a referral parameter, although with heterogeneous results. The aim of this retrospective study was to evaluate the diagnostic accuracy of [F]FDG PET/CT volumetric parameters (metabolic tumour volume (MTV) and total lesion glycolysis (TLG)) as compared to SUV to predict response to chemotherapy and clinical outcome in patients with localised EWS of bone and soft-tissue.

Methods: Twenty-eight patients with non-metastatic EWS of bone (n = 20) and soft tissues (n = 8) who underwent a [F]FDG PET/CT scan before (PET) and after induction chemotherapy (PET) were enclosed in the analysis. Values of PET metrics (SUV, MTV, TLG) at diagnosis and after neoadjuvant chemotherapy as well as the percentage change between PET and PET (ΔSUV, ΔMTV and ΔTLG) were correlated to histological response and to progression-free survival (PFS).

Results: ΔTLG (cut-off: -60%) is the best predictor for histologic response with 100% sensitivity and 77.8% specificity. MTV > 33.4 cm and TLG > 112 were also associated with a favourable histologic response (sensitivity 80% and specificity 77.8% for both). On multivariate analysis, SUV (> 3.3) and ΔTLG (< -18%) were independent predictors of worse PFS.

Conclusions: [F]FDG PET/CT could accurately predict histologic response to neoadjuvant chemotherapy in patients with EWS, also showing a possible prognostic value for future disease relapse.

Key Points: • The variation of the PET parameter tumour lesion glycolysis (TLG) can predict the histologic response to induction chemotherapy (sensitivity 100%, specificity 77.8%), in patients with Ewing sarcoma. • The percentage variation of TLG and the value of the SUVmax at PET scan after chemotherapy show a prognostic role for future disease relapse. The combination of both the parameters identifies three prognostic classes of patients with low, intermediate and high risk of disease relapse.