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We compared the usefulness of 99mTc-methyl diphosphonate (99mTc-MDP) bone scintigraphy and 18F-fluorodeoxyglucose (FDG) for positron emission tomography/computed tomography (PET/CT) in predicting histologic response in patients with osteosarcoma receiving neoadjuvant chemotherapy (NAC).We retrospectively reviewed 62 patients with high-grade osteosarcoma who had received 2 cycles of NAC and surgery. All patients underwent 99mTc-MDP bone scintigraphy and 18F-FDG PET/CT before and after NAC. 99mTc-MDP uptake in the primary tumor was measured quantitatively as the maximum tumor-to-nontumor ratio (T/NTmax) and 18F-FDG uptake was measured as the maximum standardized uptake value (SUVmax), before and after NAC. The percent changes of T/NTmax (percent changes of the maximum tumor-to-nontumor ratio [Δ%T/NTmax]) and SUVmax (percent changes of the maximum standardized uptake value [Δ%SUVmax]) after NAC were calculated and the correlations between these parameters were evaluated. After surgery, the effects of NAC were graded histopathologically (good vs poor) and the optimum cut-off values of Δ%T/NTmax and Δ%SUVmax for predicting histologic response were assessed using the receiver operating characteristic (ROC) curve analysis.Δ%T/NTmax and Δ%SUVmax were positively correlated with each other (r = 0.494, P < .01). Based on the ROC curve analysis, both Δ%T/NTmax (area under the curve [AUC] = .772, P < .01) and Δ%SUVmax (AUC = .829, P < .01) predicted good histologic response. However, there was no significant difference between the AUCs of Δ%T/NTmax and Δ%SUVmax (P = .44). The sensitivity and specificity for predicting good histologic response were 83.3% and 75.0%, for the criterion Δ%T/NTmax <–12.5%, and 80.0% and 81.3% for the criterion Δ%SUVmax <−49.0%, respectively.The 99mTc-MDP bone scan and 18F-FDG PET scan are non-inferior to each other in predicting the histologic response of osteosarcoma treatments. The 99mTc-MDP bone scan and 18F-FDG PET scan showed respective advantages with differing features. Therefore, physicians should consider which scan is appropriate for their own institute based on the advantages of each scan and the circumstances of the institute.