frequently develop acute esophageal toxicity (AET). Studies investigating the dose-effect relationship with qualitative AET scores have resulted in a wide range of dosimetric predictive parameters, while the underlying biological processes and local dose-effect relation have not been investigated. The purpose of this study was to correlate the planned dose distribution to post-CRT local esophageal FDG-PET uptake as a surrogate for local inflammation, and subsequently utilize the derived local dose-effect for AET prediction. Material/Methods: Patients treated with 24 x 2.75 Gy IMRT plus daily low dose Cisplatin and having a post-RT PET (PETpost) scan acquired within 3 months after cCRT were retrospectively selected. The outer wall of the esophagus was delineated on the planning CT and propagated to the PETpost-CT through deformable registration. The value of PETpost in relation to AET was evaluated by comparing the mean esophageal SUVof the highest 50% (SUV50%) between gr 50 Gy were associated with elevated FDG uptake, when the PETpost was acquired within 4 weeks after cCRT. After 4 weeks no dose effect was observed. The derived local dose effect model parameters, based on data acquired within 4 weeks (nZ47), were D50 Z 93 Gy and k Z 9.4. The SUV50% was significantly higher for gr 2 AET (2.2 vs. 2.7, pZ0.008). The average DPET was 54.1 Gy (1SDZ11.3) for gr<2, and 60.3 Gy (1SDZ4.3) for gr 2 AET (pZ0.001). DPET based prediction of AET gr 2 had the highest likelihood compared to Dmean (pZ0.06), V35 (pZ0.05) and V60 (pZ0.29). Conclusions: Esophageal uptake of FDG post-cCRT is associated with acute esophageal toxicity, and was only observed for RT dose above 50Gy. Predictability of grade 2 AET was improved by using the derived local relation between RT dose and PETpost SUV. With this biological response model, planning constraints onand predictability of AET can be done more accurately. Author Disclosure: J. Nijkamp: None. M. Rossi: None. J. Belderbos: None. W. Uyterlinde: None. M. Kwint: None. M. van den Heuvel: None. W. Vogel: None. J. Sonke: None.