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We evaluated whether changes in 18 F-Fluoro-D-Glucose ( 18 F-FDG)-uptake evaluated early during erlotinib treatment predict survival in non-small cell lung cancer (NSCLC) patients. Positron emission tomography (PET)/CT scans from 56 NSCLC patients before and after 7-10 days of erlotinib treatment were analyzed with four different methods: Visual evaluation and percentage change in lean body mass corrected standardized uptake values (SULs): SUL peak , SUL max and total lesion glycolysis (TLG). The semi-quantitative parameters abilities to predict progression free survival (PFS) and overall survival (OS) were compared and we found that percentage change in SUL peak , SUL max and TLG all correlated with PFS and OS with the strongest correlation found for TLG (R=0.51, P < 0.001). The highest area under the curve (AUC) for predicting OS was for TLG (0.70 (0.56-0.85)) with a sensitivity of 0.68 and a specificity of 079. All methods except visual evaluation, SUL peak at 15% and 30%, and TLG at 40% cut-off separates the survival curves for the response categories for PFS. For OS, visual evaluation and SUL max did not, whereas TLG at 4 different cut-off levels and SUL peak at the three lowest cut-off levels did., In Conclusion: Early change in 18 F-FDG-uptake during erlotinib correlated to both PFS and OS. TLG, as suggested by PERCIST 1.0, shows the strongest correlation to survival, whereas visual evaluation seems to be less sensitive at this very early time-point, but lower cut-off levels for discriminating between response categories seem to be relevant as we find that 20-25% change for both response and progression is optimal., Competing Interests: None.